Internet Engineering Task Force (IETF) W. Lin
Request for Comments: 9625 Z. Zhang
Category: Standards Track J. Drake
ISSN: 2070-1721 E. Rosen, Ed.
Juniper Networks, Inc.
J. Rabadan
Nokia
A. Sajassi
Cisco Systems
August 2024
EVPN Optimized Inter-Subnet Multicast (OISM) Forwarding
Abstract
Ethernet VPN (EVPN) provides a service that allows a single Local
Area Network (LAN), comprising a single IP subnet, to be divided into
multiple segments. Each segment may be located at a different site,
and the segments are interconnected by an IP or MPLS backbone.
Intra-subnet traffic (either unicast or multicast) always appears to
the end users to be bridged, even when it is actually carried over
the IP or MPLS backbone. When a single tenant owns multiple such
LANs, EVPN also allows IP unicast traffic to be routed between those
LANs. This document specifies new procedures that allow inter-subnet
IP multicast traffic to be routed among the LANs of a given tenant
while still making intra-subnet IP multicast traffic appear to be
bridged. These procedures can provide optimal routing of the inter-
subnet multicast traffic and do not require any such traffic to
egress a given router and then ingress that same router. These
procedures also accommodate IP multicast traffic that originates or
is destined to be external to the EVPN domain.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9625.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Terminology
1.1.1. Requirements Language
1.2. Background
1.2.1. Segments, Broadcast Domains, and Tenants
1.2.2. Inter-BD (Inter-Subnet) IP Traffic
1.2.3. EVPN and IP Multicast
1.2.4. BDs, MAC-VRFs, and EVPN Service Models
1.3. Need for EVPN-Aware Multicast Procedures
1.4. Additional Requirements That Must Be Met by the Solution
1.5. Model of Operation: Overview
1.5.1. Control Plane
1.5.2. Data Plane
2. Detailed Model of Operation
2.1. Supplementary Broadcast Domain
2.2. Detecting When a Route is for/from a Particular BD
2.3. Use of IRB Interfaces at Ingress PE
2.4. Use of IRB Interfaces at an Egress PE
2.5. Announcing Interest in (S,G)
2.6. Tunneling Frames from Ingress PEs to Egress PEs
2.7. Advanced Scenarios
3. EVPN-Aware Multicast Solution Control Plane
3.1. Supplementary Broadcast Domain (SBD) and Route Targets
3.2. Advertising the Tunnels Used for IP Multicast
3.2.1. Constructing Routes for the SBD
3.2.2. Ingress Replication
3.2.3. Assisted Replication
3.2.3.1. Automatic SBD Matching
3.2.4. BIER
3.2.5. Inclusive P2MP Tunnels
3.2.5.1. Using the BUM Tunnels as IP Multicast Inclusive
Tunnels
3.2.5.2. Using Wildcard S-PMSI A-D Routes to Advertise
Inclusive Tunnels Specific to IP Multicast
3.2.6. Selective Tunnels
3.3. Advertising SMET Routes
4. Constructing Multicast Forwarding State
4.1. Layer 2 Multicast State
4.1.1. Constructing the OIF List
4.1.2. Data Plane: Applying the OIF List to an (S,G) Frame
4.1.2.1. Eligibility of an AC to Receive a Frame
4.1.2.2. Applying the OIF List
4.2. Layer 3 Forwarding State
5. Interworking with Non-OISM EVPN PEs
5.1. IPMG Designated Forwarder
5.2. Ingress Replication
5.2.1. Ingress PE is Non-OISM
5.2.2. Ingress PE is OISM
5.3. P2MP Tunnels
6. Traffic to/from Outside the EVPN Tenant Domain
6.1. Layer 3 Interworking via EVPN OISM PEs
6.1.1. General Principles
6.1.2. Interworking with MVPN
6.1.2.1. MVPN Sources with EVPN Receivers
6.1.2.1.1. Identifying MVPN Sources
6.1.2.1.2. Joining a Flow from an MVPN Source
6.1.2.2. EVPN Sources with MVPN Receivers
6.1.2.2.1. General Procedures
6.1.2.2.2. Any-Source Multicast (ASM) Groups
6.1.2.2.3. Source on Multihomed Segment
6.1.2.3. Obtaining Optimal Routing of Traffic between MVPN
and EVPN
6.1.2.4. Selecting the MEG SBD-DR
6.1.3. Interworking with Global Table Multicast
6.1.4. Interworking with PIM
6.1.4.1. Source Inside EVPN Domain
6.1.4.2. Source Outside EVPN Domain
6.2. Interworking with PIM via an External PIM Router
7. Using an EVPN Tenant Domain as an Intermediate (Transit)
Network for Multicast Traffic
8. IANA Considerations
9. Security Considerations
10. References
10.1. Normative References
10.2. Informative References
Appendix A. Integrated Routing and Bridging
Acknowledgements
Authors' Addresses
1. Introduction
1.1. Terminology
In this document, we make frequent use of the following terminology:
OISM: Optimized Inter-Subnet Multicast. EVPN PEs that follow the
procedures of this document will be known as "OISM" Provider Edges
(PEs). EVPN PEs that do not follow the procedures of this
document will be known as "non-OISM" PEs.
IP Multicast Packet: An IP packet whose IP Destination Address field
is a multicast address that is not a link-local address. (Link-
local addresses are IPv4 addresses in the 224/24 range and IPv6
addresses in the FF02/16 range.)
IP Multicast Frame: An Ethernet frame whose payload is an IP
multicast packet (as defined above).
(S,G) Multicast Packet: An IP multicast packet whose Source IP
Address field contains S and whose IP Destination Address field
contains G.
(S,G) Multicast Frame: An IP multicast frame whose payload contains
S in its Source IP Address field and G in its IP Destination
Address field.
EVI: EVPN Instance. An EVPN instance spanning the PE devices
participating in that EVPN.
BD: Broadcast Domain. An emulated Ethernet, such that two systems
on the same BD will receive each other's link-local broadcasts.
Note that EVPN supports service models in which a single EVI
contains only one BD and service models in which a single EVI
contains multiple BDs. Both types of service models are supported
by this document. In all models, a given BD belongs to only one
EVI.
DF: Designated Forwarder. As defined in [RFC7432], an Ethernet
segment may be multihomed (attached to more than one PE). An
Ethernet segment may also contain multiple BDs of one or more
EVIs. For each such EVI, one of the PEs attached to the segment
becomes that EVI's DF for that segment. Since a BD may belong to
only one EVI, we can speak unambiguously of the BD's DF for a
given segment.
AC: Attachment Circuit. An AC connects the bridging function of an
EVPN PE to an Ethernet segment of a particular BD. ACs are not
visible at the Layer 3.
If a given Ethernet segment, attached to a given PE, contains n
BDs, we say that the PE has n ACs to that segment.
L3 Gateway: An L3 Gateway is a PE that connects an EVPN Tenant
Domain to an external multicast domain by performing both the OISM
procedures and the Layer 3 multicast procedures of the external
domain.
PEG: PIM/EVPN Gateway. An L3 Gateway that connects an EVPN Tenant
Domain to an external multicast domain whose Layer 3 multicast
procedures are those of PIM [RFC7761].
MEG: MVPN/EVPN Gateway. An L3 Gateway that connects an EVPN Tenant
Domain to an external multicast domain whose Layer 3 multicast
procedures are those of Multicast VPN (MVPN) [RFC6513] [RFC6514].
IPMG: IP Multicast Gateway. A PE that is used for interworking OISM
EVPN PEs with non-OISM EVPN PEs.
DR: Designated Router. A PE that has special responsibilities for
handling multicast on a given BD.
FHR: First Hop Router. The FHR is a PIM router [RFC7761] with
special responsibilities. It is the first multicast router to see
(S,G) packets from source S, and if G is an Any-Source Multicast
(ASM) group, the FHR is responsible for sending PIM Register
messages to the PIM Rendezvous Point (RP) for group G.
LHR: Last Hop Router. The LHR is a PIM router [RFC7761] with
special responsibilities. Generally, it is attached to a LAN, and
it determines whether there are any hosts on the LAN that need to
receive a given multicast flow. If so, it creates and sends the
PIM Join messages that are necessary to receive the flow.
EC: Extended Community. A BGP Extended Communities attribute
[RFC4360] [RFC7153] is a BGP path attribute that consists of one
or more Extended Communities.
RT: Route Target. A Route Target is a particular kind of BGP
Extended Community. A BGP Extended Community consists of a type
field, a sub-type field, and a value field. Certain type/sub-type
combinations indicate that a particular Extended Community is an
RT. RT1 and RT2 are considered to be the same RT if and only if
they have the same type, sub-type, and value fields.
C- prefix: In many documents on VPN multicast, the prefix C- appears
before any address or wildcard that refers to an address or
addresses in a tenant's address space rather than to an address of
addresses in the address space of the backbone network. This
document omits the C- prefix in many cases where it is clear from
the context that the reference is to the tenant's address space.
This document also assumes familiarity with the terminology of
[RFC4364], [RFC6514], [RFC7432], [RFC7761], [RFC9136], [RFC9251], and
[RFC9572].
1.1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Background
Ethernet VPN (EVPN) [RFC7432] provides a Layer 2 VPN (L2VPN)
solution, which allows an IP or MPLS backbone provider to offer
Ethernet service to a set of customers, known as "tenants".
In this section (as well as in [RFC9135]), we provide some essential
background information on EVPN.
1.2.1. Segments, Broadcast Domains, and Tenants
One of the key concepts of EVPN is the Broadcast Domain (BD). A BD
is essentially an emulated Ethernet. Each BD belongs to a single
tenant. A BD typically consists of multiple Ethernet segments, and
each segment may be attached to a different EVPN Provider Edge (EVPN
PE) router. EVPN PE routers are often referred to as "Network
Virtualization Endpoints (NVEs)". However, this document will use
the term "EVPN PE" or, when the context is clear, just "PE".
In this document, the term "segment" is used interchangeably with
"Ethernet Segment" or "ES", as defined in [RFC7432].
Attached to each segment are Tenant Systems (TSs). A TS may be any
type of system, physical or virtual, host or router, etc., that can
attach to an Ethernet.
When two TSs are on the same segment, traffic between them does not
pass through an EVPN PE. When two TSs are on different segments of
the same BD, traffic between them does pass through an EVPN PE.
When two TSs, say TS1 and TS2, are on the same BD, then the following
occurs:
* If TS1 knows the Media Access Control (MAC) address of TS2, TS1
can send unicast Ethernet frames to TS2. TS2 will receive the
frames unaltered.
* If TS1 broadcasts an Ethernet frame, TS2 will receive the
unaltered frame.
* If TS1 multicasts an Ethernet frame, TS2 will receive the
unaltered frame as long as TS2 has been provisioned to receive the
Ethernet multicast destination MAC address.
When we say that TS2 receives an unaltered frame from TS1, we mean
that the frame still contains TS1's MAC address and that no
alteration of the frame's payload (and consequently, no alteration of
the payload's IP header) has been made.
EVPN allows a single segment to be attached to multiple PE routers.
This is known as "EVPN multihoming". Suppose a given segment is
attached to both PE1 and PE2, and suppose PE1 receives a frame from
that segment. It may be necessary for PE1 to send the frame over the
backbone to PE2. EVPN has procedures to ensure that such a frame
cannot be sent back to its originating segment by PE2. This is
particularly important for multicast, because a frame arriving at PE1
from a given segment will already have been seen by all the systems
on that segment that need to see it. If the frame was sent back to
the originating segment by PE2, receivers on that segment would
receive the packet twice. Even worse, the frame might be sent back
to PE1, which could cause an infinite loop.
1.2.2. Inter-BD (Inter-Subnet) IP Traffic
If a given tenant has multiple BDs, the tenant may wish to allow IP
communication among these BDs. Such a set of BDs is known as an
"EVPN Tenant Domain" or just a "Tenant Domain".
If tenant systems TS1 and TS2 are not in the same BD, then they do
not receive unaltered Ethernet frames from each other. In order for
TS1 to send traffic to TS2, TS1 encapsulates an IP datagram inside an
Ethernet frame and uses Ethernet to send these frames to an IP
router. The router decapsulates the IP datagram, does the IP
processing, and re-encapsulates the datagram for Ethernet. The MAC
Source Address field now has the MAC address of the router, not of
TS1. The TTL field of the IP datagram should be decremented by
exactly 1, even if the frame needs to be sent from one PE to another.
The structure of the provider's backbone is thus hidden from the
tenants.
EVPN accommodates the need for inter-BD communication within a Tenant
Domain by providing an integrated L2/L3 service for unicast IP
traffic. EVPN's Integrated Routing and Bridging (IRB) functionality
is specified in [RFC9135]. Each BD in a Tenant Domain is assumed to
be a single IP subnet, and each IP subnet within a given Tenant
Domain is assumed to be a single BD. EVPN's IRB functionality allows
IP traffic to travel from one BD to another and ensures that proper
IP processing (e.g., TTL decrement) is done.
A brief overview of IRB, including the notion of an IRB interface,
can be found in Appendix A. As explained there, an IRB interface is
a sort of virtual interface connecting an L3 routing instance to a
BD. A BD may have multiple Attachment Circuits (ACs) to a given PE,
where each AC connects to a different Ethernet segment of the BD.
However, these ACs are not visible to the L3 routing function; from
the perspective of an L3 routing instance, a PE has just one
interface to each BD, viz., the IRB interface for that BD.
In this document, when traffic is routed out of an IRB interface, we
say it is sent down the IRB interface to the BD that the IRB is for.
In the other direction, traffic is sent up the IRB interface from the
BD to the L3 routing instance.
The L3 routing instance depicted in Appendix A is associated with a
single Tenant Domain and may be thought of as IP Virtual Routing and
Forwarding (IP-VRF) for that Tenant Domain.
1.2.3. EVPN and IP Multicast
[RFC9135] and [RFC9136] cover inter-subnet (inter-BD) IP unicast
forwarding, but they do not cover inter-subnet IP multicast
forwarding.
[RFC7432] covers intra-subnet (intra-BD) Ethernet multicast. The
intra-subnet Ethernet multicast procedures of [RFC7432] are used for
Ethernet broadcast traffic, Ethernet unicast traffic whose
Destination MAC Address field contains an unknown address, and
Ethernet traffic whose Destination MAC Address field contains an
Ethernet multicast MAC address. These three classes of traffic are
known collectively as "BUM traffic" (Broadcast, Unknown Unicast, or
Multicast traffic), and the procedures for handling BUM traffic are
known as "BUM procedures".
[RFC9251] extends the intra-subnet Ethernet multicast procedures by
adding procedures that are specific to, and optimized for, the use of
IP multicast within a subnet. However, that document does not cover
inter-subnet IP multicast.
The purpose of this document is to specify procedures for EVPN that
provide optimized IP multicast functionality within an EVPN Tenant
Domain. This document also specifies procedures that allow IP
multicast packets to be sourced from or destined to systems outside
the Tenant Domain. The entire set of procedures are referred to as
"Optimized Inter-Subnet Multicast (OISM)" procedures.
In order to support the OISM procedures specified in this document,
an EVPN PE MUST also support [RFC9135] and [RFC9251]. (However,
certain procedures in [RFC9251] are modified when OISM is supported.)
1.2.4. BDs, MAC-VRFs, and EVPN Service Models
[RFC7432] defines the notion of MAC-VRF (MAC Virtual Routing and
Forwarding). A MAC-VRF contains one or more bridge tables (see
Section 3 of [RFC7432]), each of which represents a single Broadcast
Domain.
In the IRB model (outlined in Appendix A), an L3 routing instance has
one IRB interface per BD, NOT one per MAC-VRF. This document does
not distinguish between a Broadcast Domain and a bridge table;
instead, it uses the terms interchangeably (or will use the acronym
"BD" to refer to either). The way the BDs are grouped into MAC-VRFs
is not relevant to the procedures specified in this document.
Section 6 of [RFC7432] also defines several different EVPN service
models:
* In the vlan-based service, each MAC-VRF contains one bridge table,
where the bridge table corresponds to a particular Virtual LAN
(VLAN) (see Section 3 of [RFC7432]). Thus, each VLAN is treated
as a BD.
* In the vlan bundle service, each MAC-VRF contains one bridge
table, where the bridge table corresponds to a set of VLANs.
Thus, a set of VLANs are treated as constituting a single BD.
* In the vlan-aware bundle service, each MAC-VRF may contain
multiple bridge tables, where each bridge table corresponds to one
BD. If a MAC-VRF contains several bridge tables, then it
corresponds to several BDs.
The procedures in this document are intended to work for all these
service models.
1.3. Need for EVPN-Aware Multicast Procedures
Inter-subnet IP multicast among a set of BDs can be achieved, in a
non-optimal manner, without any specific EVPN procedures. For
instance, if a particular tenant has n BDs among which it wants to
send IP multicast traffic, it can simply attach a conventional
multicast router to all n BDs. Or more generally, as long as each BD
has at least one IP multicast router, and the IP multicast routers
communicate multicast control information with each other,
conventional IP multicast procedures will work normally, and no
special EVPN functionality is needed.
However, that technique does not provide optimal routing for
multicast. In conventional multicast routing, for a given multicast
flow, there is only one multicast router on each BD that is permitted
to send traffic of that flow to the BD. If that BD has receivers for
a given flow, but the source of the flow is not on that BD, then the
flow must pass through that multicast router. This leads to the
hairpinning problem described (for unicast) in Appendix A.
For example, consider an (S,G) flow that is sourced by a TS S and
needs to be received by TSs R1 and R2. Suppose S is on a segment of
BD1, R1 is on a segment of BD2, but both are attached to PE1. Also
suppose that the tenant has a multicast router attached to a segment
of BD1 and to a segment of BD2. However, the segments to which that
router is attached are both attached to PE2. Then, the flow from S
to R would have to follow the path: S-->PE1-->PE2-->tenant multicast
router-->PE2-->PE1-->R1. Obviously, the path S-->PE1-->R would be
preferred.
+---+ +---+
|PE1+----------------------+PE2|
+---+-+ +-+---+
| \ \ / / |
BD1 BD2 BD3 BD3 BD2 BD1
| | | \ | |
S R1 R2 router
Now suppose that there is a second receiver, R2. R2 is attached to a
third BD, BD3. However, it is attached to a segment of BD3 that is
attached to PE1. And suppose that the tenant multicast router is
attached to a segment of BD3 that attaches to PE2. In this case, the
tenant multicast router will make two copies of the packet, one for
BD2 and one for BD3. PE2 will send both copies back to PE1. Not
only is the routing sub-optimal, but PE2 also sends multiple copies
of the same packet to PE1, which is a further sub-optimality.
This is only an example; many more examples of sub-optimal multicast
routing can easily be given. To eliminate sub-optimal routing and
extra copies, it is necessary to have a multicast solution that is
EVPN-aware and that can use its knowledge of the internal structure
of a Tenant Domain to ensure that multicast traffic gets routed
optimally. The procedures in this document allow us to avoid all
such sub-optimalities when routing inter-subnet multicast traffic
within a Tenant Domain.
1.4. Additional Requirements That Must Be Met by the Solution
In addition to providing optimal routing of multicast flows within a
Tenant Domain, the EVPN-aware multicast solution is intended to
satisfy the following requirements:
* The solution must integrate well with the procedures specified in
[RFC9251]. That is, an integrated set of procedures must handle
both intra-subnet multicast and inter-subnet multicast.
* With regard to intra-subnet multicast, the solution MUST maintain
the integrity of the multicast Ethernet service. This means:
- If a source and a receiver are on the same subnet, the MAC
Source Address (SA) of the multicast frame sent by the source
will not get rewritten.
- If a source and a receiver are on the same subnet, no IP
processing of the Ethernet payload is done. The IP TTL is not
decremented, the IPv4 header checksum is not changed, no
fragmentation is done, etc.
* On the other hand, if a source and a receiver are on different
subnets, the frame received by the receiver will not have the MAC
Source Address of the source, as the frame will appear to have
come from a multicast router. Also, proper processing of the IP
header is done, e.g., TTL decrements by 1, header checksum
modification, possible fragmentation, etc.
* If a Tenant Domain contains several BDs, it MUST be possible for a
multicast flow (even when the multicast group address is an ASM
address) to have sources in one of those BDs and receivers in one
or more of the other BDs without requiring the presence of any
system performing PIM RP functions [RFC7761].
* Sometimes a MAC address used by one TS on a particular BD is also
used by another TS on a different BD. Inter-subnet routing of
multicast traffic MUST NOT make any assumptions about the
uniqueness of a MAC address across several BDs.
* If two EVPN PEs attached to the same Tenant Domain both support
the OISM procedures, each may receive inter-subnet multicasts from
the other, even if the egress PE is not attached to any segment of
the BD from which the multicast packets are being sourced. It
MUST NOT be necessary to provision the egress PE with knowledge of
the ingress BD.
* There must be a procedure that allows EVPN PE routers supporting
OISM procedures to send/receive multicast traffic to/from EVPN PE
routers that support only [RFC7432] but that does not support the
OISM procedures or even the procedures of [RFC9135]. However,
when interworking with such routers (which we call "non-OISM PE
routers"), optimal routing may not be achievable.
* It MUST be possible to support scenarios in which multicast flows
with sources inside a Tenant Domain have external receivers, i.e.,
receivers that are outside the domain. It must also be possible
to support scenarios where multicast flows with external sources
(sources outside the Tenant Domain) have receivers inside the
domain.
This presupposes that unicast routes to multicast sources outside
the domain can be distributed to EVPN PEs attached to the domain
and that unicast routes to multicast sources within the domain can
be distributed outside the domain.
Of particular importance are the scenarios in which the external
sources and/or receivers are reachable via L3VPN/MVPN or via IP/
PIM.
The solution for external interworking MUST allow for deployment
scenarios in which EVPN does not need to export a host route for
every multicast source.
* The solution for external interworking must not presuppose that
the same tunneling technology is used within both the EVPN domain
and the external domain. For example, MVPN interworking must be
possible when MVPN is using MPLS Point-to-Multipoint (P2MP)
tunneling and when EVPN is using Ingress Replication (IR) or
Virtual eXtensible Local Area Network (VXLAN) tunneling.
* The solution must not be overly dependent on the details of a
small set of use cases but must be adaptable to new use cases as
they arise. (That is, the solution must be robust.)
1.5. Model of Operation: Overview
1.5.1. Control Plane
In this section, and in the remainder of this document, we assume the
reader is familiar with the procedures of IGMP / Multicast Listener
Discovery (MLD) (see [RFC3376] and [RFC3810]), by which hosts
announce their interest in receiving particular multicast flows.
Consider a Tenant Domain consisting of a set of k BDs: BD1, ..., BDk.
To support the OISM procedures, each Tenant Domain must also be
associated with a Supplementary Broadcast Domain (SBD). An SBD is
treated in the control plane as a real BD, but it does not have any
ACs. The SBD has several uses; these will be described later in this
document (see Sections 2.1 and 3).
Each PE that attaches to one or more of the BDs in a given Tenant
Domain will be provisioned to recognize that those BDs are part of
the same Tenant Domain. Note that a given PE does not need to be
configured with all the BDs of a given Tenant Domain. In general, a
PE will only be attached to a subset of the BDs in a given Tenant
Domain and will be configured only with that subset of BDs. However,
each PE attached to a given Tenant Domain must be configured with the
SBD for that Tenant Domain.
Suppose a particular segment of a particular BD is attached to PE1.
[RFC7432] specifies that PE1 must originate an Inclusive Multicast
Ethernet Tag (IMET) route for that BD and that the IMET route must be
propagated to all other PEs attached to the same BD. If the given
segment contains a host that has interest in receiving a particular
multicast flow, either an (S,G) flow or a (*,G) flow, PE1 will learn
of that interest by participating in the IGMP/MLD snooping
procedures, as specified in [RFC4541]. In this case:
* PE1 is interested in receiving the flow;
* the AC attaching the interested host to PE1 is also said to be
interested in the flow; and
* the BD containing an AC that is interested in a particular flow is
also said to be interested in that flow.
Once PE1 determines that it has an AC that is interested in receiving
a particular flow or set of flows, it originates one or more
Selective Multicast Ethernet Tag (SMET) routes [RFC9251] to advertise
that interest.
Note that each IMET or SMET route is for a particular BD. The notion
of a route being for a particular BD is explained in Section 2.2.
When OISM is being supported, the procedures of [RFC9251] are
modified as follows:
* The IMET route originated by a particular PE for a particular BD
is distributed to all other PEs attached to the Tenant Domain
containing that BD, even to those PEs that are not attached to
that particular BD.
* The SMET routes originated by a particular PE are originated on a
per-Tenant-Domain basis rather than a per-BD basis. That is, the
SMET routes are considered to be for the Tenant Domain's SBD
rather than any of its ordinary BDs. These SMET routes are
distributed to all the PEs attached to the Tenant Domain.
In this way, each PE attached to a given Tenant Domain learns,
from the other PEs attached to the same Tenant Domain, the set of
flows that are of interest to each of those other PEs.
An OISM PE that is provisioned with several BDs in the same Tenant
Domain MUST originate an IMET route for each such BD. To indicate
its support of [RFC9251], it SHOULD attach the EVPN Multicast Flags
Extended Community to each such IMET route, but it MUST attach the EC
to at least one such IMET route.
Suppose PE1 is provisioned with both BD1 and BD2 and considers them
to be part of the same Tenant Domain. It is possible that PE1 will
receive both an IMET route for BD1 and an IMET route for BD2 from
PE2. If either of these IMET routes has the EVPN Multicast Flags
Extended Community, PE1 MUST assume that PE2 is supporting the
procedures of [RFC9251] for ALL BDs in the Tenant Domain.
If a PE supports OISM functionality, it indicates that, by setting
the OISM-supported flag in the Multicast Flags Extended Community, it
attaches to some or all of its IMET routes. An OISM PE SHOULD attach
this EC with the OISM-supported flag set to all the IMET routes it
originates. However, if PE1 imports IMET routes from PE2, and at
least one of PE2's IMET routes indicates that PE2 is an OISM PE, PE1
MUST assume that PE2 is following OISM procedures.
1.5.2. Data Plane
Suppose PE1 has an AC to a segment in BD1 and PE1 receives an (S,G)
multicast frame from that AC (as defined in Section 1.1).
There may be other ACs of PE1 on which TSs have indicated an interest
(via IGMP/MLD) in receiving (S,G) multicast packets. PE1 is
responsible for sending the received multicast packet on those ACs.
There are two cases to consider:
* Intra-Subnet Forwarding: In this case, an AC with interest in
(S,G) is connected to a segment that is part of the source BD,
BD1. If the segment is not multihomed, or if PE1 is the
Designated Forwarder (DF) (see [RFC7432]) for that segment, PE1
sends the multicast frame on that AC without changing the MAC SA.
The IP header is not modified at all; in particular, the TTL is
not decremented.
* Inter-Subnet Forwarding: An AC with interest in (S,G) is connected
to a segment of BD2, where BD2 is different than BD1. If PE1 is
the DF for that segment (or if the segment is not multihomed), PE1
decapsulates the IP multicast packet, performs any necessary IP
processing (including TTL decrement), and then re-encapsulates the
packet appropriately for BD2. PE1 then sends the packet on the
AC. Note that after re-encapsulation, the MAC SA will be PE1's
MAC address on BD2. The IP TTL will have been decremented by 1.
In addition, there may be other PEs that are interested in (S,G)
traffic. Suppose PE2 is such a PE. Then, PE1 tunnels a copy of the
IP multicast frame (with its original MAC SA and with no alteration
of the payload's IP header) to PE2. The tunnel encapsulation
contains information that PE2 can use to associate the frame with an
apparent source BD. If the actual source BD of the frame is BD1,
then:
* If PE2 is attached to BD1, the tunnel encapsulation used to send
the frame to PE2 will cause PE2 to identify BD1 as the apparent
source BD.
* If PE2 is not attached to BD1, the tunnel encapsulation used to
send the frame to PE2 will cause PE2 to identify the SBD as the
apparent source BD.
Note that the tunnel encapsulation used for a particular BD will have
been advertised in an IMET route or a Selective Provider Multicast
Service Interface (S-PMSI) route [RFC9572] for that BD. That route
carries a PMSI Tunnel Attribute (PTA), which specifies how packets
originating from that BD are encapsulated. This information enables
the PE receiving a tunneled packet to identify the apparent source BD
as stated above. See Section 3.2 for more details.
When PE2 receives the tunneled frame, it will forward it on any of
its ACs that have interest in (S,G).
If PE2 determines from the tunnel encapsulation that the apparent
source BD is BD1, then:
* For those ACs that connect PE2 to BD1, the intra-subnet forwarding
procedure described above is used, except that it is now PE2, not
PE1, carrying out that procedure. Unmodified EVPN procedures from
[RFC7432] are used to ensure that a packet originating from a
multihomed segment is never sent back to that segment.
* For those ACs that do not connect to BD1, the inter-subnet
forwarding procedure described above is used, except that it is
now PE2, not PE1, carrying out that procedure.
If the tunnel encapsulation identifies the apparent source BD as the
SBD, PE2 applies the inter-subnet forwarding procedures described
above to all of its ACs that have interest in the flow.
These procedures ensure that an IP multicast frame travels from its
ingress PE to all egress PEs that are interested in receiving it.
While in transit, the frame retains its original MAC SA, and the
payload of the frame retains its original IP header. Note that in
all cases, when an IP multicast packet is sent from one BD to
another, these procedures cause its TTL to be decremented by 1.
So far, we have assumed that an IP multicast packet arrives at its
ingress PE over an AC that belongs to one of the BDs in a given
Tenant Domain. However, it is possible for a packet to arrive at its
ingress PE in other ways. Since an EVPN PE supporting IRB has an IP-
VRF, it is possible that the IP-VRF will have a VRF interface that is
not an IRB interface. For example, there might be a VRF interface
that is actually a physical link to an external Ethernet switch, a
directly attached host, or a router. When an EVPN PE, say PE1,
receives a packet through such means, we will say that the packet has
an external source (i.e., a source outside the Tenant Domain). There
are also other scenarios in which a multicast packet might have an
external source, e.g., it might arrive over an MVPN tunnel from an
L3VPN PE. In such cases, we will still refer to PE1 as the "ingress
EVPN PE".
When an EVPN PE, say PE1, receives an externally sourced multicast
packet, and there are receivers for that packet inside the Tenant
Domain, it does the following:
* Suppose PE1 has an AC in BD1 that has interest in (S,G). Then,
PE1 encapsulates the packet for BD1, filling in the MAC SA field
with PE1's own MAC address on BD1. It sends the resulting frame
on the AC.
* Suppose some other EVPN PE, say PE2, has interest in (S,G). PE1
encapsulates the packet for Ethernet, filling in the MAC SA field
with PE1's own MAC address on the SBD. PE1 then tunnels the
packet to PE2. The tunnel encapsulation will identify the
apparent source BD as the SBD. Since the apparent source BD is
the SBD, PE2 will know to treat the frame as an inter-subnet
multicast.
When IR is used to transmit IP multicast frames from an ingress EVPN
PE to a set of egress PEs, then the ingress PE has to send multiple
copies of the frame. Each copy is the original Ethernet frame;
decapsulation and IP processing take place only at the egress PE.
If a P2MP tree or Bit Index Explicit Replication (BIER) [RFC9624] is
used to transmit an IP multicast frame from an ingress PE to a set of
egress PEs, then the ingress PE only has to send one copy of the
frame to each of its next hops. Again, each egress PE receives the
original frame and does any necessary IP processing.
2. Detailed Model of Operation
The model described in Section 1.5.2 can be expressed more precisely
using the notion of IRB interface (see Appendix A). For a given
Tenant Domain:
* A given PE has one IRB interface for each BD to which it is
attached. This IRB interface connects L3 routing to that BD.
When IP multicast packets are sent or received on the IRB
interfaces, the semantics of the interface are modified from the
semantics described in Appendix A. See Section 2.3 for the
details of the modification.
* Each PE also has an IRB interface that connects L3 routing to the
SBD. The semantics of this interface is different than the
semantics of the IRB interface to the real BDs. See Section 2.3.
In this section, we assume that PIM is not enabled on the IRB
interfaces. In general, it is not necessary to enable PIM on the IRB
interfaces unless there are PIM routers on one of the Tenant Domain's
BDs or there is some other scenario requiring a Tenant Domain's L3
routing instance to become a PIM adjacency of some other system.
These cases will be discussed in Section 7.
2.1. Supplementary Broadcast Domain
Suppose a given Tenant Domain contains three BDs (BD1, BD2, and BD3)
and two PEs (PE1 and PE2). PE1 attaches to BD1 and BD2, while PE2
attaches to BD2 and BD3.
To carry out the procedures described above, all the PEs attached to
the Tenant Domain must be provisioned with the SBD for that Tenant
Domain. An RT must be associated with the SBD and provisioned on
each of those PEs. We will refer to that RT as the "SBD-RT".
A Tenant Domain is also configured with an IP-VRF [RFC9135], and the
IP-VRF is associated with an RT. This RT MAY be the same as the SBD-
RT.
Suppose an (S,G) multicast frame originating on BD1 has a receiver on
BD3. PE1 will transmit the packet to PE2 as a frame, and the
encapsulation will identify the frame's source BD as BD1. Since PE2
is not provisioned with BD1, it will treat the packet as if its
source BD were the SBD. That is, a packet can be transmitted from
BD1 to BD3 even though its ingress PE is not configured for BD3 and/
or its egress PE is not configured for BD1.
EVPN supports service models in which a given EVI can contain only
one BD. It also supports service models in which a given EVI can
contain multiple BDs. No matter which service model is being used
for a particular tenant, it is highly RECOMMENDED that an EVI
containing only the SBD be provisioned for that tenant.
If, for some reason, it is not feasible to provision an EVI that
contains only the SBD, it is possible to put the SBD in an EVI that
contains other BDs. However, in that case, the SBD-RT MUST be
different than the RT associated with any other BD. Otherwise, the
procedures of this document (as detailed in Sections 2.2 and 3.1)
will not produce correct results.
2.2. Detecting When a Route is for/from a Particular BD
In this document, we frequently say that a particular multicast route
is "from" or "for" a particular BD or is "related to" or "associated
with" a particular BD. These terms are used interchangeably.
Subsequent sections of this document explain when various routes must
be originated for particular BDs. In this section, we explain how
the PE originating a route marks the route to indicate which BD it is
for. We also explain how a PE receiving the route determines which
BD the route is for.
In EVPN, each BD is assigned an RT. An RT is a BGP Extended
Community that can be attached to the BGP routes used by the EVPN
control plane. In some EVPN service models, each BD is assigned a
unique RT. In other service models, a set of BDs (all in the same
EVI) may be assigned the same RT. The RT that is assigned to the SBD
is called the "SBD-RT".
In those service models that allow a set of BDs to share a single RT,
each BD is assigned a non-zero Tag ID. The Tag ID appears in the
Network Layer Reachability Information (NLRI) of many of the BGP
routes that are used by the EVPN control plane.
A given route may be for the SBD or an ordinary BD (a BD that is not
the SBD). An RT that has been assigned to an ordinary BD will be
known as an "ordinary BD-RT".
When constructing an IMET, SMET, S-PMSI, or Leaf [RFC9572] route that
is for a given BD, the following rules apply:
* If the route is for an ordinary BD, say BD1, then:
- the route MUST carry the ordinary BD-RT associated with BD1 and
- the route MUST NOT carry any RT that is associated with an
ordinary BD other than BD1.
* If the route is for the SBD, the route MUST carry the SBD-RT and
MUST NOT carry any RT that is associated with any other BD.
* As detailed in subsequent sections, under certain circumstances, a
route that is for BD1 may carry both the RT of BD1 and also the
SBD-RT.
The IMET route for the SBD MUST carry a Multicast Flags Extended
Community in which an OISM SBD flag is set.
The IMET route for a BD other than the SBD SHOULD carry an EVI-RT EC
as defined in [RFC9251]. The EC is constructed from the SBD-RT to
indicate the BD's corresponding SBD. This allows all PEs to check
that they have consistent SBD provisioning and allows an Assisted
Replication (AR) replicator to automatically determine a BD's
corresponding SBD without any provisioning, as explained in
Section 3.2.3.1.
When receiving an IMET, SMET, S-PMSI, or Leaf route, it is necessary
for the receiving PE to determine the BD to which the route belongs.
This is done by examining the RTs carried by the route, as well as
the Tag ID field of the route's NLRI. There are several cases to
consider. Some of these cases are error cases that arise when the
route has not been properly constructed.
When one of the error cases is detected, the route MUST be regarded
as a malformed route, and the treat-as-withdraw procedure of
[RFC7606] MUST be applied. Note that these error cases are only
detectable by EVPN procedures at the receiving PE; BGP procedures at
intermediate nodes will generally not detect the existence of such
error cases and in general SHOULD NOT attempt to do so.
Case 1: The receiving PE recognizes more than one of the route's RTs
as being an SBD-RT (i.e., the route carries SBD-RTs of more
than one Tenant Domain).
This is an error case; the route has not been properly
constructed.
Case 2: The receiving PE recognizes one of the route's RTs as being
associated with an ordinary BD and recognizes one of the
route's other RTs as being associated with a different
ordinary BD.
This is an error case; the route has not been properly
constructed.
Case 3: The receiving PE recognizes one of the route's RTs as being
associated with an ordinary BD in a particular Tenant Domain
and recognizes another of the route's RTs as being
associated with the SBD of a different Tenant Domain.
This is an error case; the route has not been properly
constructed.
Case 4: The receiving PE does not recognize any of the route's RTs
as being associated with an ordinary BD in any of its Tenant
Domains but does recognize one of the RTs as the SBD-RT of
one of its Tenant Domains.
In this case, the receiving PE associates the route with the
SBD of that Tenant Domain. This association is made even if
the Tag ID field of the route's NLRI is not the Tag ID of
the SBD.
This is a normal use case where either (a) the route is for
a BD to which the receiving PE is not attached or (b) the
route is for the SBD. In either case, the receiving PE
associates the route with the SBD.
Case 5: The receiving PE recognizes exactly one of the RTs as an
ordinary BD-RT that is associated with one of the PE's EVIs,
say EVI-1. The receiving PE also recognizes one of the RTs
as being the SBD-RT of the Tenant Domain containing EVI-1.
In this case, the route is associated with the BD in EVI-1
that is identified (in the context of EVI-1) by the Tag ID
field of the route's NLRI. (If EVI-1 contains only a single
BD, the Tag ID is likely to be zero.)
This is the case where the route is for a BD to which the
receiving PE is attached, but the route also carries the
SBD-RT. In this case, the receiving PE associates the route
with the ordinary BD, not with the SBD.
Note that according to the above rules, the mapping from BD to RT is
a many-to-one or one-to-one mapping. A route that an EVPN PE
originates for a particular BD carries that BD's RT, and an EVPN PE
that receives the route associates it with a BD as described above.
However, RTs are not used only to help identify the BD to which a
route belongs; they may also be used by BGP to determine the path
along which the route is distributed and to determine which PEs
receive the route. There may be cases where it is desirable to
originate a route for a particular BD but have that route distributed
to only some of the EVPN PEs attached to that BD. Or one might want
the route distributed to some intermediate set of systems, where it
might be modified or replaced before being propagated further. Such
situations are outside the scope of this document.
Additionally, there may be situations where it is desirable to
exchange routes among two or more different Tenant Domains (EVPN
Extranet). Such situations are outside the scope of this document.
2.3. Use of IRB Interfaces at Ingress PE
When an (S,G) multicast frame is received from an AC belonging to a
particular BD, say BD1:
1. The frame is sent unchanged to other EVPN PEs that are interested
in (S,G) traffic. The encapsulation used to send the frame to
the other EVPN PEs depends on the tunnel type being used for
multicast transmission. (For our purposes, we consider IR, AR,
and BIER to be tunnel types, even though IR, AR, and BIER do not
actually use P2MP tunnels.) At the egress PE, the apparent
source BD of the frame can be inferred from the tunnel
encapsulation. If the egress PE is not attached to the actual
source BD, it will infer that the apparent source BD is the SBD.
Note that the inter-PE transmission of a multicast frame among
EVPN PEs of the same Tenant Domain does NOT involve the IRB
interfaces as long as the multicast frame was received over an AC
attached to one of the Tenant Domain's BDs.
2. The frame is also sent up the IRB interface that attaches BD1 to
the Tenant Domain's L3 routing instance in this PE. That is, the
L3 routing instance, behaving as if it were a multicast router,
receives the IP multicast frames that arrive at the PE from its
local ACs. The L3 routing instance decapsulates the frame's
payload to extract the IP multicast packet, decrements the IP
TTL, adjusts the header checksum, and does any other necessary IP
processing (e.g., fragmentation).
3. The L3 routing instance keeps track of which BDs have local
receivers for (S,G) traffic. (A local receiver is a TS,
reachable via a local AC, that has expressed interest in (S,G)
traffic.) If the L3 routing instance has an IRB interface to
BD2, and it knows that BD2 has a LOCAL receiver interested in
(S,G) traffic, it encapsulates the packet in an Ethernet header
for BD2, putting its own MAC address in the MAC SA field. Then,
it sends the packet down the IRB interface to BD2.
If a packet is sent from the L3 routing instance to a particular BD
via the IRB interface (step 3 in the above list), and if the BD in
question is NOT the SBD, the packet is sent ONLY to LOCAL ACs of that
BD. If the packet needs to go to other PEs, it has already been sent
to them in step 1. Note that this is a change in the IRB interface
semantics from what is described in [RFC9135] and Figure 3.
If a given locally attached segment is multihomed, existing EVPN
procedures ensure that a packet is not sent by a given PE to that
segment unless the PE is the DF for that segment. Those procedures
also ensure that a packet is never sent by a PE to its segment of
origin. Thus, EVPN segment multihoming is fully supported; duplicate
delivery to a segment or looping on a segment are thereby prevented
without the need for any new procedures to be defined in this
document.
What if an IP multicast packet is received from outside the Tenant
Domain? For instance, perhaps PE1's IP-VRF for a particular Tenant
Domain also has a physical interface leading to an external switch,
host, or router and PE1 receives an IP multicast packet or frame on
that interface, or perhaps the packet is from an L3VPN or a different
EVPN Tenant Domain.
Such a packet is first processed by the L3 routing instance, which
decrements TTL and does any other necessary IP processing. Then, the
packet is sent into the Tenant Domain by sending it down the IRB
interface to the SBD of that Tenant Domain. This requires
encapsulating the packet in an Ethernet header. The MAC SA field
will contain the PE's own MAC on the SBD.
An IP multicast packet sent by the L3 routing instance down the IRB
interface to the SBD is treated as if it had arrived from a local AC,
and steps 1-3 are applied. Note that the semantics of sending a
packet down the IRB interface to the SBD are thus slightly different
than the semantics of sending a packet down other IRB interfaces. IP
multicast packets sent down the SBD's IRB interface may be
distributed to other PEs, but IP multicast packets sent down other
IRB interfaces are distributed only to local ACs.
If a PE sends a link-local multicast packet down the SBD IRB
interface, that packet will be distributed (as an Ethernet frame) to
other PEs of the Tenant Domain but will not appear on any of the
actual BDs.
2.4. Use of IRB Interfaces at an Egress PE
Suppose an egress EVPN PE receives an (S,G) multicast frame from the
frame's ingress EVPN PE. As described above, the packet will arrive
as an Ethernet frame over a tunnel from the ingress PE, and the
tunnel encapsulation will identify the source BD of the Ethernet
frame.
We define the notion of the frame's apparent source BD as follows.
If the egress PE is attached to the actual source BD, the actual
source BD is the apparent source BD. If the egress PE is not
attached to the actual source BD, the SBD is the apparent source BD.
The egress PE now takes the following steps:
1. If the egress PE has ACs belonging to the apparent source BD of
the frame, it sends the frame unchanged to any ACs of that BD
that have interest in (S,G) packets. The MAC SA of the frame is
not modified, and the IP header of the frame's payload is not
modified in any way.
2. The frame is also sent to the L3 routing instance by being sent
up the IRB interface that attaches the L3 routing instance to the
apparent source BD. Steps 2 and 3 listed in Section 2.3 are then
applied.
2.5. Announcing Interest in (S,G)
[RFC9251] defines procedures used by an egress PE to announce its
interest in a multicast flow or set of flows. If an egress PE
determines it has LOCAL receivers in a particular BD, say BD1, that
are interested in a particular set of flows, it originates one or
more SMET routes for BD1. Each SMET route specifies a particular
(S,G) or (*,G) flow. By originating a SMET route for BD1, a PE is
announcing "I have receivers for (S,G) or (*,G) in BD1". Such a SMET
route carries the RT for BD1, ensuring that it will be distributed to
all PEs that are attached to BD1.
The OISM procedures for originating SMET routes differ slightly from
those in [RFC9251]. In most cases, the SMET routes are considered to
be for the SBD rather than the BD containing local receivers. These
SMET routes carry the SBD-RT and do not carry any ordinary BD-RT.
Details on the processing of SMET routes can be found in Section 3.3.
Since the SMET routes carry the SBD-RT, every ingress PE attached to
a particular Tenant Domain will learn of all other PEs (attached to
the same Tenant Domain) that have interest in a particular set of
flows. Note that a PE that receives a given SMET route does not
necessarily have any BDs (other than the SBD) in common with the PE
that originates that SMET route.
If all the sources and receivers for a given (*,G) are in the Tenant
Domain, inter-subnet ASM traffic will be properly routed without
requiring any RPs, shared trees, or other complex aspects of
multicast routing infrastructure. Suppose, for example, that:
* PE1 has a local receiver, on BD1, for (*,G) and
* PE2 has a local source, on BD2, for (*,G).
PE1 will originate a SMET(*,G) route for the SBD, and PE2 will
receive that route, even if PE2 is not attached to BD1. PE2 will
thus know to forward (S,G) traffic to PE1. PE1 does not need to do
any source discovery. (This does assume that source S does not send
the same (S,G) datagram on two different BDs and that the Tenant
Domain does not contain two or more sources with the same IP address
S. The use of multicast sources that have IP anycast addresses is
outside the scope of this document.)
If some PE attached to the Tenant Domain does not support [RFC9251],
it will be assumed to be interested in all flows. Whether a
particular remote PE supports [RFC9251] or not is determined by the
presence of the Multicast Flags Extended Community in its IMET route;
this is specified in [RFC9251].
2.6. Tunneling Frames from Ingress PEs to Egress PEs
[RFC7432] specifies the procedures for setting up and using BUM
tunnels. A BUM tunnel is a tunnel used to carry traffic on a
particular BD if that traffic is (a) broadcast traffic, (b) unicast
traffic with an unknown Destination MAC Address, or (c) Ethernet
multicast traffic.
This document allows the BUM tunnels to be used as the default
tunnels for transmitting IP multicast frames. It also allows a
separate set of tunnels to be used, instead of the BUM tunnels, as
the default tunnels for carrying IP multicast frames. Let's call
these "IP multicast tunnels".
When the tunneling is done via IR or via BIER, this difference is of
no significance. However, when P2MP tunnels are used, there is a
significant advantage to having separate IP multicast tunnels.
It is desirable for an ingress PE to transmit a copy of a given (S,G)
multicast frame on only one P2MP tunnel. All egress PEs interested
in (S,G) packets then have to join that tunnel. If the source BD and
PE for an (S,G) frame are BD1 and PE1, respectively, and if PE2 has
receivers on BD2 for (S,G), then PE2 must join the P2MP Label
Switched Path (LSP) on which PE1 transmits the (S,G) frame. PE2 must
join this P2MP LSP even if PE2 is not attached to the source BD, BD1.
If PE1 was transmitting the multicast frame on its BD1 BUM tunnel,
then PE2 would have to join the BD1 BUM tunnel, even though PE2 has
no BD1 Attachment Circuits. This would cause PE2 to pull all the BUM
traffic from BD1, most of which it would just have to discard. Thus,
it is RECOMMENDED that the default IP multicast tunnels be distinct
from the BUM tunnels.
Notwithstanding the above, link-local IP multicast traffic MUST
always be carried on the BUM tunnels and ONLY on the BUM tunnels.
Link-local IP multicast traffic consists of IPv4 traffic with a
destination address prefix of 224/24 and IPv6 traffic with a
destination address prefix of FF02/16. In this document, the terms
"IP multicast packet" and "IP multicast frame" are defined in
Section 1.1 so as to exclude link-local traffic.
Note that it is also possible to use selective tunnels to carry
particular multicast flows (see Section 3.2). When an (S,G) frame is
transmitted on a selective tunnel, it is not transmitted on the BUM
tunnel or on the default IP multicast tunnel.
2.7. Advanced Scenarios
There are some deployment scenarios that require special procedures:
1. Some multicast sources or receivers are attached to PEs that
support [RFC7432] but do not support this document or [RFC9135].
To interoperate with these non-OISM PEs, it is necessary to have
one or more gateway PEs that interface the tunnels discussed in
this document with the BUM tunnels of the legacy PEs. This is
discussed in Section 5.
2. Sometimes multicast traffic originates from outside the EVPN
domain or needs to be sent outside the EVPN domain. This is
discussed in Section 6. An important special case of this,
integration with MVPN, is discussed in Section 6.1.2.
3. In some scenarios, one or more of the tenant systems is a PIM
router, and the Tenant Domain is used as a transit network that
is part of a larger multicast domain. This is discussed in
Section 7.
3. EVPN-Aware Multicast Solution Control Plane
3.1. Supplementary Broadcast Domain (SBD) and Route Targets
As discussed in Section 2.1, every Tenant Domain is associated with a
single SBD. Recall that a Tenant Domain is defined to be a set of
BDs that can freely send and receive IP multicast traffic to/from
each other. If an EVPN PE has one or more ACs in a BD of a
particular Tenant Domain, and if the EVPN PE supports the procedures
of this document, that EVPN PE MUST be provisioned with the SBD of
that Tenant Domain.
At each EVPN PE attached to a given Tenant Domain, there is an IRB
interface leading from the L3 routing instance of that Tenant Domain
to the SBD. However, the SBD has no ACs.
Each SBD is provisioned with an RT. All the EVPN PEs supporting a
given SBD are provisioned with that RT as an import RT. That RT MUST
NOT be the same as the RT associated with any other BD.
We will use the term "SBD-RT" to denote the RT that has been assigned
to the SBD. Routes carrying this RT will be propagated to all EVPN
PEs in the same Tenant Domain as the originator.
Section 2.2 specifies the rules by which an EVPN PE that receives a
route determines whether a received route belongs to a particular
ordinary BD or SBD.
Section 2.2 also specifies additional rules that must be followed
when constructing routes that belong to a particular BD, including
the SBD.
The SBD SHOULD be in an EVI of its own. Even if the SBD is not in an
EVI of its own, the SBD-RT MUST be different than the RT associated
with any other BD. This restriction is necessary in order for the
rules of Sections 2.2 and 3.1 to work correctly.
Note that an SBD, just like any other BD, is associated on each EVPN
PE with a MAC-VRF. Per [RFC7432], each MAC-VRF is associated with a
Route Distinguisher (RD). When constructing a route that is for an
SBD, an EVPN PE will place the RD of the associated MAC-VRF in the
Route Distinguisher field of the NLRI. (If the Tenant Domain has
several MAC-VRFs on a given PE, the EVPN PE has a choice of which RD
to use.)
If AR [RFC9574] is used, each AR-REPLICATOR for a given Tenant Domain
must be provisioned with the SBD of that Tenant Domain, even if the
AR-REPLICATOR does not have any L3 routing instances.
3.2. Advertising the Tunnels Used for IP Multicast
The procedures used for advertising the tunnels that carry IP
multicast traffic depend upon the type of tunnel being used. If the
tunnel type is neither IR, AR, nor BIER, there are procedures for
advertising both inclusive tunnels and selective tunnels.
When IR, AR, or BIER are used to transmit IP multicast packets across
the core, there are no P2MP tunnels. Once an ingress EVPN PE
determines the set of egress EVPN PEs for a given flow, the IMET
routes contain all the information needed to transport packets of
that flow to the egress PEs.
If AR is used, the ingress EVPN PE is also an AR-LEAF, and the IMET
route coming from the selected AR-REPLICATOR contains the information
needed. The AR-REPLICATOR will behave as an ingress EVPN PE when
sending a flow to the egress EVPN PEs.
If the tunneling technique requires P2MP tunnels to be set up (e.g.,
RSVP-TE P2MP, Multipoint LDP (mLDP), or PIM), some of the tunnels may
be selective tunnels and some may be inclusive tunnels.
Selective P2MP tunnels are always advertised by the ingress PE using
S-PMSI Auto-Discovery (A-D) routes [RFC9572].
For inclusive tunnels, there is a choice between using a BD's
ordinary BUM tunnel as the default inclusive tunnel for carrying IP
multicast traffic or using a separate IP multicast tunnel as the
default inclusive tunnel for carrying IP multicast. In the former
case, the inclusive tunnel is advertised in an IMET route. In the
latter case, the inclusive tunnel is advertised in a (C-*,C-*) S-PMSI
A-D route [RFC9572]. Details may be found in subsequent sections.
3.2.1. Constructing Routes for the SBD
There are situations in which an EVPN PE needs to originate IMET,
SMET, and/or S-PMSI routes for the SBD. Throughout this document, we
will refer to such routes respectively as "SBD-IMET routes", "SBD-
SMET routes", and "SBD-SPMSI routes". Subsequent sections detail the
conditions under which these routes need to be originated.
When an EVPN PE needs to originate an SBD-IMET, SBD-SMET, or SBD-
SPMSI route, it constructs the route as follows:
* The RD field of the route's NLRI is set to the RD of the MAC-VRF
that is associated with the SBD.
* The SBD-RT is attached to the route.
* The Tag ID field of the route's NLRI is set to the Tag ID that has
been assigned to the SBD. This is most likely 0 if a VLAN-based
or VLAN-bundle service is being used but non-zero if a VLAN-aware
bundle service is being used.
3.2.2. Ingress Replication
When IR is used to transport IP multicast frames of a given Tenant
Domain, each EVPN PE attached to that Tenant Domain MUST originate an
SBD-IMET route (see Section 3.2.1).
The SBD-IMET route MUST carry a PTA, and the MPLS Label field of the
PTA MUST specify a downstream-assigned MPLS label that maps uniquely
(in the context of the originating EVPN PE) to the SBD.
Following the procedures of [RFC7432], an EVPN PE MUST also originate
an IMET route for each BD to which it is attached. Each of these
IMET routes carries a PTA specifying a downstream-assigned label that
maps uniquely, in the context of the originating EVPN PE, to the BD
in question. These IMET routes need not carry the SBD-RT.
When an ingress EVPN PE needs to use IR to send an IP multicast frame
from a particular source BD to an egress EVPN PE, the ingress PE
determines whether or not the egress PE has originated an IMET route
for that BD. If so, that IMET route contains the MPLS label that the
egress PE has assigned to the source BD. The ingress PE uses that
label when transmitting the packet to the egress PE. Otherwise, the
ingress PE uses the label that the egress PE has assigned to the SBD
(in the SBD-IMET route originated by the egress).
Note that the set of IMET routes originated by a given egress PE, and
installed by a given ingress PE, may change over time. If the egress
PE withdraws its IMET route for the source BD, the ingress PE MUST
stop using the label carried in that IMET route and instead MUST use
the label carried in the SBD-IMET route from that egress PE.
Implementors must also take into account that an IMET route from a
particular PE for a particular BD may arrive after that PE's SBD-IMET
route.
3.2.3. Assisted Replication
When AR is used to transport IP multicast frames of a given Tenant
Domain, each EVPN PE (including the AR-REPLICATOR) attached to the
Tenant Domain MUST originate an SBD-IMET route (see Section 3.2.1).
An AR-REPLICATOR attached to a given Tenant Domain is considered to
be an EVPN PE of that Tenant Domain. It is attached to all the BDs
in the Tenant Domain, but it does not necessarily have L3 routing
instances.
As with IR, the SBD-IMET route carries a PTA where the MPLS Label
field specifies the downstream-assigned MPLS label that identifies
the SBD. However, the AR-REPLICATOR and AR-LEAF EVPN PEs will set
the PTA's flags differently, as per [RFC9574].
In addition, each EVPN PE originates an IMET route for each BD to
which it is attached. As in the case of IR, these routes carry the
downstream-assigned MPLS labels that identify the BDs and do not
carry the SBD-RT.
When an ingress EVPN PE, acting as AR-LEAF, needs to send an IP
multicast frame from a particular source BD to an egress EVPN PE, the
ingress PE determines whether or not there is any AR-REPLICATOR that
originated an IMET route for that BD. After the AR-REPLICATOR
selection (if there are more than one), the AR-LEAF uses the label
contained in the IMET route of the AR-REPLICATOR when transmitting
packets to it. The AR-REPLICATOR receives the packet and, based on
the procedures specified in [RFC9574] and in Section 3.2.2 of this
document, transmits the packets to the egress EVPN PEs using the
labels contained in the received IMET routes for either the source BD
or the SBD.
If an ingress AR-LEAF for a given BD has not received any IMET route
for that BD from an AR-REPLICATOR, the ingress AR-LEAF follows the
procedures in Section 3.2.2.
3.2.3.1. Automatic SBD Matching
Each PE needs to know a BD's corresponding SBD. Configuring that
information in each BD is one way, but it requires repetitive
configuration and consistency checking (to make sure that all the BDs
of the same tenant are configured with the same SBD). A better way
is to configure the SBD info in the L3 routing instance so that all
related BDs will derive the SBD information.
An AR-REPLICATOR also needs to know the same information, though it
does not necessarily have an L3 routing instance. However, from the
EVI-RT EC in a BD's IMET route, an AR-REPLICATOR can derive the
corresponding SBD of that BD without any configuration.
3.2.4. BIER
When BIER is used to transport multicast packets of a given Tenant
Domain, and a given EVPN PE attached to that Tenant Domain is a
possible ingress EVPN PE for traffic originating outside that Tenant
Domain, the given EVPN PE MUST originate an SBD-IMET route (see
Section 3.2.1).
In addition, IMET routes that are originated for other BDs in the
Tenant Domain MUST carry the SBD-RT.
Each IMET route (including but not limited to the SBD-IMET route)
MUST carry a PTA. The MPLS Label field of the PTA MUST specify an
upstream-assigned MPLS label that maps uniquely (in the context of
the originating EVPN PE) to the BD for which the route is originated.
Suppose an ingress EVPN PE, say PE1, needs to use BIER to tunnel an
IP multicast frame to a set of egress EVPN PEs. And suppose the
frame's source BD is BD1. The frame is encapsulated as follows:
* A four-octet MPLS label stack entry [RFC3032] is prepended to the
frame. The Label field is set to the upstream-assigned label that
PE1 has assigned to BD1.
* The resulting MPLS packet is then encapsulated in a BIER
encapsulation [RFC8296] [RFC9624]. The BIER BitString is set to
identify the egress EVPN PEs. The BIER Proto field is set to the
value for "MPLS packet with an upstream-assigned label at top of
the stack".
Note: It is possible that the packet being tunneled from PE1
originated outside the Tenant Domain. In this case, the actual
source BD, BD1, is considered to be the SBD, and the upstream-
assigned label it carries will be the label that PE1 assigned to the
SBD and advertised in its SBD-IMET route.
Suppose an egress PE, say PE2, receives such a BIER packet. The
BFIR-id field of the BIER header allows PE2 to determine that the
ingress PE is PE1. There are then two cases to consider:
1. PE2 has received and installed an IMET route for BD1 from PE1.
In this case, the BIER packet will be carrying the upstream-
assigned label that is specified in the PTA of that IMET route.
This enables PE2 to determine the apparent source BD (as defined
in Section 2.4).
2. PE2 has not received and installed an IMET route for BD1 from
PE1.
In this case, PE2 will not recognize the upstream-assigned label
carried in the BIER packet. PE2 MUST discard the packet.
Further details on the use of BIER to support EVPN can be found in
[RFC9624].
3.2.5. Inclusive P2MP Tunnels
3.2.5.1. Using the BUM Tunnels as IP Multicast Inclusive Tunnels
The procedures in this section apply only when:
a) it is desired to use the BUM tunnels to carry IP multicast
traffic across the backbone and
b) the BUM tunnels are P2MP tunnels (i.e., neither IR, AR, nor BIER
are being used to transport the BUM traffic).
In this case, an IP multicast frame (whether inter-subnet or intra-
subnet) will be carried across the backbone in the BUM tunnel
belonging to its source BD. Each EVPN PE attached to a given Tenant
Domain needs to join the BUM tunnels for every BD in the Tenant
Domain, even those BDs to which the EVPN PE is not locally attached.
This ensures that an IP multicast packet from any source BD can reach
all PEs attached to the Tenant Domain.
Note that this will cause all the BUM traffic from a given BD in a
Tenant Domain to be sent to all PEs that attach to that Tenant
Domain, even the PEs that don't attach to the given BD. To avoid
this, it is RECOMMENDED that the BUM tunnels not be used as IP
multicast inclusive tunnels and that the procedures of
Section 3.2.5.2 be used instead.
If a PE is a possible ingress EVPN PE for traffic originating outside
the Tenant Domain, the PE MUST originate an SBD-IMET route (see
Section 3.2.1). This route MUST carry a PTA specifying the P2MP
tunnel used for transmitting IP multicast packets that originate
outside the Tenant Domain. All EVPN PEs of the Tenant Domain MUST
join the tunnel specified in the PTA of an SBD-IMET route:
* If the tunnel is an RSVP-TE P2MP tunnel, the originator of the
route MUST use RSVP-TE P2MP procedures to add each PE of the
Tenant Domain to the tunnel, even PEs that have not originated an
SBD-IMET route.
* If the tunnel is an mLDP or PIM tunnel, each PE importing the SBD-
IMET route MUST add itself to the tunnel, using mLDP or PIM
procedures, respectively.
Whether or not a PE originates an SBD-IMET route, it will of course
originate an IMET route for each BD to which it is attached. Each of
these IMET routes MUST carry the SBD-RT, as well as the RT for the BD
to which it belongs.
If a received IMET route is not the SBD-IMET route, it will also be
carrying the RT for its source BD. The route's NLRI will carry the
Tag ID for the source BD. From the RT and the Tag ID, any PE
receiving the route can determine the route's source BD.
If the MPLS Label field of the PTA contains zero, the specified P2MP
tunnel is used only to carry frames of a single source BD.
If the MPLS Label field of the PTA does not contain zero, it MUST
contain an upstream-assigned MPLS label that maps uniquely (in the
context of the originating EVPN PE) to the source BD (or in the case
of an SBD-IMET route, to the SBD). The tunnel may then be used to
carry frames of multiple source BDs. The apparent source BD of a
particular packet is inferred from the label carried by the packet.
IP multicast traffic originating outside the Tenant Domain is
transmitted with the label corresponding to the SBD, as specified in
the ingress EVPN PE's SBD-IMET route.
3.2.5.2. Using Wildcard S-PMSI A-D Routes to Advertise Inclusive
Tunnels Specific to IP Multicast
The procedures of this section apply when (and only when) it is
desired to transmit IP multicast traffic on an inclusive tunnel but
not on the same tunnel used to transmit BUM traffic.
However, these procedures do NOT apply when the tunnel type is IR or
BIER, EXCEPT in the case where it is necessary to interwork between
non-OISM PEs and OISM PEs, as specified in Section 5.
Each EVPN PE attached to the given Tenant Domain MUST originate an
SBD-SPMSI A-D route. The NLRI of that route MUST contain (C-*,C-*)
(see [RFC6625]). Additional rules for constructing that route are
given in Section 3.2.1.
In addition, an EVPN PE MUST originate an S-PMSI A-D route containing
(C-*,C-*) in its NLRI for each of the other BDs, in the given Tenant
Domain, to which it is attached. All such routes MUST carry the SBD-
RT. This ensures that those routes are imported by all EVPN PEs
attached to the Tenant Domain.
A PE receiving these routes follows the procedures of Section 2.2 to
determine which BD the route is for.
If the MPLS Label field of the PTA contains zero, the specified
tunnel is used only to carry frames of a single source BD.
If the MPLS Label field of the PTA does not contain zero, it MUST
specify an upstream-assigned MPLS label that maps uniquely (in the
context of the originating EVPN PE) to the source BD. The tunnel may
be used to carry frames of multiple source BDs, and the apparent
source BD for a particular packet is inferred from the label carried
by the packet.
The EVPN PE advertising these S-PMSI A-D routes is specifying the
default tunnel that it will use (as ingress PE) for transmitting IP
multicast packets. The upstream-assigned label allows an egress PE
to determine the apparent source BD of a given packet.
3.2.6. Selective Tunnels
An ingress EVPN PE for a given multicast flow or set of flows can
always assign the flow to a particular P2MP tunnel by originating an
S-PMSI A-D route whose NLRI identifies the flow or set of flows. The
NLRI of the route could be (C-*,C-G) or (C-S,C-G). The S-PMSI A-D
route MUST carry the SBD-RT so that it is imported by all EVPN PEs
attached to the Tenant Domain.
An S-PMSI A-D route is for a particular source BD. It MUST carry the
RT associated with that BD, and it MUST have the Tag ID for that BD
in its NLRI.
When an EVPN PE imports an S-PMSI A-D route, it applies the rules of
Section 2.2 to associate the route with a particular BD.
Each such route MUST contain a PTA, as specified in Section 3.2.5.2.
An egress EVPN PE interested in the specified flow or flows MUST join
the specified tunnel. Procedures for joining the specified tunnel
are specific to the tunnel type. (Note that if the tunnel type is
RSVP-TE P2MP LSP, the Leaf Information Required (LIR) flag of the PTA
SHOULD NOT be set. An ingress OISM PE knows which OISM EVPN PEs are
interested in any given flow and hence can add them to the RSVP-TE
P2MP tunnel that carries such flows.)
If the PTA does not specify a non-zero MPLS label, the apparent
source BD of any packets that arrive on that tunnel is considered to
be the BD associated with the route that carries the PTA. If the PTA
does specify a non-zero MPLS label, the apparent source BD of any
packets that arrive on that tunnel carrying the specified label is
considered to be the BD associated with the route that carries the
PTA.
It should be noted that, when either IR or BIER is used, there is no
need for an ingress PE to use S-PMSI A-D routes to assign specific
flows to selective tunnels. The procedures of Section 3.3, along
with the procedures of Sections 3.2.2, 3.2.3, and 3.2.4, provide the
functionality of selective tunnels without the need to use S-PMSI A-D
routes.
3.3. Advertising SMET Routes
[RFC9251] allows an egress EVPN PE to express its interest in a
particular multicast flow or set of flows by originating a SMET
route. The NLRI of the SMET route identifies the flow or set of
flows as (C-*,C-*), (C-*,C-G), or (C-S,C-G).
Each SMET route belongs to a particular BD. The Tag ID for the BD
appears in the NLRI of the route, and the route carries the RT
associated with that BD. From this <RT, tag> pair, other EVPN PEs
can identify the BD to which a received SMET route belongs.
(Remember though that the route may be carrying multiple RTs.)
There are three cases to consider:
Case 1: It is known that no BD of a Tenant Domain contains a
multicast router.
In this case, an egress PE advertises its interest in a flow
or set of flows by originating a SMET route that belongs to
the SBD. We refer to this as an SBD-SMET route. The SBD-
SMET route carries the SBD-RT and has the Tag ID for the SBD
in its NLRI. SMET routes for the individual BDs are not
needed, because there is no need for a PE that receives a
SMET route to send a corresponding IGMP/MLD Join message on
any of its ACs.
Case 2: It is known that more than one BD of a Tenant Domain may
contain a multicast router.
This is much like Case 1. An egress PE advertises its
interest in a flow or set of flows by originating an SBD-
SMET route. The SBD-SMET route carries the SBD-RT and has
the Tag ID for the SBD in its NLRI.
In this case, it is important to be sure that SMET routes
for the individual BDs are not originated. For example,
suppose that PE1 had local receivers for a given flow on
both BD1 and BD2 and that it originated SMET routes for both
those BDs. Then, PEs receiving those SMET routes might send
IGMP/MLD Joins on both those BDs. This could cause
externally sourced multicast traffic to enter the Tenant
Domain at both BDs, which could result in duplication of
data.
Note that if it is possible that more than one BD contains a
tenant multicast router, then in order to receive multicast
data originating from outside EVPN, the PEs MUST follow the
procedures of Section 6.
Case 3: It is known that only a single BD of a Tenant Domain
contains a multicast router.
Suppose that an egress PE is attached to a BD on which there
might be a tenant multicast router. (The tenant router is
not necessarily on a segment that is attached to that PE.)
And suppose that the PE has one or more ACs attached to that
BD, which are interested in a given multicast flow. In this
case, in addition to the SMET route for the SBD, the egress
PE MAY originate a SMET route for that BD. This will enable
the ingress PE(s) to send IGMP/MLD messages on ACs for the
BD, as specified in [RFC9251]. As long as that is the only
BD on which there is a tenant multicast router, there is no
possibility of duplication of data.
This document does not specify procedures for dynamically determining
which of the three cases applies to a given deployment; the PEs of a
given Tenant Domain MUST be provisioned to know which case applies.
As detailed in [RFC9251], a SMET route carries flags indicating
whether IGMP (v1, v2, or v3) or MLD (v1 or v2) messages should be
triggered on the ACs of the BD to which the SMET route belongs. For
IGMP v3 and MLD v2, the Include/Exclude (IE) flag also indicates
whether the source information in the SMET route is of an Include
Group type or Exclude Group type. If an SBD PE needs to generate
IGMP/MLD reports (as it is the case in Section 6.2) or the route is
for an (S, G) state, the value of the flags MUST be set according to
the rules in [RFC9251]. Otherwise, the flags SHOULD be set to 0.
Note that a PE only needs to originate the set of SBD-SMET routes
that are needed in order to receive multicast traffic that the PE is
interested in. Suppose PE1 has ACs attached to BD1 that are
interested in (C-*,C-G) traffic and ACs attached to BD2 that are
interested in (C-S,C-G) traffic. A single SBD-SMET route specifying
(C-*,C-G) will attract all the necessary flows.
As another example, suppose the ACs attached to BD1 are interested in
(C-*,C-G) but not in (C-S,C-G), while the ACs attached to BD2 are
interested in (C-S,C-G). A single SBD-SMET route specifying
(C-*,C-G) will pull in all the necessary flows.
In other words, to determine the set of SBD-SMET routes that have to
be sent for a given C-G, the PE has to merge the IGMP/MLD state for
all the BDs (of the given Tenant Domain) to which it is attached.
Per [RFC9251], importing a SMET route for a particular BD will cause
the IGMP/MLD state to be instantiated for the IRB interface to that
BD. This also applies when the BD is the SBD.
However, traffic that originates in one of the actual BDs of a
particular Tenant Domain MUST NOT be sent down the IRB interface that
connects the L3 routing instance of that Tenant Domain to the SBD.
That would cause duplicate delivery of traffic, since such traffic
will have already been distributed throughout the Tenant Domain.
Therefore, when setting up the IGMP/MLD state based on SBD-SMET
routes, care must be taken to ensure that the IRB interface to the
SBD is not added to the Outgoing Interface (OIF) list if the traffic
originates within the Tenant Domain.
There are some multicast scenarios that make use of anycast sources.
For example, two different sources may share the same anycast IP
address, say S1, and each may transmit an (S1,G) multicast flow. In
such a scenario, the two (S1,G) flows are typically identical.
Ordinary PIM procedures will cause only one of the flows to be
delivered to each receiver that has expressed interest in either
(*,G) or (S1,G). However, the OISM procedures described in this
document will result in both of the (S1,G) flows being distributed in
the Tenant Domain, and duplicate delivery will result. Therefore, if
there are receivers for (*,G) in a given Tenant Domain, there MUST
NOT be anycast sources for G within that Tenant Domain. (This
restriction could be lifted by defining additional procedures;
however, that is outside the scope of this document.)
4. Constructing Multicast Forwarding State
4.1. Layer 2 Multicast State
An EVPN PE maintains Layer 2 multicast state for each BD to which it
is attached. Note that this is used for forwarding IP multicast
frames based on the inner IP header. The state is learned through
IGMP/MLD snooping [RFC4541] and procedures in this document.
Let PE1 be an EVPN PE and BD1 be a BD to which it is attached. At
PE1, BD1's Layer 2 multicast state for a given (C-S,C-G) or (C-*,C-G)
governs the disposition of an IP multicast packet that is received by
BD1's Layer 2 multicast function on an EVPN PE.
An IP multicast (S,G) packet is considered to have been received by
BD1's Layer 2 multicast function in PE1 in the following cases:
* The packet is the payload of an Ethernet frame received by PE1
from an AC that attaches to BD1.
* The packet is the payload of an Ethernet frame whose apparent
source BD is BD1, which is received by the PE1 over a tunnel from
another EVPN PE.
* The packet is received from BD1's IRB interface (i.e., has been
transmitted by PE1's L3 routing instance down BD1's IRB
interface).
According to the procedures of this document, all transmissions of IP
multicast packets from one EVPN PE to another are done at Layer 2.
That is, the packets are transmitted as Ethernet frames, according to
the Layer 2 multicast state.
Each Layer 2 multicast state (S,G) or (*,G) contains a set of
outgoing interfaces (an OIF list). The disposition of an (S,G)
multicast frame received by BD1's Layer 2 multicast function is
determined as follows:
* The OIF list is taken from BD1's Layer 2 (S,G) state, or if there
is no such (S,G) state, then it is taken from BD1's (*,G) state.
(If neither state exists, the OIF list is considered to be null.)
* The rules of Section 4.1.2 are applied to the OIF list. This will
generally result in the frame being transmitted to some, but not
all, elements of the OIF list.
Note that there is no Reverse Path Forwarding (RPF) check at Layer 2.
4.1.1. Constructing the OIF List
In this document, we have extended the procedures of [RFC9251] so
that IMET and SMET routes for a particular BD are distributed not
just to PEs that attach to that BD but to PEs that attach to any BD
in the Tenant Domain. In this way, each PE attached to a given
Tenant Domain learns, from another PE attached to the same Tenant
Domain, the set of flows that are of interest to each of those other
PEs. (If some PE attached to the Tenant Domain does not support
[RFC9251], it will be assumed to be interested in all flows. Whether
or not a particular remote PE supports [RFC9251] is determined by the
presence of an Extended Community in its IMET route; this is
specified in [RFC9251].) If a set of remote PEs are interested in a
particular flow, the tunnels used to reach those PEs are added to the
OIF list of the multicast states corresponding to that flow.
An EVPN PE may run IGMP/MLD snooping procedures [RFC4541] on each of
its ACs in order to determine the set of flows of interest to each
AC. (An AC is said to be interested in a given flow if it connects
to a segment that has tenant systems interested in that flow.) If
IGMP/MLD procedures are not being run on a given AC, that AC is
considered to be interested in all flows. For each BD, the set of
ACs interested in a given flow is determined, and the ACs of that set
are added to the OIF list of that BD's multicast state for that flow.
The OIF list for each multicast state must also contain the IRB
interface for the BD to which the state belongs.
Implementors should note that the OIF list of a multicast state will
change from time to time as ACs and/or remote PEs either become
interested in or lose interest in particular multicast flows.
4.1.2. Data Plane: Applying the OIF List to an (S,G) Frame
When an (S,G) multicast frame is received by the Layer 2 multicast
function of a given EVPN PE, say PE1, its disposition depends upon
(a) the way it was received, (b) the OIF list of the corresponding
multicast state (see Section 4.1.1), (c) the eligibility of an AC to
receive a given frame (see Section 4.1.2.1), and (d) its apparent
source BD (see Section 3.2 for information about determining the
apparent source BD of a frame received over a tunnel from another
PE).
4.1.2.1. Eligibility of an AC to Receive a Frame
A given (S,G) multicast frame is eligible to be transmitted by a
given PE, say PE1, on a given AC, say AC1, only if one of the
following conditions holds:
1. Ethernet Segment Identifier (ESI) labels are being used, PE1 is
the DF for the segment to which AC1 is connected, and the frame
did not originate from that same segment (as determined by the
ESI label).
2. The ingress PE for the frame is a remote PE, say PE2, local bias
is being used, and PE2 is not connected to the same segment as
AC1.
4.1.2.2. Applying the OIF List
Assume a given (S,G) multicast frame has been received by a given PE,
say PE1. PE1 determines the apparent source BD of the frame, finds
the Layer 2 (S,G) state for that BD (or the (*,G) state if there is
no (S,G) state), and uses the OIF list from that state. (Note that
if PE1 is not attached to the actual source BD, the apparent source
BD will be the SBD.)
If PE1 has determined the frame's apparent source BD to be BD1 (which
may or may not be the SBD), then the following cases should be
considered:
1. The frame was received by PE1 from a local AC, say AC1, that
attaches to BD1.
a. The frame MUST be sent on all local ACs of BD1 that appear in
the OIF list, except for AC1 itself.
b. The frame MUST also be delivered to any other EVPN PEs that
have interest in it. This is achieved as follows:
i. If (a) AR is being used, (b) PE1 is an AR-LEAF, and (c)
the OIF list is non-null, PE1 MUST send the frame to the
AR-REPLICATOR.
ii. Otherwise, the frame MUST be sent on all tunnels in the
OIF list.
c. The frame MUST be sent to the local L3 routing instance by
being sent up the IRB interface of BD1. It MUST NOT be sent
up any other IRB interfaces.
2. The frame was received by PE1 over a tunnel from another PE.
(See Section 3.2 for the rules to determine the apparent source
BD of a packet received from another PE. Note that if PE1 is not
attached to the source BD, it will regard the SBD as the apparent
source BD.)
a. The frame MUST be sent on all local ACs in the OIF list that
connect to BD1 and that are eligible (per Section 4.1.2.1) to
receive the frame.
b. The frame MUST be sent up the IRB interface of the apparent
source BD. (Note that this may be the SBD.) The frame MUST
NOT be sent up any other IRB interfaces.
c. If PE1 is not an AR-REPLICATOR, it MUST NOT send the frame to
any other EVPN PEs. However, if PE1 is an AR-REPLICATOR, it
MUST send the frame to all tunnels in the OIF list, except
for the tunnel over which the frame was received.
3. The frame was received by PE1 from the BD1 IRB interface (i.e.,
the frame has been transmitted by PE1's L3 routing instance down
the BD1 IRB interface), and BD1 is NOT the SBD.
a. The frame MUST be sent on all local ACs in the OIF list that
are eligible, as per Section 4.1.2.1, to receive the frame.
b. The frame MUST NOT be sent to any other EVPN PEs.
c. The frame MUST NOT be sent up any IRB interfaces.
4. The frame was received from the SBD IRB interface (i.e., has been
transmitted by PE1's L3 routing instance down the SBD IRB
interface).
a. The frame MUST be sent on all tunnels in the OIF list. This
causes the frame to be delivered to any other EVPN PEs that
have interest in it.
b. The frame MUST NOT be sent on any local ACs.
c. The frame MUST NOT be sent up any IRB interfaces.
4.2. Layer 3 Forwarding State
If an EVPN PE is performing IGMP/MLD procedures on the ACs of a given
BD, it processes those messages at Layer 2 to help form the Layer 2
multicast state. It also sends those messages up that BD's IRB
interface to the L3 routing instance of a particular Tenant Domain.
This causes the (C-S,C-G) or (C-*,C-G) L3 state to be created/
updated.
A Layer 3 multicast state has both an Input Interface (IIF) and an
OIF list.
For a (C-S,C-G) state, if the source BD is present on the PE, the IIF
is set to the IRB interface that attaches to that BD. Otherwise, the
IIF is set to the SBD IRB interface.
For (C-*,C-G) states, traffic can arrive from any BD, so the IIF
needs to be set to a wildcard value meaning "any IRB interface".
The OIF list of these states includes one or more of the IRB
interfaces of the Tenant Domain. In general, maintenance of the OIF
list does not require any EVPN-specific procedures. However, there
is one EVPN-specific rule:
If the IIF is one of the IRB interfaces (or the wildcard meaning
"any IRB interface"), then the SBD IRB interface MUST NOT be added
to the OIF list. Traffic originating from within a particular
EVPN Tenant Domain must not be sent down the SBD IRB interface, as
such traffic has already been distributed to all EVPN PEs attached
to that Tenant Domain.
Please also see Section 6.1.1, which states a modification of this
rule for the case where OISM is interworking with external Layer 3
multicast routing.
5. Interworking with Non-OISM EVPN PEs
It is possible that a given Tenant Domain will be attached to both
OISM PEs and non-OISM PEs. Inter-subnet IP multicast should be
possible and fully functional even if not all PEs attaching to a
Tenant Domain can be upgraded to support OISM functionality.
Note that the non-OISM PEs are not required to have IRB support or
support for [RFC9251]. However, it is advantageous for the non-OISM
PEs to support [RFC9251].
In this section, we will use the following terminology:
PE-S: The ingress PE for an (S,G) flow.
PE-R: An egress PE for an (S,G) flow.
BD-S: The source BD for an (S,G) flow. PE-S must have one or more
ACs attached to BD-S, at least one of which attaches to host S.
BD-R: A BD that contains a host interested in the flow. The host is
attached to PE-R via an AC that belongs to BD-R.
To allow OISM PEs to interwork with non-OISM PEs, a given Tenant
Domain needs to contain one or more IP Multicast Gateways (IPMGs).
An IPMG is an OISM PE with special responsibilities regarding the
interworking between OISM and non-OISM PEs.
If a PE is functioning as an IPMG, it MUST signal this fact by
setting the IPMG flag in the Multicast Flags EC that it attaches to
its IMET routes. An IPMG SHOULD attach this EC, with the IPMG flag
set, to all IMET routes it originates. Furthermore, if PE1 imports
any IMET route from PE2 that has the EC present with the IPMG flag
set, then the PE1 will assume that PE2 is an IPMG.
An IPMG Designated Forwarder (IPMG-DF) selection procedure is used to
ensure that there is exactly one active IPMG-DF for any given BD at
any given time. Details of the IPMG-DF selection procedure are in
Section 5.1. The IPMG-DF for a given BD, say BD-S, has special
functions to perform when it receives (S,G) frames on that BD:
* If the frames are from a non-OISM PE-S:
- The IPMG-DF forwards them to OISM PEs that do not attach to
BD-S but have interest in (S,G).
Note that OISM PEs that do attach to BD-S will have received
the frames on the BUM tunnel from the non-OISM PE-S.
- The IPMG-DF forwards them to non-OISM PEs that have interest in
(S,G) on ACs that do not belong to BD-S.
Note that if a non-OISM PE has multiple BDs (other than BD-S)
with interest in (S,G), it will receive one copy of the frame
for each such BD. This is necessary because the non-OISM PEs
cannot move IP multicast traffic from one BD to another.
* If the frames are from an OISM PE, the IPMG-DF forwards them to
non-OISM PEs that have interest in (S,G) on ACs that do not belong
to BD-S.
If a non-OISM PE has interest in (S,G) on an AC belonging to BD-S,
it will have received a copy of the (S,G) frame, encapsulated for
BD-S, from the OISM PE-S (see Section 3.2.2). If the non-OISM PE
has interest in (S,G) on one or more ACs belonging to BD-
R1,...,BD-Rk where the BD-Ri are distinct from BD-S, the IPMG-DF
needs to send it a copy of the frame for each BD-Ri.
If an IPMG receives a frame on a BD for which it is not the IPMG-DF,
it just follows normal OISM procedures.
This section specifies several sets of procedures:
* the procedures that the IPMG-DF for a given BD needs to follow
when receiving, on that BD, an IP multicast frame from a non-OISM
PE;
* the procedures that the IPMG-DF for a given BD needs to follow
when receiving, on that BD, an IP multicast frame from an OISM PE;
and
* the procedures that an OISM PE needs to follow when receiving, on
a given BD, an IP multicast frame from a non-OISM PE, when the
OISM PE is not the IPMG-DF for that BD.
To enable OISM/non-OISM interworking in a given Tenant Domain, the
Tenant Domain MUST have some EVPN PEs that can function as IPMGs. An
IPMG must be configured with the SBD. It must also be configured
with every BD of the Tenant Domain that exists on any of the non-OISM
PEs of that domain. (Operationally, it may be simpler to configure
the IPMG with all the BDs of the Tenant Domain.)
Of course, a non-OISM PE only needs to be configured with BDs for
which it has ACs. An OISM PE that is not an IPMG only needs to be
configured with the SBD and with the BDs for which it has ACs.
An IPMG MUST originate a wildcard SMET route (with (C-*,C-*) in the
NLRI) for each BD in the Tenant Domain. This will cause it to
receive all the IP multicast traffic that is sourced in the Tenant
Domain. Note that non-OISM nodes that do not support [RFC9251] will
send all the multicast traffic from a given BD to all PEs attached to
that BD, even if those PEs do not originate a SMET route.
The interworking procedures vary somewhat depending upon whether
packets are transmitted from PE to PE via IR or via P2MP tunnels. In
this section, we do not consider the use of BIER due to the low
likelihood of there being a non-OISM PE that supports BIER.
5.1. IPMG Designated Forwarder
Every PE that is eligible for selection as an IPMG-DF for a
particular BD originates both an IMET route for that BD and an SBD-
IMET route. As stated in Section 5, these SBD-IMET routes carry a
Multicast Flags EC with the IPMG flag set.
These SBD-IMET routes SHOULD also carry a DF Election EC. The DF
Election EC and its use is specified in [RFC8584]. When the route is
originated, the AC-DF bit in the DF Election EC SHOULD NOT be set.
This bit is not used when selecting an IPMG-DF, i.e., it MUST be
ignored by the receiver of an SBD-IMET route.
In the context of a given Tenant Domain, to select the IPMG-DF for a
particular BD, say BD1, the IPMGs of the Tenant Domain perform the
following procedures:
* From the set of received SBD-IMET routes for the given Tenant
Domain, determine the candidate set of PEs that support IPMG
functionality for that domain.
* From that candidate set, eliminate any PEs from which an IMET
route for BD1 has not been received.
* Select a DF election algorithm as specified in [RFC8584]. Some of
the possible algorithms can be found, e.g., in [RFC8584],
[RFC7432], and [EVPN-DF].
* Apply the DF election algorithm (see [RFC8584]) to the candidate
set of PEs. The winner becomes the IPMG-DF for BD1.
Note that even if a given PE supports MEG (Section 6.1.2) and/or PEG
(Section 6.1.4) functionality, as well as IPMG functionality, its
SBD-IMET routes carry only one DF Election EC.
5.2. Ingress Replication
The procedures of this section are used when IR is used to transmit
packets from one PE to another.
When a non-OISM PE-S transmits a multicast frame from BD-S to another
PE, say PE-R, PE-S will use the encapsulation specified in the BD-S
IMET route that was originated by PE-R. This encapsulation will
include the label that appears in the MPLS Label field of the PTA of
the IMET route. If the tunnel type is VXLAN, the label is actually a
Virtual Network Identifier (VNI); for other tunnel types, the label
is an MPLS label. In either case, the frames are transmitted with a
label that was assigned to a particular BD by the PE-R to which the
frame is being transmitted.
To support OISM/non-OISM interworking, an OISM PE-R MUST originate,
for each of its BDs, both an IMET route and an (C-*,C-*) S-PMSI A-D
route. Note that even when IR is being used, interworking between
OISM and non-OISM PEs requires the OISM PEs to follow the rules of
Section 3.2.5.2, as modified below.
Non-OISM PEs will not understand S-PMSI A-D routes. So when a non-
OISM PE-S transmits an IP multicast frame with a particular source BD
to an IPMG, it encapsulates the frame using the label specified in
that IPMG's BD-S IMET route. (This is just the procedure of
[RFC7432].)
The (C-*,C-*) S-PMSI A-D route originated by a given OISM PE will
have a PTA that specifies IR.
* If MPLS tunneling is being used, the MPLS Label field SHOULD
contain a non-zero value, and the LIR flag SHOULD be zero. (The
case where the MPLS Label field is zero or the LIR flag is set is
outside the scope of this document.)
* If the tunnel encapsulation is VXLAN, the MPLS Label field MUST
contain a non-zero value, and the LIR flag MUST be zero.
When an OISM PE-S transmits an IP multicast frame to an IPMG, it will
use the label specified in that IPMG's (C-*,C-*) S-PMSI A-D route.
When a PE originates both an IMET route and a (C-*,C-*) S-PMSI A-D
route, the values of the MPLS Label field in the respective PTAs must
be distinct. Further, each MUST map uniquely (in the context of the
originating PE) to the route's BD.
As a result, an IPMG receiving an MPLS-encapsulated IP multicast
frame can always tell by the label whether the frame's ingress PE is
an OISM PE or a non-OISM PE. When an IPMG receives a VXLAN-
encapsulated IP multicast frame, it may need to determine the
identity of the ingress PE from the outer IP encapsulation; it can
then determine whether the ingress PE is an OISM PE or a non-OISM PE
by looking at the IMET route from that PE.
Suppose an IPMG receives an IP multicast frame from another EVPN PE
in the Tenant Domain and the IPMG is not the IPMG-DF for the frame's
source BD. Then, the IPMG performs only the ordinary OISM functions;
it does not perform the IPMG-specific functions for that frame. In
the remainder of this section, when we discuss the procedures applied
by an IPMG when it receives an IP multicast frame, we are presuming
that the source BD of the frame is a BD for which the IPMG is the
IPMG-DF.
We have two basic cases to consider: (1) a frame's ingress PE is a
non-OISM node and (2) a frame's ingress PE is an OISM node.
5.2.1. Ingress PE is Non-OISM
In this case, a non-OISM PE, say PE-S, has received an (S,G)
multicast frame over an AC that is attached to a particular BD, say
BD-S. By virtue of normal EVPN procedures, PE-S has sent a copy of
the frame to every PE-R (both OISM and non-OISM) in the Tenant Domain
that is attached to BD-S. If the non-OISM node supports [RFC9251],
only PEs that have expressed interest in (S,G) receive the frame.
The IPMG will have expressed interest via a (C-*,C-*) SMET route and
thus receives the frame.
Any OISM PE (including an IPMG) receiving the frame will apply normal
OISM procedures. As a result, it will deliver the frame to any of
its local ACs (in BD-S or in any other BD) that have interest in
(S,G).
An OISM PE that is also the IPMG-DF for a particular BD, say BD-S,
has additional procedures that it applies to frames received on BD-S
from non-OISM PEs:
1. When the IPMG-DF for BD-S receives an (S,G) frame from a non-OISM
node, it MUST forward a copy of the frame to every OISM PE that
is NOT attached to BD-S but has interest in (S,G). The copy sent
to a given OISM PE-R must carry the label that PE-R has assigned
to the SBD in an S-PMSI A-D route. The IPMG MUST NOT do any IP
processing of the frame's IP payload. TTL decrement and other IP
processing will be done by PE-R, per the normal OISM procedures.
There is no need for the IPMG to include an ESI label in the
frame's tunnel encapsulation, because it is already known that
the frame's source BD has no presence on PE-R. There is also no
need for the IPMG to modify the frame's MAC SA.
2. In addition, when the IPMG-DF for BD-S receives an (S,G) frame
from a non-OISM node, it may need to forward copies of the frame
to other non-OISM nodes. Before it does so, it MUST decapsulate
the (S,G) packet and do the IP processing (e.g., TTL decrement).
Suppose PE-R is a non-OISM node that has an AC to BD-R, where
BD-R is not the same as BD-S, and that AC has interest in (S,G).
The IPMG must then encapsulate the (S,G) packet (after the IP
processing has been done) in an Ethernet header. The MAC SA
field will have the MAC address of the IPMG's IRB interface for
BD-R. The IPMG then sends the frame to PE-R. The tunnel
encapsulation will carry the label that PE-R advertised in its
IMET route for BD-R. There is no need to include an ESI label,
as the source and destination BDs are known to be different.
Note that if a non-OISM PE-R has several BDs (other than BD-S)
with local ACs that have interest in (S,G), the IPMG will send it
one copy for each such BD. This is necessary because the non-
OISM PE cannot move packets from one BD to another.
There may be deployment scenarios in which every OISM PE is
configured with every BD that is present on any non-OISM PE. In such
scenarios, the procedures of item 1 above will not actually result in
the transmission of any packets. Hence, if it is known a priori that
this deployment scenario exists for a given Tenant Domain, the
procedures of item 1 above can be disabled.
5.2.2. Ingress PE is OISM
In this case, an OISM PE, say PE-S, has received an (S,G) multicast
frame over an AC that attaches to a particular BD, say BD-S.
By virtue of receiving all the IMET routes for BD-S, PE-S will know
all the PEs attached to BD-S. By virtue of normal OISM procedures:
* PE-S will send a copy of the frame to every OISM PE-R (including
the IPMG) in the Tenant Domain that is attached to BD-S and has
interest in (S,G). The copy sent to a given PE-R carries the
label that the PE-R has assigned to BD-S in its (C-*,C-*) S-PMSI
A-D route.
* PE-S will also transmit a copy of the (S,G) frame to every OISM
PE-R that has interest in (S,G) but is not attached to BD-S. The
copy will contain the label that the PE-R has assigned to the SBD.
(As specified in Section 5.2.1, an IPMG is assumed to have
indicated interest in all multicast flows.)
* PE-S will also transmit a copy of the (S,G) frame to every non-
OISM PE-R that is attached to BD-S. It does this using the label
advertised by that PE-R in its IMET route for BD-S.
The PE-Rs follow their normal procedures. An OISM PE that receives
the (S,G) frame on BD-S applies the OISM procedures to deliver the
frame to its local ACs as necessary. A non-OISM PE that receives the
(S,G) frame on BD-S delivers the frame only to its local BD-S ACs as
necessary.
Suppose that a non-OISM PE-R has interest in (S,G) on a BD that is
different than BD-S, say BD-R. If the non-OISM PE-R is attached to
BD-S, the OISM PE-S will send it the original (S,G) multicast frame,
but the non-OISM PE-R will not be able to send the frame to ACs that
are not in BD-S. If PE-R is not even attached to BD-S, the OISM PE-S
will not send it a copy of the frame at all, because PE-R is not
attached to the SBD. In these cases, the IPMG needs to relay the
(S,G) multicast traffic from OISM PE-S to non-OISM PE-R.
When the IPMG-DF for BD-S receives an (S,G) frame from an OISM PE-S,
it has to forward it to every non-OISM PE-R that has interest in
(S,G) on a BD-R that is different than BD-S. The IPMG MUST
decapsulate the IP multicast packet, do the IP processing, re-
encapsulate it for BD-R (changing the MAC SA to the IPMG's own MAC
address for BD-R), and send a copy of the frame to PE-R. Note that a
given non-OISM PE-R will receive multiple copies of the frame if it
has multiple BDs on which there is interest in the frame.
5.3. P2MP Tunnels
When IR is used to distribute the multicast traffic among the EVPN
PEs, the procedures described in Section 5.2 ensure that there will
be no duplicate delivery of multicast traffic. That is, no egress PE
will ever send a frame twice on any given AC. If P2MP tunnels are
being used to distribute the multicast traffic, it is necessary to
have additional procedures to prevent duplicate delivery.
At the present time, it is not clear that there will be a use case in
which OISM nodes need to interwork with non-OISM nodes that use P2MP
tunnels. If it is determined that there is such a use case,
procedures for P2MP may be specified in a separate document.
6. Traffic to/from Outside the EVPN Tenant Domain
In this section, we discuss scenarios where a multicast source
outside a given EVPN Tenant Domain sends traffic to receivers inside
the domain (as well as, possibly, to receivers outside the domain).
This requires the OISM procedures to interwork with various Layer 3
multicast routing procedures.
In this section, we assume that the Tenant Domain is not being used
as an intermediate transit network for multicast traffic; that is, we
do not consider the case where the Tenant Domain contains multicast
routers that will receive traffic from sources outside the domain and
forward the traffic to receivers outside the domain. The transit
scenario is considered in Section 7.
We can divide the non-transit scenarios into two classes:
1. One or more of the EVPN PE routers provide the functionality
needed to interwork with Layer 3 multicast routing procedures.
2. A single BD in the Tenant Domain contains external multicast
routers (tenant multicast routers), and those tenant multicast
routers are used to interwork, on behalf of the entire Tenant
Domain, with Layer 3 multicast routing procedures.
6.1. Layer 3 Interworking via EVPN OISM PEs
6.1.1. General Principles
Sometimes it is necessary to interwork an EVPN Tenant Domain with an
external Layer 3 multicast domain (the external domain), e.g., a PIM
or MVPN domain. This is needed to allow EVPN tenant systems to
receive multicast traffic from sources (external sources) outside the
EVPN Tenant Domain. It is also needed to allow receivers (external
receivers) outside the EVPN Tenant Domain to receive traffic from
sources inside the Tenant Domain.
In order to allow interworking between an EVPN Tenant Domain and an
external domain, one or more OISM PEs must be L3 Gateways. An L3
Gateway participates both in the OISM procedures and in the L3
multicast routing procedures of the external domain, as shown in the
following figure.
src1 rcvr1
| |
R1 RP R2
PIM/MVPN
Domain
+---+ +---+
-----|GW1|----------------------|GW2|----
+---+ +---+
| \ \ / / |
| \ \ / / |
BD1 BD2 SBD SBD BD2 BD1
EVPN Domain
SBD SBD
/ \
/ \
+---+ +---+
|PE1| |PE2|
+---+ +---+
| \ / |
BD1 BD2 BD2 BD1
| | | |
src2 rcvr2 src3 rcvr3
Figure 1: Interworking via OISM PEs
An L3 Gateway that has interest in receiving (S,G) traffic must be
able to determine the best route to S. If an L3 Gateway has interest
in (*,G), it must be able to determine the best route to G's RP. In
these interworking scenarios, the L3 Gateway must be running a Layer
3 unicast routing protocol. Via this protocol, it imports unicast
routes (either IP routes or VPN-IP routes) from routers other than
EVPN PEs. And since there may be multicast sources inside the EVPN
Tenant Domain, the EVPN PEs also need to export, either as IP routes
or as VPN-IP routes (depending upon the external domain), unicast
routes to those sources.
When selecting the best route to a multicast source or RP, an L3
Gateway might have a choice between an EVPN route and an IP/VPN-IP
route. When such a choice exists, the L3 Gateway SHOULD always
prefer the EVPN route. This will ensure that when traffic originates
in the Tenant Domain and has a receiver in the Tenant Domain, the
path to that receiver will remain within the EVPN Tenant Domain, even
if the source is also reachable via a routed path. This also
provides protection against sub-optimal routing that might occur if
two EVPN PEs export IP/VPN-IP routes and each imports the other's IP/
VPN-IP routes.
Section 4.2 discusses the way Layer 3 multicast states are
constructed by OISM PEs. These Layer 3 multicast states have IRB
interfaces as their IIF and OIF list entries and are the basis for
interworking OISM with other Layer 3 multicast procedures such as
MVPN or PIM. From the perspective of the Layer 3 multicast
procedures running in a given L3 Gateway, an EVPN Tenant Domain is a
set of IRB interfaces.
When interworking an EVPN Tenant Domain with an external domain, the
L3 Gateway's Layer 3 multicast states will not only have IRB
interfaces as IIF and OIF list entries but also other interfaces that
lead outside the Tenant Domain. For example, when interworking with
MVPN, the multicast states may have MVPN tunnels as well as IRB
interfaces as IIF or OIF list members. When interworking with PIM,
the multicast states may have PIM-enabled non-IRB interfaces as IIF
or OIF list members.
As long as a Tenant Domain is not being used as an intermediate
transit network for IP multicast traffic, it is not necessary to
enable PIM on its IRB interfaces.
In general, an L3 Gateway has the following responsibilities:
* It exports, to the external domain, unicast routes to those
multicast sources in the EVPN Tenant Domain that are locally
attached to the L3 Gateway.
* It imports, from the external domain, unicast routes to multicast
sources that are in the external domain.
* It executes the procedures necessary to draw externally sourced
multicast traffic that is of interest to locally attached
receivers in the EVPN Tenant Domain. When such traffic is
received, the traffic is sent down the IRB interfaces of the BDs
on which the locally attached receivers reside.
One of the L3 Gateways in a given Tenant Domain becomes the DR for
the SBD (see Section 6.1.2.4). This L3 Gateway has the following
additional responsibilities:
* It exports, to the external domain, unicast routes to multicast
sources in the EVPN Tenant Domain that are not locally attached to
any L3 Gateway.
* It imports, from the external domain, unicast routes to multicast
sources that are in the external domain.
* It executes the procedures necessary to draw externally sourced
multicast traffic that is of interest to receivers in the EVPN
Tenant Domain that are not locally attached to an L3 Gateway.
When such traffic is received, the traffic is sent down the SBD
IRB interface. OISM procedures already described in this document
will then ensure that the IP multicast traffic gets distributed
throughout the Tenant Domain to any EVPN PEs that have interest in
it. Thus, to an OISM PE that is not an L3 Gateway, the externally
sourced traffic will appear to have been sourced on the SBD.
In order for this to work, some special care is needed when an L3
Gateway creates or modifies a Layer 3 (*,G) multicast state. Suppose
group G has both external sources (sources outside the EVPN Tenant
Domain) and internal sources (sources inside the EVPN Tenant Domain).
Section 4.2 states that when there are internal sources, the SBD IRB
interface must not be added to the OIF list of the (*,G) state.
Traffic from internal sources will already have been delivered to all
the EVPN PEs that have interest in it. However, if the OIF list of
the (*,G) state does not contain its SBD IRB interface, then traffic
from external sources will not get delivered to other EVPN PEs.
One way of handling this is the following. When an L3 Gateway
receives (S,G) traffic that is from an interface other than IRB, and
the traffic corresponds to a Layer 3 (*,G) state, the L3 Gateway can
create (S,G) state. The IIF will be set to the external interface
over which the traffic is expected. The OIF list will contain the
SBD IRB interface, as well as the IRB interfaces of any other BDs
attached to the PEG DR that have locally attached receivers with
interest in the (S,G) traffic. The (S,G) state will ensure that the
external traffic is sent down the SBD IRB interface. The following
text will assume this procedure; however, other implementation
techniques may also be possible.
If a particular BD is attached to several L3 Gateways, one of the L3
Gateways becomes the DR for that BD (see Section 6.1.2.4). If the
interworking scenario requires FHR functionality, it is generally the
DR for a particular BD that is responsible for performing that
functionality on behalf of the source hosts on that BD (e.g., if the
interworking scenario requires that PIM Register messages be sent by
an FHR, the DR for a given BD would send the PIM Register messages
for sources on that BD). Although, note that the DR for the SBD does
not perform FHR functionality on behalf of external sources.
An optional alternative is to have each L3 Gateway perform FHR
functionality for locally attached sources. Then, the DR would only
have to perform FHR functionality on behalf of sources that are
locally attached to itself AND sources that are not attached to any
L3 Gateway.
Note that if it is possible that more than one BD contains a tenant
multicast router, then a PE receiving a SMET route for that BD MUST
NOT reconstruct IGMP/MLD Join Reports from the SMET route and MUST
NOT transmit any such IGMP/MLD Join Reports on its local ACs
attaching to that BD. Otherwise, multicast traffic may be
duplicated.
6.1.2. Interworking with MVPN
In this section, we specify the procedures necessary to allow EVPN
PEs running OISM procedures to interwork with L3VPN PEs that run BGP-
based MVPN [RFC6514] procedures. More specifically, the procedures
herein allow a given EVPN Tenant Domain to become part of an L3VPN/
MVPN and support multicast flows where either of the following
occurs:
* The source of a given multicast flow is attached to an Ethernet
segment whose BD is part of an EVPN Tenant Domain, and one or more
receivers of the flow are attached to the network via L3VPN/MVPN.
(Other receivers may be attached to the network via EVPN.)
* The source of a given multicast flow is attached to the network
via L3VPN/MVPN, and one or more receivers of the flow are attached
to an Ethernet segment that is part of an EVPN Tenant Domain.
(Other receivers may be attached via L3VPN/MVPN.)
In this interworking model, existing L3VPN/MVPN PEs are unaware that
certain sources or receivers are part of an EVPN Tenant Domain. The
existing L3VPN/MVPN nodes run only their standard procedures and are
entirely unaware of EVPN. Interworking is achieved by having some or
all of the EVPN PEs function as L3 Gateways running L3VPN/MVPN
procedures, as detailed in the following subsections.
In this section, we assume that there are no tenant multicast routers
on any of the EVPN-attached Ethernet segments. (Of course, there may
be multicast routers in the L3VPN.) Consideration of the case where
there are tenant multicast routers is addressed in Section 7.
To support MVPN/EVPN interworking, we introduce the notion of an
MVPN/EVPN Gateway (MEG).
A MEG is an L3 Gateway (see Section 6.1.1); hence, it is both an OISM
PE and an L3VPN/MVPN PE. For a given EVPN Tenant Domain, it will
have an IP-VRF. If the Tenant Domain is part of an L3VPN/MVPN, the
IP-VRF also serves as an L3VPN VRF [RFC4364]. The IRB interfaces of
the IP-VRF are considered to be VRF interfaces of the L3VPN VRF. The
L3VPN VRF may also have other local VRF interfaces that are not EVPN
IRB interfaces.
The VRF on the MEG will import VPN-IP routes [RFC4364] from other
L3VPN PE routers. It will also export VPN-IP routes to other L3VPN
PE routers. In order to do so, it must be appropriately configured
with the RTs used in the L3VPN to control the distribution of the
VPN-IP routes. In general, these RTs will be different than the RTs
used for controlling the distribution of EVPN routes, as there is no
need to distribute EVPN routes to L3VPN-only PEs and no reason to
distribute L3VPN/MVPN routes to EVPN-only PEs.
Note that the RDs in the imported VPN-IP routes will not necessarily
conform to the EVPN rules (as specified in [RFC7432]) for creating
RDs. Therefore, a MEG MUST NOT expect the RDs of the VPN-IP routes
to be of any particular format other than what is required by the
L3VPN/MVPN specifications.
The VPN-IP routes that a MEG exports to L3VPN are subnet routes and/
or host routes for the multicast sources that are part of the EVPN
Tenant Domain. The exact set of routes that need to be exported is
discussed in Section 6.1.2.2.
Each IMET route originated by a MEG SHOULD carry a Multicast Flags
Extended Community with the MEG flag set, indicating that the
originator of the IMET route is a MEG. However, PE1 will consider
PE2 to be a MEG if PE1 imports at least one IMET route from PE2 that
carries the Multicast Flags EC with the MEG flag set.
All the MEGs of a given Tenant Domain attach to the SBD of that
domain, and one of them is selected to be the SBD's Designated Router
(the MEG SBD-DR) for the domain. The selection procedure is
discussed in Section 6.1.2.4.
In this model of operation, MVPN procedures and EVPN procedures are
largely independent. In particular, there is no assumption that MVPN
and EVPN use the same kind of tunnels. Thus, no special procedures
are needed to handle the common scenarios where, e.g., EVPN uses
VXLAN tunnels but MVPN uses MPLS P2MP tunnels, or where EVPN uses IR
but MVPN uses MPLS P2MP tunnels.
Similarly, no special procedures are needed to prevent duplicate data
delivery on Ethernet segments that are multihomed.
The MEG does have some special procedures (described below) for
interworking between EVPN and MVPN; these have to do with selection
of the Upstream PE for a given multicast source, with the exporting
of VPN-IP routes and with the generation of MVPN C-multicast routes
triggered by the installation of SMET routes.
6.1.2.1. MVPN Sources with EVPN Receivers
6.1.2.1.1. Identifying MVPN Sources
Consider a multicast source S. It is possible that a MEG will import
both an EVPN unicast route to S and a VPN-IP route (or an ordinary IP
route), where the prefix length of each route is the same. In order
to draw (S,G) multicast traffic for any group G, the MEG SHOULD use
the EVPN route rather than the VPN-IP or IP route to determine the
Upstream PE (see Section 5 of [RFC6513]).
Doing so ensures that when an EVPN tenant system desires to receive a
multicast flow from another EVPN tenant system, the traffic from the
source to that receiver stays within the EVPN domain. This prevents
problems that might arise if there is a unicast route via L3VPN to S
but no multicast routers along the routed path. This also prevents
problem that might arise as a result of the fact that the MEGs will
import each others' VPN-IP routes.
In Section 6.1.2.1.2, we describe the procedures to be used when the
selected route to S is a VPN-IP route.
6.1.2.1.2. Joining a Flow from an MVPN Source
Consider a tenant system, say R, on a particular BD, say BD-R.
Suppose R wants to receive (S,G) multicast traffic, where source S is
not attached to any PE in the EVPN Tenant Domain but is attached to
an MVPN PE.
* Suppose R is on a singly homed Ethernet segment of BD-R and that
segment is attached to PE1, where PE1 is a MEG. PE1 learns via
IGMP/MLD listening that R is interested in (S,G). PE1 determines
from its VRF that there is no route to S within the Tenant Domain
(i.e., no EVPN RT-2 route matching on S's IP address) but that
there is a route to S via L3VPN (i.e., the VRF contains a subnet
or host route to S that was received as a VPN-IP route). Thus,
PE1 originates (if it hasn't already) an MVPN C-multicast Source
Tree Join (S,G) route. The route is constructed according to
normal MVPN procedures.
The Layer 2 multicast state is constructed as specified in
Section 4.1.
In the Layer 3 multicast state, the IIF is the appropriate MVPN
tunnel, and the IRB interface to BD-R is added to the OIF list.
When PE1 receives (S,G) traffic from the appropriate MVPN tunnel,
it performs IP processing of the traffic and then sends the
traffic down its IRB interface to BD-R. Following normal OISM
procedures, the (S,G) traffic will be encapsulated for Ethernet
and sent on the AC to which R is attached.
* Suppose R is on a singly homed Ethernet segment of BD-R and that
segment is attached to PE1, where PE1 is an OISM PE but is NOT a
MEG. PE1 learns via IGMP/MLD listening that R is interested in
(S,G). PE1 follows normal OISM procedures, originating an SBD-
SMET route for (S,G); this route will be received by all the MEGs
of the Tenant Domain, including the MEG SBD-DR. From PE1's IMET
routes, the MEG SBD-DR can determine whether or not PE1 is itself
a MEG. If PE1 is not a MEG, the MEG SBD-DR will originate (if it
hasn't already) an MVPN C-multicast Source Tree Join (S,G) route.
This will cause the MEG SBD-DR to receive (S,G) traffic on an MVPN
tunnel.
The Layer 2 multicast state is constructed as specified in
Section 4.1.
In the Layer 3 multicast state, the IIF is the appropriate MVPN
tunnel, and the IRB interface to the SBD is added to the OIF list.
When the MEG SBD-DR receives (S,G) traffic on an MVPN tunnel, it
performs IP processing of the traffic and then sends the traffic
down its IRB interface to the SBD. Following normal OISM
procedures, the traffic will be encapsulated for Ethernet and
delivered to all PEs in the Tenant Domain that have interest in
(S,G), including PE1.
* If R is on a multihomed Ethernet segment of BD-R, one of the PEs
attached to the segment will be its DF (following normal EVPN
procedures), and the DF will know (via IGMP/MLD listening or the
procedures of [RFC9251]) that a tenant system reachable via one of
its local ACs to BD-R is interested in (S,G) traffic. The DF is
responsible for originating an SBD-SMET route for (S,G), following
normal OISM procedures. If the DF is a MEG, it MUST originate the
corresponding MVPN C-multicast Source Tree Join (S,G) route; if
the DF is not a MEG, the MEG SBD-DR SBD MUST originate the
C-multicast route when it receives the SMET route.
Optionally, if the non-DF is a MEG, it MAY originate the
corresponding MVPN C-multicast Source Tree Join (S,G) route. This
will cause the traffic to flow to both the DF and the non-DF, but
only the DF will forward the traffic out an AC. This allows for
quicker recovery if the DF's local AC to R fails.
* If R is attached to a non-OISM PE, it will receive the traffic via
an IPMG, as specified in Section 5.
If an EVPN-attached receiver is interested in (*,G) traffic, and if
it is possible for there to be sources of (*,G) traffic that are
attached only to L3VPN nodes, the MEGs will have to know the group-
to-RP mappings. That will enable them to originate MVPN C-multicast
Shared Tree Join (*,G) routes and to send them toward the RP. (Since
we are assuming in this section that there are no tenant multicast
routers attached to the EVPN Tenant Domain, the RP must be attached
via L3VPN. Alternatively, the MEG itself could be configured to
function as an RP for group G.)
The Layer 2 multicast states are constructed as specified in
Section 4.1.
In the Layer 3 (*,G) multicast state, the IIF is the appropriate MVPN
tunnel. A MEG will add its IRB interfaces to the (*,G) OIF list for
any BDs containing locally attached receivers. If there are
receivers attached to other EVPN PEs, then whenever (S,G) traffic
from an external source matches a (*,G) state, the MEG will create
(S,G) state, with the MVPN tunnel as the IIF, the OIF list copied
from the (*,G) state, and the SBD IRB interface added to the OIF
list. (Please see the discussion in Section 6.1.1 regarding the
inclusion of the SBD IRB interface in a (*,G) state; the SBD IRB
interface is only used in the OIF list for traffic from external
sources.)
Normal MVPN procedures will then result in the MEG getting the (*,G)
traffic from all the multicast sources for G that are attached via
L3VPN. This traffic arrives on MVPN tunnels. When the MEG removes
the traffic from these tunnels, it does the IP processing. If there
are any receivers on a given BD, say BD-R, that are attached via
local EVPN ACs, the MEG sends the traffic down its BD-R IRB
interface. If there are any other EVPN PEs that are interested in
the (*,G) traffic, the MEG sends the traffic down the SBD IRB
interface. Normal OISM procedures then distribute the traffic as
needed to other EVPN PEs.
6.1.2.2. EVPN Sources with MVPN Receivers
6.1.2.2.1. General Procedures
Consider the case where an EVPN tenant system S is sending IP
multicast traffic to group G and there is a receiver R for the (S,G)
traffic that is attached to the L3VPN but not attached to the EVPN
Tenant Domain. (In this document, we assume that the L3VPN-/MVPN-
only nodes will not have any special procedures to deal with the case
where a source is inside an EVPN domain.)
In this case, an L3VPN PE through which R can be reached has to send
an MVPN C-multicast Join (S,G) route to one of the MEGs that is
attached to the EVPN Tenant Domain. For this to happen, the L3VPN PE
must have imported a VPN-IP route for S (either a host route or a
subnet route) from a MEG.
If a MEG determines that there is multicast source transmitting on
one of its ACs, the MEG SHOULD originate a VPN-IP host route for that
source. This determination SHOULD be made by examining the IP
multicast traffic that arrives on the ACs. (It MAY be made by
provisioning.) A MEG SHOULD NOT export a VPN-IP host route for any
IP address that is not known to be a multicast source (unless it has
some other reason for exporting such a route). The VPN-IP host route
for a given multicast source MUST be withdrawn if the source goes
silent for a configurable period of time or if it can be determined
that the source is no longer reachable via a local AC.
A MEG SHOULD also originate a VPN-IP subnet route for each of the BDs
in the Tenant Domain.
VPN-IP routes exported by a MEG must carry any attributes or Extended
Communities that are required by L3VPN and MVPN. In particular, a
VPN-IP route exported by a MEG must carry a VRF Route Import Extended
Community corresponding to the IP-VRF from which it is imported and a
Source AS Extended Community.
As a result, if S is attached to a MEG, the L3VPN nodes will direct
their MVPN C-multicast Join routes to that MEG. Normal MVPN
procedures will cause the traffic to be delivered to the L3VPN nodes.
The Layer 3 multicast state for (S,G) will have the MVPN tunnel on
its OIF list. The IIF will be the IRB interface leading to the BD
containing S.
If S is not attached to a MEG, the L3VPN nodes will direct their
C-multicast Join routes to whichever MEG appears to be on the best
route to S's subnet. Upon receiving the C-multicast Join, that MEG
will originate an EVPN SMET route for (S,G). As a result, the MEG
will receive the (S,G) traffic at Layer 2 via the OISM procedures.
The (S,G) traffic will be sent up the appropriate IRB interface, and
the Layer 3 MVPN procedures will ensure that the traffic is delivered
to the L3VPN nodes that have requested it. The Layer 3 multicast
state for (S,G) will have the MVPN tunnel in the OIF list, and the
IIF will be one of the following:
* If S belongs to a BD that is attached to the MEG, the IIF will be
the IRB interface to that BD.
* Otherwise, the IIF will be the SBD IRB interface.
Note that this works even if S is attached to a non-OISM PE, per the
procedures of Section 5.
6.1.2.2.2. Any-Source Multicast (ASM) Groups
Suppose the MEG SBD-DR learns that one of the PEs in its Tenant
Domain is interested in (*,G) traffic, where G is an ASM group. If
there are no tenant multicast routers, the MEG SBD-DR SHOULD perform
the First Hop Router (FHR) functionality for group G on behalf of the
Tenant Domain, as described in [RFC7761]. This means that the MEG
SBD-DR must know the identity of the RP for each group, must send
Register messages to the RP, etc.
If the MEG SBD-DR is to be the FHR for the Tenant Domain, it must see
all the multicast traffic that is sourced from within the domain and
destined to an ASM group address. The MEG can ensure this by
originating an SBD-SMET route for (*,*).
(As a possible optimization, an SBD-SMET route for (*, any ASM group)
may be defined in a separate document.)
In some deployment scenarios, it may be preferred that the MEG that
receives the (S,G) traffic over an AC be the one providing the FHR
functionality. This behavior is OPTIONAL. If this option is used,
it MUST be ensured that the MEG DR does not provide the FHR
functionality for (S,G) traffic that is attached to another MEG; FHR
functionality for (S,G) traffic from a particular source S MUST be
provided by only a single router.
Other deployment scenarios are also possible. For example, one might
want to configure the MEGs themselves to be RPs. In this case, the
RPs would have to exchange with each other information about which
sources are active. The method exchanging such information is
outside the scope of this document.
6.1.2.2.3. Source on Multihomed Segment
Suppose S is attached to a segment that is all-active multihomed to
PE1 and PE2. If S is transmitting to two groups, say G1 and G2, it
is possible that PE1 will receive the (S,G1) traffic from S, whereas
PE2 will receive the (S,G2) traffic from S.
This creates an issue for MVPN/EVPN interworking, because there is no
way to cause L3VPN/MVPN nodes to select PE1 as the ingress PE for
(S,G1) traffic while selecting PE2 as the ingress PE for (S,G2)
traffic.
However, the following procedure ensures that the IP multicast
traffic will still flow, even if the L3VPN/MVPN nodes pick the wrong
EVPN PE as the Upstream PE for, e.g., the (S,G1) traffic.
Suppose S is on an Ethernet segment, belonging to BD1, that is
multihomed to both PE1 and PE2, where PE1 is a MEG. And suppose that
IP multicast traffic from S to G travels over the AC that attaches
the segment to PE2. If PE1 receives a C-multicast Source Tree Join
(S,G) route, it MUST originate a SMET route for (S,G). Normal OISM
procedures will then cause PE2 to send the (S,G) traffic to PE1 on an
EVPN IP multicast tunnel. Normal OISM procedures will also cause PE1
to send the (S,G) traffic up its BD1 IRB interface. Normal MVPN
procedures will then cause PE1 to forward the traffic on an MVPN
tunnel. In this case, the routing is not optimal, but the traffic
does flow correctly.
6.1.2.3. Obtaining Optimal Routing of Traffic between MVPN and EVPN
The routing of IP multicast traffic between MVPN nodes and EVPN nodes
will be optimal as long as there is a MEG along the optimal route.
There are various deployment strategies that can be used to obtain
optimal routing between MVPN and EVPN.
In one such scenario, a Tenant Domain will have a small number of
strategically placed MEGs. For example, a data center may have a
small number of MEGs that connect it to a wide-area network. Then,
the optimal route into or out of the data center would be through the
MEGs.
In this scenario, the MEGs do not need to originate VPN-IP host
routes for the multicast sources; they only need to originate VPN-IP
subnet routes. The internal structure of the EVPN is completely
hidden from the MVPN node. EVPN actions, such as MAC Mobility and
Mass Withdrawal [RFC7432], have zero impact on the MVPN control
plane.
While this deployment scenario provides the most optimal routing and
has the least impact on the installed based of MVPN nodes, it does
complicate network planning considerations.
Another way of providing routing that is close to optimal is to turn
each EVPN PE into a MEG. Then, routing of MVPN-to-EVPN traffic is
optimal. However, routing of EVPN-to-MVPN traffic is not guaranteed
to be optimal when a source host is on a multihomed Ethernet segment
(as discussed in Section 6.1.2.2.)
The obvious disadvantage of this method is that it requires every
EVPN PE to be a MEG.
The procedures specified in this document allow an operator to add
MEG functionality to any subset of its EVPN OISM PEs. This allows an
operator to make whatever trade-offs deemed appropriate between
optimal routing and MEG deployment.
6.1.2.4. Selecting the MEG SBD-DR
Every PE that is eligible for selection as the MEG SBD-DR originates
an SBD-IMET route. As stated in Section 5, these SBD-IMET routes
carry a Multicast Flags EC with the MEG flag set.
These SBD-IMET routes SHOULD also carry a DF Election EC. The DF
Election EC and its use are specified in [RFC8584]. When the route
is originated, the AC-DF bit in the DF Election EC SHOULD be set to
zero. This bit is not used when selecting a MEG SBD-DR, i.e., it
MUST be ignored by the receiver of an SBD-IMET route.
In the context of a given Tenant Domain, to select the MEG SBD-DR,
the MEGs of the Tenant Domain perform the following procedure:
* From the set of received SBD-IMET routes for the given Tenant
Domain, determine the candidate set of PEs that support MEG
functionality for that domain.
* Select a DF election algorithm as specified in [RFC8584]. Some of
the possible algorithms can be found, e.g., in [RFC7432],
[RFC8584], and [EVPN-DF].
* Apply the DF election algorithm (see [RFC8584]) to the candidate
set of PEs. The winner becomes the MEG SBD-DR.
Note that if a given PE supports IPMG (Section 6.1.2) or PEG
(Section 6.1.4) functionality as well as MEG functionality, its SBD-
IMET routes carry only one DF Election EC.
6.1.3. Interworking with Global Table Multicast
If multicast service to the outside sources and/or receivers is
provided via the BGP-based Global Table Multicast (GTM) procedures of
[RFC7716], the procedures of Section 6.1.2 can easily be adapted for
EVPN/GTM interworking. The way to adapt the MVPN procedures to GTM
is explained in [RFC7716].
6.1.4. Interworking with PIM
As discussed, there may be receivers in an EVPN Tenant Domain that
are interested in multicast flows whose sources are outside the EVPN
Tenant Domain. Or there may be receivers outside an EVPN Tenant
Domain that are interested in multicast flows whose sources are
inside the Tenant Domain.
If the outside sources and/or receivers are part of an MVPN, see the
procedures for interworking that are covered in Section 6.1.2.
There are also cases where an external source or receiver are
attached via IP and the Layer 3 multicast routing is done via PIM.
In this case, the interworking between the PIM domain and the EVPN
Tenant Domain is done at L3 Gateways that perform PIM/EVPN Gateway
(PEG) functionality. A PEG is very similar to a MEG, except that its
Layer 3 multicast routing is done via PIM rather than via BGP.
If external sources or receivers for a given group are attached to a
PEG via a Layer 3 interface, that interface should be treated as a
VRF interface attached to the Tenant Domain's L3VPN VRF. The Layer 3
multicast routing instance for that Tenant Domain will either run PIM
on the VRF interface or listen for IGMP/MLD messages on that
interface. If the external receiver is attached elsewhere on an IP
network, the PE has to enable PIM on its interfaces to the backbone
network. In both cases, the PE needs to perform PEG functionality,
and its IMET routes must carry the Multicast Flags EC with the PEG
flag set.
For each BD on which there is a multicast source or receiver, one of
the PEGs will become the PEG DR. DR selection can be done using the
same procedures specified in Section 6.1.2.4, except with PEG
substituted for MEG.
As long as there are no tenant multicast routers within the EVPN
Tenant Domain, the PEGs do not need to run PIM on their IRB
interfaces.
6.1.4.1. Source Inside EVPN Domain
If a PEG receives a PIM Join (S,G) from outside the EVPN Tenant
Domain, it may find it necessary to create (S,G) state. The PE needs
to determine whether S is within the Tenant Domain. If S is not
within the EVPN Tenant Domain, the PE carries out normal Layer 3
multicast routing procedures. If S is within the EVPN Tenant Domain,
the IIF of the (S,G) state is set as follows:
* If S is on a BD that is attached to the PE, the IIF is the PE's
IRB interface to that BD.
* If S is not on a BD that is attached to the PE, the IIF is the
PE's IRB interface to the SBD.
When the PE creates such an (S,G) state, it MUST originate (if it
hasn't already) an SBD-SMET route for (S,G). This will cause it to
pull the (S,G) traffic via Layer 2. When the traffic arrives over an
EVPN tunnel, it gets sent up an IRB interface where the Layer 3
multicast routing determines the packet's disposition. The SBD-SMET
route is withdrawn when the (S,G) state no longer exists (unless
there is some other reason for not withdrawing it).
If there are no tenant multicast routers within the EVPN Tenant
Domain, there cannot be an RP in the Tenant Domain, so a PEG does not
have to handle externally arriving PIM Join (*,G) messages.
The PEG DR for a particular BD MUST act as the a First Hop Router for
that BD. It will examine all (S,G) traffic on the BD, and whenever G
is an ASM group, the PEG DR will send Register messages to the RP for
G. This means that the PEG DR will need to pull all the (S,G)
traffic originating on a given BD by originating a SMET (*,*) route
for that BD. If a PEG DR is the DR for all the BDs, it SHOULD
originate just an SBD-SMET (*,*) route rather than a SMET (*,*) route
for each BD.
The rules for exporting IP routes to multicast sources are the same
as those specified for MEGs in Section 6.1.2.2, except that the
exported routes will be IP routes rather than VPN-IP routes, and it
is not necessary to attach the VRF Route Import EC or the Source AS
EC.
When a source is on a multihomed segment, the same issue discussed in
Section 6.1.2.2.3 exists. Suppose S is on an Ethernet segment,
belonging to BD1, that is multihomed to both PE1 and PE2, where PE1
is a PEG. And suppose that IP multicast traffic from S to G travels
over the AC that attaches the segment to PE2. If PE1 receives an
external PIM Join (S,G) route, it MUST originate a SMET route for
(S,G). Normal OISM procedures will cause PE2 to send the (S,G)
traffic to PE1 on an EVPN IP multicast tunnel. Normal OISM
procedures will also cause PE1 to send the (S,G) traffic up its BD1
IRB interface. Normal PIM procedures will then cause PE1 to forward
the traffic along a PIM tree. In this case, the routing is not
optimal, but the traffic does flow correctly.
6.1.4.2. Source Outside EVPN Domain
By means of normal OISM procedures, a PEG learns whether there are
receivers in the Tenant Domain that are interested in receiving (*,G)
or (S,G) traffic. The PEG must determine whether or not S (or the RP
for G) is outside the EVPN Tenant Domain. If so, and if there is a
receiver on BD1 interested in receiving such traffic, the PEG DR for
BD1 is responsible for originating a PIM Join (S,G) or Join (*,G)
control message.
An alternative would be to allow any PEG that is directly attached to
a receiver to originate the PIM Joins. Then, the PEG DR would only
have to originate PIM Joins on behalf of receivers that are not
attached to a PEG. However, if this is done, it is necessary for the
PEGs to run PIM on all their IRB interfaces so that the PIM Assert
procedures can be used to prevent duplicate delivery to a given BD.
The IIF for the Layer 3 (S,G) or (*,G) state is determined by normal
PIM procedures. If a receiver is on BD1, and the PEG DR is attached
to BD1, its IRB interface to BD1 is added to the OIF list. This
ensures that any receivers locally attached to the PEG DR will
receive the traffic. If there are receivers attached to other EVPN
PEs, then whenever (S,G) traffic from an external source matches a
(*,G) state, the PEG will create (S,G) state. The IIF will be set to
whatever external interface the traffic is expected to arrive on
(copied from the (*,G) state), the OIF list is copied from the (*,G)
state, and the SBD IRB interface is added to the OIF list.
6.2. Interworking with PIM via an External PIM Router
Section 6.1 describes how to use an OISM PE router as the gateway to
a non-EVPN multicast domain when the EVPN Tenant Domain is not being
used as an intermediate transit network for multicast. An
alternative approach is to have one or more external PIM routers
(perhaps operated by a tenant) on one of the BDs of the Tenant
Domain. We will refer to this BD as the "gateway BD".
In this model:
* The EVPN Tenant Domain is treated as a stub network attached to
the external PIM routers.
* The external PIM routers follow normal PIM procedures and provide
the FHR and LHR functionality for the entire Tenant Domain.
* The OISM PEs do not run PIM.
* There MUST NOT be more than one gateway BD.
* If an OISM PE not attached to the gateway BD has interest in a
given multicast flow, it conveys that interest, following normal
OISM procedures, by originating an SBD-SMET route for that flow.
* If a PE attached to the gateway BD receives an SBD-SMET, it may
need to generate and transmit a corresponding IGMP/MLD Join on one
or more of its ACs. (Procedures for generating an IGMP/MLD Join
as a result of receiving a SMET route are given in [RFC9251].)
The PE MUST know which BD is the gateway BD and MUST NOT transmit
an IGMP/MLD Join to any other BDs. Furthermore, even if a
particular AC is part of that BD, the PE SHOULD NOT transmit an
IGMP/MLD Join on that AC unless there is an external PIM router
attached via that AC.
As a result, IGMP/MLD messages will be received by the external
PIM routers on the gateway BD, and those external PIM routers will
send PIM Join messages externally as required. Traffic for the
given multicast flow will then be received by one of the external
PIM routers, and that traffic will be forwarded by that router to
the gateway BD.
The normal OISM procedures will then cause the given multicast
flow to be tunneled to any PEs of the EVPN Tenant Domain that have
interest in the flow. PEs attached to the gateway BD will see the
flow as originating from the gateway BD, and other PEs will see
the flow as originating from the SBD.
* An OISM PE attached to a gateway BD MUST set its Layer 2 multicast
state to indicate that each AC to the gateway BD has interest in
all multicast flows. It MUST also originate a SMET route for
(*,*). The procedures for originating SMET routes are discussed
in Section 2.5.
This will cause the OISM PEs attached to the gateway BD to receive
all the IP multicast traffic that is sourced within the EVPN
Tenant Domain and to transmit that traffic to the gateway BD,
where the external PIM routers will receive it. This enables the
external PIM routers to perform FHR functions on behalf of the
entire Tenant Domain. (Of course, if the gateway BD has a
multihomed segment, only the PE that is the DF for that segment
will transmit the multicast traffic to the segment.)
7. Using an EVPN Tenant Domain as an Intermediate (Transit) Network for
Multicast Traffic
In this section, we consider the scenario where one or more BDs of an
EVPN Tenant Domain are being used to carry IP multicast traffic for
which the source and at least one receiver are not part the Tenant
Domain. That is, one or more BDs of the Tenant Domain are
intermediate links of a larger multicast tree created by PIM.
We define a "tenant multicast router" as a multicast router, running
PIM, that:
1. is attached to one or more BDs of the Tenant Domain but
2. is not an EVPN PE router.
In order for an EVPN Tenant Domain to be used as a transit network
for IP multicast, one or more of its BDs must have tenant multicast
routers, and an OISM PE attached to such a BD MUST be provisioned to
enable PIM on its IRB interface to that BD. (This is true even if
none of the tenant routers is on a segment attached to the PE.)
Further, all the OISM PEs (even ones not attached to a BD with tenant
multicast routers) MUST be provisioned to enable PIM on their SBD IRB
interfaces.
If PIM is enabled on a particular BD, the DR selection procedure of
Section 6.1.2.4 MUST be replaced by the normal PIM DR Election
procedure of [RFC7761]. Note that this may result in one of the
tenant routers being selected as the DR rather than one of the OISM
PE routers. In this case, First Hop Router and Last Hop Router
functionality will not be performed by any of the EVPN PEs.
A PIM control message on a particular BD is considered to be a link-
local multicast message and, as such, is sent transparently from PE
to PE via the BUM tunnel for that BD. This is true whether the
control message was received from an AC or from the local Layer 3
routing instance via an IRB interface.
A PIM Join/Prune message contains three fields that are relevant to
the present discussion:
* Upstream Neighbor
* Group Address (G)
* Source Address (S), omitted in the case of (*,G) Join/Prune
messages
We will generally speak of a PIM Join as a Join (S,G) or a Join (*,G)
message and will use the term "Join (X,G)" to mean either "Join
(S,G)" or "Join (*,G)". In the context of a Join (X,G), we will use
the term "X" to mean "S" in the case of (S,G) or "G's RP" in the case
of (*,G).
Suppose BD1 contains two tenant multicast routers, say C1 and C2.
Suppose C1 is on a segment attached to PE1 and C2 is on a segment
attached to PE2. When C1 sends a PIM Join (X,G) to BD1, the Upstream
Neighbor field might be set to PE1, PE2, or C2. C1 chooses the
Upstream Neighbor based on its unicast routing. Typically, it will
choose the PIM router on BD1 that is closest (according to the
unicast routing) to X as the Upstream Neighbor. Note that this will
not necessarily be PE1. PE1 may not even be visible to the unicast
routing algorithm used by the tenant routers. Even if it is, it is
unlikely to be the PIM router that is closest to X. So we need to
consider the following two cases:
1. C1 sends a PIM Join (X,G) to BD1, with PE1 as the Upstream
Neighbor.
PE1's PIM routing instance will receive the Join arrive on the
BD1 IRB interface. If X is not within the Tenant Domain, PE1
handles the Join according to normal PIM procedures. This will
generally result in PE1 selecting an Upstream Neighbor and
sending it a Join (X,G).
If X is within the Tenant Domain but is attached to some other
PE, PE1 sends (if it hasn't already) an SBD-SMET route for (X,G).
The IIF of the Layer 3 (X,G) state will be the SBD IRB interface,
and the OIF list will include the IRB interface to BD1.
The SBD-SMET route will pull the (X,G) traffic to PE1, and the
(X,G) state will result in the (X,G) traffic being forwarded to
C1.
If X is within the Tenant Domain but is attached to PE1 itself,
no SBD-SMET route is sent. The IIF of the Layer 3 (X,G) state
will be the IRB interface to X's BD, and the OIF list will
include the IRB interface to BD1.
2. C1 sends a PIM Join (X,G) to BD1, with either PE2 or C2 as the
Upstream Neighbor.
PE1's PIM routing instance will receive the Join arrive on the
BD1 IRB interface. If neither X nor Upstream Neighbor is within
the Tenant Domain, PE1 handles the Join according to normal PIM
procedures. This will NOT result in PE1 sending a Join (X,G).
If either X or Upstream Neighbor is within the Tenant Domain, PE1
sends (if it hasn't already) an SBD-SMET route for (X,G). The
IIF of the Layer 3 (X,G) state will be the SBD IRB interface, and
the OIF list will include the IRB interface to BD1.
The SBD-SMET route will pull the (X,G) traffic to PE1, and the
(X,G) state will result in the (X,G) traffic being forwarded to
C1.
8. IANA Considerations
IANA has assigned new flags in the "Multicast Flags Extended
Community" registry under the "Border Gateway Protocol (BGP) Extended
Communities" registry as shown below.
+=====+================+===========+===================+
| Bit | Name | Reference | Change Controller |
+=====+================+===========+===================+
| 7 | OISM SBD | RFC 9625 | IETF |
+-----+----------------+-----------+-------------------+
| 9 | IPMG | RFC 9625 | IETF |
+-----+----------------+-----------+-------------------+
| 10 | MEG | RFC 9625 | IETF |
+-----+----------------+-----------+-------------------+
| 11 | PEG | RFC 9625 | IETF |
+-----+----------------+-----------+-------------------+
| 12 | OISM-supported | RFC 9625 | IETF |
+-----+----------------+-----------+-------------------+
Table 1: Multicast Flags Extended Community Registry
9. Security Considerations
This document uses protocols and procedures defined in the normative
references and inherits the security considerations of those
references.
This document adds flags or Extended Communities (ECs) to a number of
BGP routes in order to signal that particular nodes support the OISM,
IPMG, MEG, and/or PEG functionalities that are defined in this
document. Incorrect addition, removal, or modification of those
flags and/or ECs will cause the procedures defined herein to
malfunction, in which case loss or diversion of data traffic is
possible. Implementations should provide tools to easily debug
configuration mistakes that cause the signaling of incorrect
information.
The interworking with non-OISM networks described in Sections 5 and 6
requires gateway functions in multiple redundant PEs, among which one
of them is elected as Designated Forwarder for a given BD (or SBD).
The election of the MEG or PEG DR, as well as the IPMG Designated
Forwarder, makes use of the Designated Forwarder election procedures
[RFC8584]. An attacker with access to one of these Gateways may
influence such election and therefore modify the forwarding of
multicast traffic between the OISM network and the external domain.
The operator should be especially careful with the protection of
these gateways by making sure the management interfaces to access the
gateways are only allowed to authorized operators.
The document also introduces the concept of per-Tenant-Domain
dissemination for the SMET routes, as opposed to per-BD distribution
in [RFC9251]. That is, a SMET route triggered by the reception of an
IGMP/MLD Join in BD-1 on PE1 needs to be distributed and imported by
all PEs of the Tenant Domain, even to those PEs that are not attached
to BD-1. This means that an attacker with access to only one BD in a
PE of the Tenant Domain might force the advertisement of SMET routes
and impact the resources of all the PEs of the Tenant Domain, as
opposed to only the PEs of that particular BD (as in [RFC9251]). The
implementation should provide ways to filter/control the client IGMP/
MLD reports that are received by the attached hosts.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,
<https://www.rfc-editor.org/info/rfc3032>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
February 2006, <https://www.rfc-editor.org/info/rfc4360>.
[RFC6625] Rosen, E., Ed., Rekhter, Y., Ed., Hendrickx, W., and R.
Qiu, "Wildcards in Multicast VPN Auto-Discovery Routes",
RFC 6625, DOI 10.17487/RFC6625, May 2012,
<https://www.rfc-editor.org/info/rfc6625>.
[RFC7153] Rosen, E. and Y. Rekhter, "IANA Registries for BGP
Extended Communities", RFC 7153, DOI 10.17487/RFC7153,
March 2014, <https://www.rfc-editor.org/info/rfc7153>.
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8584] Rabadan, J., Ed., Mohanty, S., Ed., Sajassi, A., Drake,
J., Nagaraj, K., and S. Sathappan, "Framework for Ethernet
VPN Designated Forwarder Election Extensibility",
RFC 8584, DOI 10.17487/RFC8584, April 2019,
<https://www.rfc-editor.org/info/rfc8584>.
[RFC9135] Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
Rabadan, "Integrated Routing and Bridging in Ethernet VPN
(EVPN)", RFC 9135, DOI 10.17487/RFC9135, October 2021,
<https://www.rfc-editor.org/info/rfc9135>.
[RFC9136] Rabadan, J., Ed., Henderickx, W., Drake, J., Lin, W., and
A. Sajassi, "IP Prefix Advertisement in Ethernet VPN
(EVPN)", RFC 9136, DOI 10.17487/RFC9136, October 2021,
<https://www.rfc-editor.org/info/rfc9136>.
[RFC9251] Sajassi, A., Thoria, S., Mishra, M., Patel, K., Drake, J.,
and W. Lin, "Internet Group Management Protocol (IGMP) and
Multicast Listener Discovery (MLD) Proxies for Ethernet
VPN (EVPN)", RFC 9251, DOI 10.17487/RFC9251, June 2022,
<https://www.rfc-editor.org/info/rfc9251>.
[RFC9572] Zhang, Z., Lin, W., Rabadan, J., Patel, K., and A.
Sajassi, "Updates to EVPN Broadcast, Unknown Unicast, or
Multicast (BUM) Procedures", RFC 9572,
DOI 10.17487/RFC9572, May 2024,
<https://www.rfc-editor.org/info/rfc9572>.
[RFC9574] Rabadan, J., Ed., Sathappan, S., Lin, W., Katiyar, M., and
A. Sajassi, "Optimized Ingress Replication Solution for
Ethernet VPNs (EVPNs)", RFC 9574, DOI 10.17487/RFC9574,
May 2024, <https://www.rfc-editor.org/info/rfc9574>.
10.2. Informative References
[EVPN-DF] Rabadan, J., Sathappan, S., Lin, W., Drake, J., and A.
Sajassi, "Preference-based EVPN DF Election", Work in
Progress, Internet-Draft, draft-ietf-bess-evpn-pref-df-13,
9 October 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-bess-evpn-pref-df-13>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[RFC4541] Christensen, M., Kimball, K., and F. Solensky,
"Considerations for Internet Group Management Protocol
(IGMP) and Multicast Listener Discovery (MLD) Snooping
Switches", RFC 4541, DOI 10.17487/RFC4541, May 2006,
<https://www.rfc-editor.org/info/rfc4541>.
[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
2012, <https://www.rfc-editor.org/info/rfc6513>.
[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
<https://www.rfc-editor.org/info/rfc6514>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
[RFC7716] Zhang, J., Giuliano, L., Rosen, E., Ed., Subramanian, K.,
and D. Pacella, "Global Table Multicast with BGP Multicast
VPN (BGP-MVPN) Procedures", RFC 7716,
DOI 10.17487/RFC7716, December 2015,
<https://www.rfc-editor.org/info/rfc7716>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>.
[RFC9624] Zhang, Z., Przygienda, T., Sajassi, A., and J. Rabadan,
"EVPN Broadcast, Unknown Unicast, or Multicast (BUM) Using
Bit Index Explicit Replication (BIER)", RFC 9624,
DOI 10.17487/RFC9624, August 2024,
<https://www.rfc-editor.org/info/rfc9624>.
Appendix A. Integrated Routing and Bridging
This appendix provides a short tutorial on the interaction of routing
and bridging. First, it shows a model, where bridging and routing
are performed in separate devices. Then, it shows the model
specified in [RFC9135], where a single device contains both routing
and bridging functions. The latter model is presupposed in the body
of this document.
Figure 2 shows the model where a router only does routing and has no
L2 bridging capabilities. There are two LANs: LAN1 and LAN2. LAN1
is realized by switch1, and LAN2 is realized by switch2. The router
has an interface, lan1, that attaches to LAN1 (via switch1) and an
interface, lan2, that attaches to LAN2 (via switch2). Each interface
is configured, as an IP interface, with an IP address and a subnet
mask.
+-------+ +--------+ +-------+
| | lan1| |lan2 | |
H1 -----+Switch1+--------+ Router1+--------+Switch2+------H3
| | | | | |
H2 -----| | | | | |
+-------+ +--------+ +-------+
|_________________| |__________________|
LAN1 LAN2
Figure 2: Conventional Router with LAN Interfaces
IP traffic (unicast or multicast) that remains within a single subnet
never reaches the router. For instance, if H1 emits an Ethernet
frame with H2's MAC address in the Ethernet Destination Address
field, the frame will go from H1 to Switch1 to H2 without ever
reaching the router. Since the frame is never seen by a router, the
IP datagram within the frame remains entirely unchanged, e.g., its
TTL is not decremented. The Ethernet Source and Destination MAC
addresses are not changed either.
If H1 wants to send a unicast IP datagram to H3, which is on a
different subnet, H1 has to be configured with the IP address of a
default router. Let's assume that H1 is configured with an IP
address of Router1 as its default router address. H1 compares H3's
IP address with its own IP address and IP subnet mask and determines
that H3 is on a different subnet. So the packet has to be routed.
H1 uses ARP to map Router1's IP address to a MAC address on LAN1. H1
then encapsulates the datagram in an Ethernet frame, using Router1's
MAC address as the destination MAC address, and sends the frame to
Router1.
Router1 then receives the frame over its lan1 interface. Router1
sees that the frame is addressed to it, so it removes the Ethernet
encapsulation and processes the IP datagram. The datagram is not
addressed to Router1, so it must be forwarded further. Router1 does
a lookup of the datagram's IP Destination Address field and
determines that the destination (H3) can be reached via Router1's
lan2 interface. Router1 now performs the IP processing of the
datagram: it decrements the IP TTL, adjusts the IP header checksum
(if present), may fragment the packet as necessary, etc. Then, the
datagram (or its fragments) is encapsulated in an Ethernet header,
with Router1's MAC address on LAN2 as the MAC Source Address and H3's
MAC address on LAN2 (which Router1 determines via ARP) as the
Destination MAC Address. Finally, the packet is sent on the lan2
interface.
If H1 has an IP multicast datagram to send (i.e., an IP datagram
whose Destination Address field is an IP Multicast Address), it
encapsulates it in an Ethernet frame whose Destination MAC Address is
computed from the IP Destination Address.
If H2 is a receiver for that multicast address, H2 will receive a
copy of the frame, unchanged, from H1. The MAC Source Address in the
Ethernet encapsulation does not change, the IP TTL field does not get
decremented, etc.
If H3 is a receiver for that multicast address, the datagram must be
routed to H3. In order for this to happen, Router1 must be
configured as a multicast router, and it must accept traffic sent to
Ethernet multicast addresses. Router1 will receive H1's multicast
frame on its lan1 interface, remove the Ethernet encapsulation, and
determine how to dispatch the IP datagram based on Router1's
multicast forwarding states. If Router1 knows that there is a
receiver for the multicast datagram on LAN2, it makes a copy of the
datagram, decrements the TTL (and performs any other necessary IP
processing), and then encapsulates the datagram in the Ethernet frame
for LAN2. The MAC Source Address for this frame will be Router1's
MAC Source Address on LAN2. The Destination MAC Address is computed
from the IP Destination Address. Finally, the frame is sent on
Router1's LAN2 interface.
Figure 3 shows an integrated router/bridge that supports the routing/
bridging integration model of [RFC9135].
+------------------------------------------+
| Integrated Router/Bridge |
+-------+ +--------+ +-------+
| | IRB1| L3 |IRB2 | |
H1 -----+ BD1 +--------+Routing +--------+ BD2 +------H3
| | |Instance| | |
H2 -----| | | | | |
+-------+ +--------+ +-------+
|___________________| |____________________|
LAN1 LAN2
Figure 3: Integrated Router/Bridge
In Figure 3, a single device consists of one or more L3 Routing
Instances. The routing/forwarding tables of a given routing instance
is known as an IP-VRF [RFC9135]. In the context of EVPN, it is
convenient to think of each routing instance as representing the
routing of a particular tenant. Each IP-VRF is attached to one or
more interfaces.
When several EVPN PEs have a routing instance of the same Tenant
Domain, those PEs advertise IP routes to the attached hosts. This is
done as specified in [RFC9135].
The integrated router/bridge shown in Figure 3 also attaches to a
number of Broadcast Domains (BDs). Each BD performs the functions
that are performed by the bridges in Figure 2. To the L3 routing
instance, each BD appears to be a LAN. The interface attaching a
particular BD to a particular IP-VRF is known as an "IRB interface".
From the perspective of L3 routing, each BD is a subnet. Thus, each
IRB interface is configured with a MAC address (which is the router's
MAC address on the corresponding LAN), as well as an IP address and
subnet mask.
The integrated router/bridge shown in Figure 3 may have multiple ACs
to each BD. These ACs are visible only to the bridging function, not
to the routing instance. To the L3 routing instance, there is just
one interface to each BD.
If the L3 routing instance represents the IP routing of a particular
tenant, the BDs attached to that routing instance are BDs belonging
to that same tenant.
Bridging and routing now proceed exactly as in the case of Figure 2,
except that BD1 replaces Switch1, BD2 replaces Switch2, interface
IRB1 replaces interface lan1, and interface IRB2 replaces interface
lan2.
It is important to understand that an IRB interface connects an L3
routing instance to a BD, NOT to a MAC-VRF (see [RFC7432] for the
definition of MAC-VRF). A MAC-VRF may contain several BDs, as long
as no MAC address appears in more than one BD. From the perspective
of the L3 routing instance, each individual BD is an individual IP
subnet; whether or not each BD has its own MAC-VRF is irrelevant to
the L3 routing instance.
Figure 4 illustrates IRB when a pair of BDs (subnets) are attached to
two different PE routers. In this example, each BD has two segments,
and one segment of each BD is attached to one PE router.
+------------------------------------------+
| Integrated Router/Bridges |
+-------+ +--------+ +-------+
| | IRB1| |IRB2 | |
H1 -----+ BD1 +--------+ PE1 +--------+ BD2 +------H3
|(Seg-1)| |(L3 Rtg)| |(Seg-1)|
H2 -----| | | | | |
+-------+ +--------+ +-------+
|___________________| | |____________________|
LAN1 | LAN2
|
|
+-------+ +--------+ +-------+
| | IRB1| |IRB2 | |
H4 -----+ BD1 +--------+ PE2 +--------+ BD2 +------H5
|(Seg-2)| |(L3 Rtg)| |(Seg-2)|
| | | | | |
+-------+ +--------+ +-------+
Figure 4: Integrated Router/Bridges with Distributed Subnet
If H1 needs to send an IP packet to H4, it determines from its IP
address and subnet mask that H4 is on the same subnet as H1.
Although H1 and H4 are not attached to the same PE router, EVPN
provides Ethernet communication among all hosts that are on the same
BD. Thus, H1 uses ARP to find H4's MAC address and sends an Ethernet
frame with H4's MAC address in the Destination MAC Address field.
The frame is received at PE1, but since the Destination MAC address
is not PE1's MAC address, PE1 assumes that the frame is to remain on
BD1. Therefore, the packet inside the frame is NOT decapsulated and
is NOT sent up the IRB interface to PE1's routing instance. Rather,
standard EVPN intra-subnet procedures (as detailed in [RFC7432]) are
used to deliver the frame to PE2, which then sends it to H4.
If H1 needs to send an IP packet to H5, it determines from its IP
address and subnet mask that H5 is NOT on the same subnet as H1.
Assuming that H1 has been configured with the IP address of PE1 as
its default router, H1 sends the packet in an Ethernet frame with
PE1's MAC address in its Destination MAC Address field. PE1 receives
the frame and sees that the frame is addressed to it. Thus, PE1
sends the frame up its IRB1 interface to the L3 routing instance.
Appropriate IP processing is done, e.g., TTL decrement. The L3
routing instance determines that the next hop for H5 is PE2, so the
packet is encapsulated (e.g., in MPLS) and sent across the backbone
to PE2's routing instance. PE2 will see that the packet's
destination, H5, is on BD2 segment-2 and will send the packet down
its IRB2 interface. This causes the IP packet to be encapsulated in
an Ethernet frame with PE2's MAC address (on BD2) in the Source
Address field and H5's MAC address in the Destination Address field.
Note that if H1 has an IP packet to send to H3, the forwarding of the
packet is handled entirely within PE1. PE1's routing instance sees
the packet arrive on its IRB1 interface and then transmits the packet
by sending it down its IRB2 interface.
Often, all the hosts in a particular Tenant Domain will be
provisioned with the same value of the default router IP address.
This IP address can be provisioned as an anycast address in all the
EVPN PEs attached to that Tenant Domain. Thus, although all hosts
are provisioned with the same default router address, the actual
default router for a given host will be one of the PEs attached to
the same Ethernet segment as the host. This provisioning method
ensures that IP packets from a given host are handled by the closest
EVPN PE that supports IRB.
In the topology of Figure 4, one could imagine that H1 is configured
with a default router address that belongs to PE2 but not to PE1.
Inter-subnet routing would still work, but IP packets from H1 to H3
would then follow the non-optimal path H1-->PE1-->PE2-->PE1-->H3.
Sending traffic on this sort of path, where it leaves a router and
then comes back to the same router, is sometimes known as
"hairpinning". Similarly, if PE2 supports IRB but PE1 dos not, the
same non-optimal path from H1 to H3 would have to be followed. To
avoid hairpinning, each EVPN PE needs to support IRB.
It is worth pointing out the way IRB interfaces interact with
multicast traffic. Referring again to Figure 4, suppose PE1 and PE2
are functioning as IP multicast routers. Also, suppose that H3
transmits a multicast packet and both H1 and H4 are interested in
receiving that packet. PE1 will receive the packet from H3 via its
IRB2 interface. The Ethernet encapsulation from BD2 is removed, the
IP header processing is done, and the packet is then re-encapsulated
for BD1, with PE1's MAC address in the MAC Source Address field.
Then, the packet is sent down the IRB1 interface. Layer 2 procedures
(as defined in [RFC7432]) would then be used to deliver a copy of the
packet locally to H1 and remotely to H4.
Please be aware that this document modifies the semantics, described
in the previous paragraph, of sending/receiving multicast traffic on
an IRB interface. This is explained in Section 1.5.1 and subsequent
sections.
Acknowledgements
The authors thank Vikram Nagarajan and Princy Elizabeth for their
work on Sections 6.2 and 3.2.3.1. The authors also benefited
tremendously from discussions with Aldrin Isaac on EVPN multicast
optimizations.
Authors' Addresses
Wen Lin
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America
Email: wlin@juniper.net
Zhaohui Zhang
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America
Email: zzhang@juniper.net
John Drake
Juniper Networks, Inc.
1194 N. Mathilda Ave
Sunnyvale, CA 94089
United States of America
Email: jdrake@juniper.net
Eric C. Rosen (editor)
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886
United States of America
Email: erosen52@gmail.com
Jorge Rabadan
Nokia
777 E. Middlefield Road
Mountain View, CA 94043
United States of America
Email: jorge.rabadan@nokia.com
Ali Sajassi
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
United States of America
Email: sajassi@cisco.com