RFC7177: Transparent Interconnection of Lots of Links (TRILL): Adjacency
Download in PDF format Download in text format
Obsoletes:
RFC6327
Internet Engineering Task Force (IETF) D. Eastlake 3rd
Request for Comments: 7177 Huawei
Obsoletes: 6327 R. Perlman
Updates: 6325 Intel Labs
Category: Standards Track A. Ghanwani
ISSN: 2070-1721 Dell
H. Yang
Cisco
V. Manral
Ionos Corp.
May 2014
Transparent Interconnection of Lots of Links (TRILL): Adjacency
Abstract
The IETF Transparent Interconnection of Lots of Links (TRILL)
protocol supports arbitrary link technologies between TRILL switches,
including point-to-point links and multi-access Local Area Network
(LAN) links that can have multiple TRILL switches and end stations
attached. TRILL uses Intermediate System to Intermediate System
(IS-IS) routing. This document specifies the establishment,
reporting, and termination of IS-IS adjacencies between TRILL
switches, also known as RBridges (Routing Bridges). It also concerns
four other link-local aspects of TRILL: Designated RBridge (DRB)
selection, MTU (Maximum Transmission Unit) testing, pseudonode
creation, and BFD (Bidirectional Forwarding Detection) session
bootstrapping in connection with adjacency. State diagrams are
included where appropriate. This document obsoletes RFC 6327 and
updates RFC 6325.
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 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7177.
Eastlake, et al. Standards Track [Page 1]
RFC 7177 TRILL: Adjacency May 2014
Copyright Notice
Copyright (c) 2014 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Eastlake, et al. Standards Track [Page 2]
RFC 7177 TRILL: Adjacency May 2014
Table of Contents
1. Introduction ....................................................4
1.1. Content and Precedence .....................................5
1.2. Terminology and Acronyms ...................................5
2. The TRILL Hello Environment and Purposes ........................6
2.1. RBridge Interconnection by Ethernet ........................6
2.2. Handling Native Frames .....................................7
2.3. Zero or Minimal Configuration ..............................8
2.4. MTU Robustness .............................................8
2.5. Purposes of the TRILL Hello Protocol .......................9
3. Adjacency State Machinery ......................................10
3.1. TRILL Hellos, Ports, and VLANs ............................10
3.2. Adjacency Table Entries and States ........................11
3.3. Adjacency and Hello Events ................................12
3.4. Adjacency State Diagram and Table .........................15
3.5. Multiple Parallel Links ...................................17
3.6. Insufficient Space in Adjacency Table .....................17
4. LAN Ports and DRB State ........................................17
4.1. Port Table Entries and DRB Election State .................18
4.2. DRB Election Events .......................................19
4.2.1. DRB Election Details ...............................20
4.2.2. Change in DRB ......................................20
4.2.3. Change in Designated VLAN ..........................21
4.3. Port State Table and Diagram ..............................21
5. MTU Matching ...................................................22
6. BFD-Enabled TLV and BFD Session Bootstrapping ..................24
7. Pseudonodes ....................................................25
8. More TRILL Hello Details .......................................26
8.1. Contents of TRILL Hellos ..................................27
8.2. Transmitting TRILL Hellos .................................27
8.2.1. TRILL Neighbor TLVs ................................28
8.3. Receiving TRILL Hellos ....................................29
9. Multiple Ports on the Same Broadcast Link ......................29
10. IANA Considerations ...........................................30
11. Security Considerations .......................................30
Appendix A. Changes from RFC 6327 .................................31
Appendix B. Changes to RFC 6325 ...................................31
Normative References...............................................32
Informative References.............................................33
Acknowledgements...................................................34
Eastlake, et al. Standards Track [Page 3]
RFC 7177 TRILL: Adjacency May 2014
1. Introduction
The IETF Transparent Interconnection of Lots of Links (TRILL)
protocol [RFC6325] provides optimal pair-wise data frame forwarding
without configuration, safe forwarding even during transient loops,
and support for transmission of both unicast and multicast traffic
taking advantage of multiple paths in multi-hop networks with
arbitrary topology. End stations are connected to TRILL switches
with Ethernet, but TRILL switches can be interconnected with
arbitrary link technology. TRILL accomplishes this by using [IS-IS]
(Intermediate System to Intermediate System) link-state routing
[RFC1195] [RFC7176] and a header in TRILL Data packets that includes
a hop count. The design supports data labeling by Virtual Local Area
Networks (VLANs) and fine-grained labels [RFC7172] as well as
optimization of the distribution of multi-destination frames based on
data label and multicast groups. Devices that implement TRILL are
called TRILL switches or RBridges (Routing Bridges).
This document provides detailed specifications for five of the link-
local aspects of the TRILL protocol used on broadcast links (also
called LAN or multi-access links) and for the three of these aspects
that are also used on point-to-point (P2P) links. It includes state
diagrams and implementation details where appropriate. Alternative
implementations that interoperate on the wire are permitted.
The scope of this document is limited to the following aspects of the
TRILL protocol; their applicability, along with the most relevant
section of this document, are as shown here:
LAN P2P Section Link-Local Aspect
--- --- ------- ----------------------------------------------
X X 3 Adjacency formation and dissolution
X 4 DRB (Designated RBridge) election
X X 5 MTU (Maximum Transmission Unit) matching
X X 6 1-hop BFD (Bidirectional Forwarding Detection)
for adjacency
X 7 Creation and use of pseudonodes [IS-IS]
Table 1: LAN/P2P Applicability
There is no DRB (also known as DIS (Designated Intermediate System))
election and no pseudonode creation on links configured as point-to-
point.
For other aspects of the TRILL base protocol, see [RFC6325],
[RFC6439], [RFC7180], and [ESADI].
Eastlake, et al. Standards Track [Page 4]
RFC 7177 TRILL: Adjacency May 2014
This document obsoletes [RFC6327]. See Appendix A for a summary of
changes from [RFC6327]. This document updates [RFC6325] as described
in Appendix B.
1.1. Content and Precedence
In cases of conflict between this document and [RFC6325], this
document prevails.
Section 2 below explains the rationale for the differences between
the TRILL Hello protocol and the Layer 3 IS-IS Hello protocol in
light of the environment for which the TRILL protocol is designed.
It also describes the purposes of the TRILL Hello protocol.
Section 3 describes the adjacency state machine, its states, and its
relevant events.
Section 4 describes the Designated RBridge (DRB) election state
machine for RBridge LAN ports, its states, and its relevant events.
Section 5 describes MTU testing and matching on a TRILL link.
Section 6 discusses one-hop BFD session bootstrapping in connection
with adjacency.
Section 7 discusses pseudonode creation and use on LAN links.
Section 8 provides more details on the reception and transmission of
TRILL Hellos.
Section 9 discusses the case of multiple ports from one RBridge on
the same link.
1.2. Terminology and Acronyms
This document uses the acronyms defined in [RFC6325], supplemented by
the following additional acronyms:
BFD - Bidirectional Forwarding Detection [RFC7175].
SNPA - Subnetwork Point of Attachment [IS-IS].
TRILL switch - an alternative name for an RBridge.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Eastlake, et al. Standards Track [Page 5]
RFC 7177 TRILL: Adjacency May 2014
2. The TRILL Hello Environment and Purposes
[IS-IS] has subnetwork-independent functions and subnetwork-dependent
functions. Currently, Layer 3 use of IS-IS supports two types of
subnetworks: (1) point-to-point link subnetworks between routers and
(2) general broadcast (LAN) subnetworks. Because of the differences
between the environment of Layer 3 routers and the environment of
TRILL RBridges, instead of the subnetwork-dependent functions used at
Layer 3, which are specified in Sections 8.2 and 8.4 of [IS-IS], the
TRILL protocol uses modified subnetwork-dependent functions for
point-to-point subnetworks and broadcast (LAN) subnetworks. The
differences between the TRILL and Layer 3 environments are described
in Sections 2.1 through 2.4 followed by a summation, in Section 2.5,
of the purposes of the TRILL Hello protocol.
2.1. RBridge Interconnection by Ethernet
TRILL supports the interconnection of RBridges by multi-access LAN
links such as Ethernet. Because this includes general bridged LANs
[802.1Q], the links between RBridges may contain devices or services
that can restrict VLAN connectivity, such as [802.1Q] bridges or
carrier Ethernet services. In addition, RBridge Ethernet ports, like
[802.1Q] ports, can be configured to restrict input/output on a VLAN
basis.
For this reason, TRILL Data and TRILL IS-IS packets are sent on
Ethernet links in a Designated VLAN that is assumed to provide
connectivity between all RBridges on the link. The Designated VLAN
is dictated for a LAN link by the elected Designated RBridge on that
link (DRB, equivalent to the Designated Intermediate System at
Layer 3). On an RBridge Ethernet port configured as point-to-point,
TRILL Data and IS-IS packets are sent in that port's Desired
Designated VLAN, regardless of the state of any other ports on the
link. Connectivity on an Ethernet link configured as point-to-point
generally depends on both ends being configured with the same Desired
Designated VLAN. Because TRILL Data packets flow between RBridges on
an Ethernet link only in the link's Designated VLAN, adjacency for
routing calculations is based only on connectivity characteristics in
that VLAN.
(Non-Ethernet links, such as PPP [RFC6361] generally do not have any
Outer.VLAN labeling, so the Designated VLAN for such links has no
effect.)
Eastlake, et al. Standards Track [Page 6]
RFC 7177 TRILL: Adjacency May 2014
2.2. Handling Native Frames
This section discusses the handling of "native" frames as defined in
Section 1.4 of [RFC6325]. As such, this section is not applicable to
point-to-point links between TRILL switches or any link where all the
TRILL switch ports on the link have been configured as "trunk ports"
by setting the end-station service disable bit for the port (see
Section 4.9.1 of [RFC6325]).
Layer 3 data packets, such as IP packets, are already "tamed" when
they are originated by an end station: they include a hop count and
Layer 3 source and destination address fields. Furthermore, for
ordinary data packets, there is no requirement to preserve any outer
Layer 2 addressing, and, if the packets are unicast, they are
explicitly addressed to their first-hop router.
In contrast, TRILL switches must accept, transport, and deliver
"untamed" native frames: native frames that lack a hop count field
usable by TRILL and have Layer 2 MAC (Media Access Control) addresses
that indicate their source and destination. These Layer 2 addresses
must be preserved for delivery to the native frame's Layer 2
destination. One resulting difference is that RBridge ports
providing native frame service must receive in promiscuous MAC
address mode, while Layer 3 router ports typically receive in a
selective MAC address mode.
TRILL handles these requirements by having, on the link where an end
station originates a native frame, one RBridge "ingress" such a
locally originated native frame by adding a TRILL Header that
includes a hop count, thus converting it to a TRILL Data packet.
This augmented packet is then routed to one RBridge on the link
having the destination end station for the frame (or one RBridge on
each such link if it is a multi-destination frame). Such final
RBridges perform an "egress" function, removing the TRILL Header and
delivering the original frame to its destination(s). (For the
purposes of TRILL, a Layer 3 router is an end station.)
Care must be taken to avoid a loop that would involve egressing a
native frame and then re-ingressing it because, while it is in native
form, it would not be protected by a hop count and could loop
forever. Such a loop could, for multi-destination frames, even
involve multiplication of the number of frames each time around and
would likely saturate all links involved within milliseconds. For
TRILL, safety against such loops for a link is more important than
transient loss of data connectivity on that link.
Eastlake, et al. Standards Track [Page 7]
RFC 7177 TRILL: Adjacency May 2014
The primary TRILL defense mechanism against such loops, which is
mandatory, is to assure that, as far as practically possible, there
is only a single RBridge that is in charge of ingressing and
egressing native frames from and to a link where TRILL is offering
end-station service. This is the Designated RBridge and Appointed
Forwarder mechanism initially specified in the TRILL base protocol
[RFC6325], discussed in Section 2.5 below, and further specified in
both Section 4 below and [RFC6439].
2.3. Zero or Minimal Configuration
TRILL provides connectivity and least-cost paths with zero
configuration. For additional services, it strives to require only
minimal configuration; however, services that require configuration
when offered by [802.1Q] bridges, such as non-default VLANs or
priority, will require configuration. This differs from Layer 3
routing where routers typically need to be configured as to the
subnetworks connected to each port, etc., to provide service.
2.4. MTU Robustness
TRILL IS-IS needs to be robust against links with reasonably
restricted MTUs, including links that accommodate only the classic
Ethernet frame size, despite the addition of reasonable headers such
as VLAN tags. Such robustness is particularly required for TRILL
Hellos to assure correct adjacency and the election of a unique DRB
on LAN links.
TRILL will also be used inside data centers where it is common for
all or most of the links and switches to support frames substantially
larger than the classic Ethernet maximum size. For example, they may
have an MTU adequate to comfortably handle Fiber Channel over
Ethernet frames, for which T11 recommends a 2,500-byte MTU [FCoE], or
even 9K byte jumbo frames. It would be beneficial for a TRILL campus
with such a large MTU to be able to safely make use of it.
These needs are met by a mandatory maximum on the size of TRILL
Hellos and by the optional use of MTU testing as described below.
Eastlake, et al. Standards Track [Page 8]
RFC 7177 TRILL: Adjacency May 2014
2.5. Purposes of the TRILL Hello Protocol
There are three purposes for the TRILL Hello protocol. They are
listed below, along with a reference to the section of this document
in which each is discussed:
1. To determine which RBridge neighbors have acceptable connectivity
to be reported as part of the topology (Section 3)
2. To elect a unique Designated RBridge on broadcast (LAN) links
(Section 4)
3. To determine the MTU with which it is possible to safely
communicate with each RBridge neighbor (Section 5)
In Layer 3 IS-IS, all three of these functions are combined. Hellos
may be padded to the maximum length (see [RFC3719], Section 6) so
that a router neighbor is not discovered if it is impossible to
communicate with it using maximum-sized Layer 3 IS-IS packets. Also,
even if Hellos from a neighbor R2 are received by R1, if connectivity
to R2 is not 2-way (i.e., R2 does not list R1 in R2's Hello), then R1
does not consider R2 as a Designated Intermediate System (Designated
Router) candidate. Because of this logic, it is possible at Layer 3
for multiple Designated Routers to be elected on a LAN, with each
representing the LAN as a pseudonode. It appears to the topology as
if the LAN is now two or more separate LANs. Although this is
surprising, this does not cause problems for Layer 3.
In contrast, this behavior is not acceptable for TRILL, since in
TRILL it is important that all RBridges on a link know about each
other, and on broadcast (LAN) links that they choose a single RBridge
to be the DRB to control the native frame ingress and egress.
Otherwise, multiple RBridges might ingress/egress the same native
frame, forming loops that are not protected by the hop count in the
TRILL Header as discussed above.
The TRILL Hello protocol is best understood by focusing separately on
each of these three functions listed above, which we do in
Sections 3, 4, and 5.
One other issue with TRILL LAN Hellos is to ensure that subsets of
the information can appear in any single message, and be processable,
in the spirit of IS-IS Link State PDUs (LSPs) and Complete Sequence
Number PDUs (CSNPs). LAN TRILL Hello packets, even though they are
not padded, can become very large. An example where this might be
the case is when some sort of backbone technology interconnects
hundreds of TRILL sites over what would appear to TRILL to be a giant
Ethernet, where the RBridges connected to that cloud will perceive
Eastlake, et al. Standards Track [Page 9]
RFC 7177 TRILL: Adjacency May 2014
that backbone to be a single link with hundreds of neighbors. Thus,
the TRILL LAN Hello uses a different Neighbor TLV [RFC7176] that
lists neighbors seen for a range of MAC (SNPA) addresses.
3. Adjacency State Machinery
Each RBridge port has associated with it a port state, as discussed
in Section 4, and a table of zero or more adjacencies (if the port is
configured as point-to-point, zero, or one) as discussed in this
section. The states such adjacencies can have, the events that cause
adjacency state changes, the actions associated with those state
changes, a state table, and a state diagram are given below.
3.1. TRILL Hellos, Ports, and VLANs
The determination of adjacencies on links is made using TRILL Hellos
(see Section 8), an optional MTU test (see Section 5), and,
optionally, BFD (see Section 6) and/or other connectivity tests. If
the MAC (SNPA) addresses of more than one RBridge port on a broadcast
link are the same, all but one of such ports are put in the Suspended
state (see Section 4) and do not participate in the link, except to
monitor whether they should stay suspended. If the two ports on a
point-to-point link have MAC (SNPA) addresses, it does not affect
TRILL operation if they are the same. (PPP ports, for example, do
not have MAC addresses [RFC6361].)
The following items MUST be the same for all TRILL Hellos issued by
an RBridge on a particular Ethernet port, regardless of the VLAN in
which the Hello is sent:
- Source MAC address,
- Priority to be the DRB,
- Desired Designated VLAN,
- Port ID, and,
- if included, BFD-Enabled TLV [RFC6213] and PORT-TRILL-VER
sub-TLV [RFC7176].
Of course, the priority, Desired Designated VLAN, and possibly the
inclusion or value of the PORT-TRILL-VER sub-TLV, and/or BFD-Enabled
TLV can change on occasion, but then the new value(s) must similarly
be used in all TRILL Hellos on the LAN port, regardless of VLAN.
Eastlake, et al. Standards Track [Page 10]
RFC 7177 TRILL: Adjacency May 2014
On broadcast links:
Because bridges acting as glue on an Ethernet broadcast link might
be configured in such a way that some VLANs are partitioned, it is
necessary for RBridges to transmit Hellos on Ethernet links with
multiple VLAN tags. The conceptually simplest solution may have
been to have RBridges transmit up to 4,094 times as many Hellos,
one with each legal VLAN ID enabled at each port, but this would
obviously have deleterious performance implications. So, the
TRILL protocol specifies that if RB1 knows it is not the DRB, it
transmits its Hellos on only a limited set of VLANs. Only an
RBridge that believes itself to be the DRB on a broadcast Ethernet
link "sprays" its TRILL Hellos on all of its enabled VLANs at the
port. And in both cases, an RBridge can be configured to send
Hellos on only a subset of those VLANs. The details are given in
[RFC6325], Section 4.4.3.
On point-to-point links:
If the link technology is VLAN sensitive, such as Ethernet, an
RBridge sends TRILL Hellos only in the Desired Designated VLAN for
which it is configured.
3.2. Adjacency Table Entries and States
Every adjacency is in one of four states, whether it is one of the
adjacencies on a broadcast link or the one possible adjacency on a
point-to-point link. An RBridge participates in LSP synchronization
at a port as long as it has one or more adjacencies out of that port
that are in the 2-Way or Report state.
Down: This is a virtual state for convenience in creating state
diagrams and tables. It indicates that the adjacency is
nonexistent, and there is no entry in the adjacency table for it.
Detect: A neighbor RBridge has been detected through receipt of a
TRILL Hello, but either 2-way connectivity has not been confirmed
or the detection was on an Ethernet link in a VLAN other than the
Designated VLAN.
2-Way: 2-way connectivity to the neighbor has been found and, if the
link is Ethernet, it was found on the Designated VLAN, but some
enabled test, such as the link MTU meeting the minimum campus
requirement or BFD confirming link connectivity, has not yet
succeeded.
Eastlake, et al. Standards Track [Page 11]
RFC 7177 TRILL: Adjacency May 2014
Report: There is 2-way connectivity to the neighbor (on the
Designated VLAN if an Ethernet link); all enabled tests have
succeeded, including, if enabled, MTU and/or BFD testing. This
state will cause adjacency to be reported in an LSP (with
appropriate provision for a pseudonode, if any, as described in
Section 7).
For an adjacency in any of the three non-Down states (Detect, 2-Way,
or Report), there will be an adjacency table entry. That entry will
give the state of the adjacency and will also include the information
listed below.
o The address, if any, of the neighbor, the Port ID, and the System
ID in the received Hellos. Together, these three quantities
uniquely identify the adjacency on a broadcast link.
o One or more Hello holding timers. For a point-to-point adjacency,
there is a single Hello holding timer. For a broadcast LAN
adjacency, there are exactly two Hello holding timers: a
Designated VLAN holding timer and a non-Designated VLAN holding
timer. Each timer consists of a 16-bit unsigned integer number
of seconds.
o If the adjacency is on a broadcast link, the 7-bit unsigned
priority of the neighbor to be the DRB.
o The 5 bytes of data from the PORT-TRILL-VER received in the most
recent TRILL Hello from the neighbor RBridge.
o The VLAN that the neighbor RBridge wants to be the Designated VLAN
on the link, called the Desired Designated VLAN.
o For an adjacency table at an RBridge that supports BFD, a flag
indicating whether the last received TRILL Hello from the neighbor
RBridge contained a BFD-Enabled TLV (see Section 6).
3.3. Adjacency and Hello Events
The following events can change the state of an adjacency:
A0. Receive a TRILL Hello for a broadcast LAN adjacency whose source
MAC address (SNPA) is equal to that of the port on which it is
received. This is a special event that cannot occur on a port
configured as point-to-point and is handled as described
immediately after this list of events. It does not appear in the
state transition table or diagram.
Eastlake, et al. Standards Track [Page 12]
RFC 7177 TRILL: Adjacency May 2014
A1. Receive a TRILL Hello (other than an A0 event) such that:
- If received on an Ethernet port, it was received in the
Designated VLAN.
- If received for a broadcast LAN adjacency, it contains a TRILL
Neighbor TLV that explicitly lists the receiving port's (SNPA)
address.
- If received for a point-to-point adjacency, it contains a
Three-Way Handshake TLV with the receiver's System ID and
Extended Circuit ID.
A2. Event A2 is not possible for a port configured as point-to-point.
Receive a TRILL Hello (other than an A0 event) such that either
- The port is Ethernet and the Hello was not on the Designated
VLAN (any TRILL Neighbor TLV in such a Hello is ignored), or
- The Hello does not contain a TRILL Neighbor TLV covering an
address range that includes the receiver's (SNPA) address.
A3. Receive a TRILL Hello (other than an A0 event) such that:
- If received on an Ethernet port, it was received in the
Designated VLAN.
- If received for a broadcast LAN adjacency, it contains one or
more TRILL Neighbor TLVs covering an address range that
includes the receiver's (SNPA) address and none of which list
the receiver.
- If received for a point-to-point adjacency, it contains a
Three-Way Handshake TLV with either the System ID or Extended
Circuit ID or both not equal to that of the receiver.
A4. Either
(1) the Hello holding timer expires on a point-to-point
adjacency, or
(2) on a broadcast LAN adjacency,
(2a) both Hello timers expire simultaneously or
(2b) one Hello timer expires when the other Hello timer is
already in the expired state.
Eastlake, et al. Standards Track [Page 13]
RFC 7177 TRILL: Adjacency May 2014
A5. For a broadcast LAN adjacency, the Designated VLAN Hello holding
timer expires, but the non-Designated VLAN Hello holding timer
still has time left until it expires. This event cannot occur
for a point-to-point adjacency.
A6. MTU if enabled, BFD if enabled, and all other enabled
connectivity tests successful.
A7. MTU if enabled, BFD if enabled, and all other enabled
connectivity tests were successful but one or more now fail.
A8. The RBridge port goes operationally down.
For the special A0 event, the Hello is examined to determine if it
has a higher priority than the port on which it is received such that
the sending port should be the DRB as described in Section 4.2.1. If
the Hello is of lower priority than the receiving port, it is
discarded with no further action. If it is of higher priority than
the receiving port, then any adjacencies for the receiving port are
discarded (transitioned to the Down state), and the port is suspended
as described in Section 4.2.
The receipt of a TRILL Hello that is not an event A0 causes the
following actions (except where the Hello would have created a new
adjacency table entry but both the adjacency table is full and the
Hello is too low priority to displace an existing entry as described
in Section 3.6). The Designated VLAN referred to is the Designated
VLAN dictated by the DRB determined without taking the received TRILL
LAN Hello into account (see Section 4) for a broadcast LAN and the
local Desired Designated VLAN for a port configured as point-to-
point.
o If the receipt of a Hello creates a new adjacency table entry, the
neighbor RBridge MAC (SNPA) address (if any), Port ID, and System
ID are set from the Hello.
o For a point-to-point adjacency, the Hello holding timer is set
from the Holding Time field of the Hello. For a broadcast link
adjacency, the appropriate Hello holding timer for that adjacency,
depending on whether or not the Hello was received in the
Designated VLAN, is set to the Holding Time field of the Hello and
if the receipt of the LAN Hello is creating a new adjacency table
entry, the other timer is set to expired.
o For a broadcast link adjacency, the priority of the neighbor
RBridge to be the DRB is set to the priority field of the LAN
Hello.
Eastlake, et al. Standards Track [Page 14]
RFC 7177 TRILL: Adjacency May 2014
o For a broadcast link adjacency, the VLAN that the neighbor RBridge
wants to be the Designated VLAN on the link is set from the Hello.
o The 5 bytes of PORT-TRILL-VER data are set from that sub-TLV in
the Hello or set to zero if that sub-TLV does not occur in the
Hello.
o For a broadcast link, if the creation of a new adjacency table
entry or the priority update above changes the results of the DRB
election on the link, the appropriate RBridge port event (D2 or
D3) occurs, after the above actions, as described in Section 4.2.
o For a broadcast link adjacency, if there is no change in the DRB,
but the neighbor Hello is from the DRB and has a changed
Designated VLAN from the previous Hello received from the DRB, the
result is a change in Designated VLAN for the link as specified in
Section 4.2.3.
An event A4 resulting in the adjacency transitioning to the Down
state may also result in an event D3 as described in Section 4.2.
Concerning events A6 and A7, if none of MTU, BFD, or other testing is
enabled, A6 is considered to occur immediately upon the adjacency
entering the 2-Way state, and A7 cannot occur.
See further TRILL Hello receipt details in Section 8.
3.4. Adjacency State Diagram and Table
The table below shows the transitions between the states defined
above, based on the events defined above:
| Event | Down | Detect | 2-Way | Report |
+-------+--------+--------+--------+--------+
| A1 | 2-Way | 2-Way | 2-Way | Report |
| A2 | Detect | Detect | 2-Way | Report |
| A3 | Detect | Detect | Detect | Detect |
| A4 | N/A | Down | Down | Down |
| A5 | N/A | Detect | Detect | Detect |
| A6 | N/A | N/A | Report | Report |
| A7 | N/A | N/A | 2-Way | 2-Way |
| A8 | Down | Down | Down | Down |
Table 2: Adjacency State Table
Eastlake, et al. Standards Track [Page 15]
RFC 7177 TRILL: Adjacency May 2014
"N/A" indicates that the event to the left is not applicable in the
state at the top of the column. These events affect only a single
adjacency. The special A0 event transitions all adjacencies to Down,
as explained immediately after the list of adjacency events in
Section 3.3.
The diagram below presents the same information as that in the state
table:
+---------------+
| Down |<--------+
+---------------+ |
| | ^ | |
A2,A3| |A8| |A1 |
| +--+ | |
| +-----------|---+
V | |
+----------------+ A4,A8 | |
+----->| Detect |------->| |
| +----------------+ | |
| | | ^ | |
| A1| |A2,A3,A5 | | |
| | +---------+ | |
| | | |
| | +------------|---+
| | | |
| V V |
|A3,A5 +----------------+ A4,A8 |
|<-----| 2-Way |------->|
| +----------------+ |
| | ^ | ^ |
| A6| | |A1,A2,A7| |
| | | +--------+ |
| | | |
| | |A7 |
| V | |
|A3,A5 +-------------+ A4,A8 |
|<-----| Report |---------->|
+-------------+
| ^
|A1,A2,A6 |
+---------+
Figure 1: Adjacency State Diagram
Eastlake, et al. Standards Track [Page 16]
RFC 7177 TRILL: Adjacency May 2014
3.5. Multiple Parallel Links
There can be multiple parallel adjacencies between neighbor RBridges
that are visible to TRILL. (Multiple low-level links that have been
bonded together by technologies such as link aggregation [802.1AX]
appear to TRILL as a single link over which only a single TRILL
adjacency can be established.)
Any such links that have pseudonodes (see Section 7) are
distinguished in the topology; such adjacencies, if they are in the
Report state, appear in LSPs as per Section 7. However, there can be
multiple parallel adjacencies without pseudonodes because they are
point-to-point adjacencies or LAN adjacencies for which a pseudonode
is not being created. Such parallel, non-pseudonode adjacencies in
the Report state appear in LSPs as a single adjacency. The cost of
such an adjacency MAY be adjusted downwards to account for the
parallel paths. Multipathing across such parallel connections can be
freely done for unicast TRILL Data traffic on a per-flow basis but is
restricted for multi-destination traffic, as described in
Section 4.5.2 (point 3) of [RFC6325] and Appendix C of [RFC6325].
3.6. Insufficient Space in Adjacency Table
If the receipt of a TRILL Hello would create a new adjacency table
entry (that is, would transition an adjacency out of the Down state),
there may be no space for the new entry. For ports that are
configured as point-to-point and can thus only have zero or one
adjacency not in the Down state, it is RECOMMENDED that space be
reserved for one adjacency so that this condition cannot occur.
When there is adjacency table space exhaustion, the DRB election
priority (see Section 4.2.1) of the new entry that would be created
is compared with the DRB election priority for the existing entries.
If the new entry is higher priority than the lowest priority existing
entry, it replaces the lowest priority existing entry, which is
transitioned to the Down state.
4. LAN Ports and DRB State
This section specifies the DRB election process in TRILL at a
broadcast (LAN) link port. Since there is no such election when a
port is configured as point-to-point, this section does not apply in
that case.
Eastlake, et al. Standards Track [Page 17]
RFC 7177 TRILL: Adjacency May 2014
The information at an RBridge associated with each of its broadcast
LAN ports includes the following:
o Enablement bit, which defaults to enabled.
o The 5 bytes of PORT-TRILL-VER sub-TLV data used in TRILL Hellos
sent on the port.
o SNPA address (commonly a 48-bit MAC address) of the port.
o Port ID, used in TRILL Hellos sent on the port.
o The Holding Time, used in TRILL Hellos sent on the port.
o The priority to be the DRB, used in TRILL LAN Hellos sent on the
port.
o BFD support. If the port supports BFD, a BFD Enabled flag that
controls whether or not a BFD-Enabled TLV is included in TRILL
Hellos sent on the port.
o The DRB state of the port, determined as specified below.
o A 16-bit unsigned Suspension Timer, measured in seconds.
o The Desired Designated VLAN. The VLAN this RBridge wants to be
the Designated VLAN for the link out of this port, used in TRILL
Hellos sent on the port if the link is Ethernet.
o A table of zero or more adjacencies (see Section 3).
4.1. Port Table Entries and DRB Election State
The TRILL equivalent of the DIS (Designated Intermediate System) on a
broadcast link is the DRB or Designated RBridge. The DRB election
state machinery is described below.
Each RBridge port that is not configured as point-to-point is in one
of the following four DRB states:
Down: The port is operationally down. It might be administratively
disabled or down at the link layer. In this state, there will be
no adjacencies for the port, and no TRILL Hellos or other TRILL
IS-IS PDUs or TRILL Data packets are accepted or transmitted.
Suspended: Operation of the port is suspended because there is a
higher priority port on the link with the same MAC (SNPA) address.
This is the same as the Down state, with the exception that TRILL
Eastlake, et al. Standards Track [Page 18]
RFC 7177 TRILL: Adjacency May 2014
Hellos are accepted for the sole purpose of determining whether to
change the value of the Suspension Timer for the port as described
below.
DRB: The port is the DRB and can receive and transmit TRILL Data
packets.
Not DRB: The port is deferring to another port on the link, which it
believes is the DRB, but can still receive and transmit TRILL Data
packets.
4.2. DRB Election Events
The following events can change the DRB state of a port. Note that
this is only applicable to broadcast links. There is no DRB state or
election at a port configured to be point-to-point.
D1. The port becomes enabled or the Suspension Timer expires while
the port is in the Suspended state.
D2. The adjacency table for the port changes, and there are now
entries for one or more other RBridge ports on the link that
appear to be higher priority to be the DRB than the local port.
D3. The port is not Down or Suspended, and the adjacency table for
the port changes, so there are now no entries for other RBridge
ports on the link that appear to be higher priority to be the DRB
than the local port.
D4. A TRILL LAN Hello is received that has the same MAC address
(SNPA) as the receiving port and higher priority to be the DRB as
described for event A0.
D5. The port becomes operationally down.
Event D1 is considered to occur on RBridge boot if the port is
administratively and link-layer enabled.
Event D4 causes the port to enter the Suspended state and all
adjacencies for the port to be discarded (transitioned to the Down
state). If the port was in some state other than Suspended, the
Suspension Timer is set to the Holding Time in the Hello that causes
event D4. If it was in the Suspended state, the Suspension Timer is
set to the maximum of its current value and the Holding Time in the
Hello that causes event D4.
Eastlake, et al. Standards Track [Page 19]
RFC 7177 TRILL: Adjacency May 2014
4.2.1. DRB Election Details
Events D2 and D3 constitute losing and winning the DRB election at
the port, respectively.
The candidates for election are the local RBridge and all RBridges
with which there is an adjacency on the port in an adjacency state
other than the Down state. The winner is the RBridge with highest
priority to be the DRB, as determined from the 7-bit priority field
in that RBridge's Hellos received and the local port's priority to be
the DRB field, with MAC (SNPA) address as a tiebreaker, Port ID as a
secondary tiebreaker, and System ID as a tertiary tiebreaker. These
fields are compared as unsigned integers, with the larger magnitude
being considered higher priority.
Resorting to the secondary and tertiary tiebreakers should only be
necessary in rare circumstances when multiple ports have the same
priority and MAC (SNPA) address and some of them are not yet
suspended. For example, RB1, which has low priority to be the DRB on
the link, could receive Hellos from two other ports on the link that
have the same MAC address as each other and are higher priority to be
the DRB. One of these two ports with the same MAC address will be
suspended and cease sending Hellos, and the Hello from it received by
RB1 will eventually time out. But, in the meantime, RB1 can use the
tiebreakers to determine which port is the DRB and thus which port's
Hello to believe for such purposes as setting the Designated VLAN on
the link.
4.2.2. Change in DRB
Events D2 and D3 result from a change in the apparent DRB on the
link. Unnecessary DRB changes should be avoided, especially on links
offering native frame service, as a DRB change will generally cause a
transient interruption to native frame service.
If a change in the DRB on the link changes the Designated VLAN on an
Ethernet link, the actions specified in Section 4.2.3 are taken.
If an RBridge changes in either direction between being the DRB and
not being the DRB at a port, this will generally change the VLANs on
which that RBridge sends Hellos through that port, as specified in
Section 4.4.3 of [RFC6325].
Eastlake, et al. Standards Track [Page 20]
RFC 7177 TRILL: Adjacency May 2014
4.2.3. Change in Designated VLAN
Unnecessary changes in the Designated VLAN on an Ethernet link should
be avoided because a change in the Designated VLAN can cause a
transient interruption to adjacency and thus to TRILL Data forwarding
on the link. When practical, all RBridge ports on a link should be
configured with the same Desired Designated VLAN so that if the
winner of the DRB election changes for any reason, the Designated
VLAN will remain the same.
If an RBridge detects a change in Designated VLAN on an Ethernet
link, then, for all adjacency table entries for a port to that link,
the RBridge takes the following steps, in the order given.
o The non-Designated VLAN Hello holding timer is set to the maximum
of its time to expiration and the current time to expiration of
the Designated VLAN Hello holding timer.
o The Designated VLAN Hello holding timer is then set to expired (if
necessary), and an event A5 occurs for the adjacency (see
Section 3.3).
If the Designated VLAN for a link changes, this will generally change
the VLANs on which Hellos are sent by an RBridge port on that link as
specified in Section 4.4.3 of [RFC6325].
4.3. Port State Table and Diagram
The table below shows the transitions between the DRB states defined
above, based on the events defined above:
| Event | Down | Suspended | DRB | Not DRB |
+-------+--------+-----------+-----------+-----------+
| D1 | DRB | DRB | N/A | N/A |
| D2 | N/A | N/A | Not DRB | Not DRB |
| D3 | N/A | N/A | DRB | DRB |
| D4 | N/A | Suspended | Suspended | Suspended |
| D5 | Down | Down | Down | Down |
Table 3: Port State Table
"N/A" indicates that the event to the left is not applicable in the
state at the top of the column.
Eastlake, et al. Standards Track [Page 21]
RFC 7177 TRILL: Adjacency May 2014
The diagram below presents the same information as in the state
table:
+-------------+
| Down |<--------------+
+-+---+-------+ ^ |
| | ^ | |
D1| |D5 | | |
| +---+ |D5 |
| | |
| +--------+----+ |
| | Suspended |<---|---+
| +-+-----+-----+ | |
| D1| ^ | ^ | |
| | | |D4 | | |
| | | +---+ | |
| | | | |
| | |D4 | |
V V | | |
+---------------+-+ D5 | |
| DRB |---------->| |
+--------+--+-----+ | |
^ | | ^ | |
| D2| |D3| | |
| | +--+ | |
| | D4 | |
|D3 | +-----------------|---+
| V | |
+----+-------+-+ D5 |
| Not DRB |-------------->|
+----+---------+
| ^
|D2 |
+----+
Figure 2: Port State Diagram
5. MTU Matching
The purpose of MTU testing is to ensure that the links used in the
campus topology can pass TRILL IS-IS packets, particularly LSP PDUs,
at the TRILL campus MTU. The LSP PDUs generated at a TRILL switch
could, as part of the flooding process, be sent over any adjacency in
the campus. To assure correct operation of IS-IS, an LSP PDU must be
able to reach every RBridge in the IS-IS reachable campus; this might
be impossible if the PDU exceeded the MTU of an adjacency that was
part of the campus topology.
Eastlake, et al. Standards Track [Page 22]
RFC 7177 TRILL: Adjacency May 2014
An RBridge, RB1, determines the desired campus link MTU by
calculating the minimum of its originatingL1LSPBufferSize and the
originatingL1LSPBufferSize of other RBridges in the campus, as
advertised in the link-state database, but not less than 1,470 bytes.
Although originatingL1LSPBufferSize in Layer 3 [IS-IS] is limited to
the range 512 to 1,492 bytes inclusive, in TRILL it is limited to the
range 1,470 to 65,535 bytes inclusive. (See Section 5 of [RFC7180].)
Although MTU testing is optional, it is mandatory for an RBridge to
respond to an MTU-probe PDU with an MTU-ack PDU [RFC6325] [RFC7176].
The use of multicast or unicast for MTU-probe and MTU-ack is an
implementation choice. However, the burden on the link is generally
minimized by the following: (1) multicasting MTU-probes when a
response from all other RBridges on the link is desired, such as when
initializing or reconfirming MTU, (2) unicasting MTU-probes when a
response from a single RBridge is desired, such as one that has just
been detected on the link, and (3) unicasting all MTU-ack packets.
RB1 can test the MTU size to RB2 as described in Section 4.3.2 of
[RFC6325]. For this purpose, MTU testing is only done in the
Designated VLAN. An adjacency that fails the MTU test at the campus
MTU will not enter the Report state, or, if the adjacency is in that
state, it leaves that state. Thus, an adjacency failing the MTU test
at the campus minimum MTU will not be reported by the RBridge
performing the test. Since inclusion in least-cost route computation
requires the adjacency to be reported by both ends, as long as the
RBridge at either end of the adjacency notices the MTU failure, it
will not be so used.
If RB1 tests MTU size, it reports the largest size for which the MTU
test succeeds or a flag indicating that it fails at the campus MTU.
This report always appears with the neighbor in RB1's TRILL Neighbor
TLV. RB1 MAY also report this with the adjacency in an Extended
Reachability TLV in RB1's LSP. RB1 MAY choose to test MTU sizes
greater than the desired campus MTU as well as the desired
campus MTU.
Most types of TRILL IS-IS packets, such as LSPs, can make use of the
campus MTU. The exceptions are TRILL Hellos, which must be kept
small for loop safety, and the MTU PDUs, whose size must be adjusted
appropriately for the tests being performed.
Eastlake, et al. Standards Track [Page 23]
RFC 7177 TRILL: Adjacency May 2014
6. BFD-Enabled TLV and BFD Session Bootstrapping
When the adjacency between RBridges reaches the 2-Way state, TRILL
Hellos will already have been exchanged. If an RBridge supports BFD
[RFC7175], it will have learned whether the other RBridge has BFD
enabled by whether or not a BFD-Enabled TLV [RFC6213] was included in
its Hellos. In addition, TRILL Hellos include a nickname of the
sending RBridge [RFC7176] so that information will be available to
the receiving RBridge.
The BFD-Enabled TLVs in TRILL Hellos will look like the following:
+-+-+-+-+-+-+-+-+
| Type=148 | (1 byte)
+-+-+-+-+-+-+-+-+
| Length=3*n | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | MT ID=0 | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLPID=0xC0 | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-...
| possible additional (3*(n-1) bytes)
| topology/NLPID pairs
+-+-+-+-+-+-+-...
Figure 3: BFD-Enabled TLV Example/Format
Type = 148 for BFD Enabled [RFC6213].
Length will be 3 times the number of topology and protocol
pairs in the TLV.
MT ID is a topology ID [RFC5120] that will be zero unless
multi-topology is being supported [MT].
NLPID is a Network Layer Protocol ID [RFC6328] and will be 0xC0
for TRILL, but additional topology and protocol pairs could
conceivably be listed.
An RBridge port initiates a one-hop BFD session with another RBridge
if the following conditions are met: (1) it has BFD enabled, (2) it
has an adjacency to another RBridge in the 2-Way or Report state, and
(3) the Hellos it receives indicate that the other RBridge also has
BFD enabled. Either (a) BFD was enabled on both RBridge ports when
the adjacency changed to the 2-Way or Report state, (b) the adjacency
was already in the 2-Way or Report state and BFD was enabled on one
RBridge port when BFD had been enabled on the other, or (c) BFD was
simultaneously enabled on both RBridge ports.
Eastlake, et al. Standards Track [Page 24]
RFC 7177 TRILL: Adjacency May 2014
In such a BFD session, BFD is encapsulated as specified in [RFC7175].
The egress nickname to be used will have been learned from received
Hellos. On a point-to-point link, the Any-RBridge nickname [RFC7180]
can also be used as egress, since support of that nickname is
required by support of RBridge Channel [RFC7178] and support of
RBridge Channel is required for support of BFD over TRILL.
The rare case of transient nickname conflict (due to the network
operator configuring a conflict, new connectivity to a previously
isolated RBridge, or the like) can cause transient failure of an
ongoing BFD session. This can be avoided in the one-hop point-to-
point case by using the Any-RBridge egress nickname. In cases where
Any-RBridge cannot be used as the egress nickname and a transient
nickname conflict is detected for the intended destination of a BFD
session, initiation of the session SHOULD be delayed until the
conflict is resolved.
If a one-hop BFD session is initiated when the adjacency is in the
2-Way state, the adjacency MUST NOT advance to the Report state until
BFD and any other enabled connectivity tests (including MTU, if
enabled) have succeeded, as specified in Section 3.
If a one-hop BFD session is established when the adjacency is in the
Report state, due to enablement at the RBridges, then, to minimize
unnecessary topology changes, the adjacency MUST remain in the Report
state unless and until the BFD session (or some other enabled
connectivity test) fails.
7. Pseudonodes
This section only applies to broadcast links, as there is no DRB and
there cannot be a pseudonode [IS-IS] for a link configured as point-
to-point. The Designated RBridge (DRB), determined as described
above, controls whether a pseudonode will be used on a link.
If the DRB sets the bypass pseudonode bit in its TRILL LAN Hellos,
the RBridges on the link (including the DRB) just directly report all
their adjacencies on the LAN that are in the Report state. If the
DRB does not set the bypass pseudonode bit in its TRILL Hellos, then
(1) the DRB reports in its LSP its adjacency to the pseudonode, (2)
the DRB sends LSPs on behalf of the pseudonode in which it reports
adjacency to all other RBridges on the link where it sees that
adjacency in the Report state, and (3) all other RBridges on the link
report their adjacency to the pseudonode if they see their adjacency
to the DRB as being in the Report state and do not report any other
adjacencies on the link. Setting the bypass pseudonode bit has no
effect on how LSPs are flooded on a link. It only affects what LSPs
are generated.
Eastlake, et al. Standards Track [Page 25]
RFC 7177 TRILL: Adjacency May 2014
It is anticipated that many links between RBridges will actually be
point-to-point even in cases where the link technology supports
operation as a multi-access broadcast link, in which case using a
pseudonode merely adds to the complexity. For example, if RB1 and
RB2 are the only RBridges on the link, and RB1 is the DRB, then if
RB1 creates a pseudonode -- for example, RB1.25 -- that is used,
there are then 3 LSPs: RB1.25, RB1, and RB2, where RB1.25 reports
connectivity to RB1 and RB2, and RB1 and RB2 each just say they are
connected to RB1.25. However, if DRB RB1 sets the bypass pseudonode
bit in its Hellos, then there will be only 2 LSPs: RB1 and RB2, each
reporting connectivity to each other.
A DRB SHOULD set the bypass pseudonode bit in its Hellos if it has
not seen at least two simultaneous adjacencies in the Report state
since it last rebooted or was reset by network management.
8. More TRILL Hello Details
This section provides further details on the receipt, transmission,
and content of TRILL Hellos. Unless otherwise stated, it applies to
both LAN and point-to-point Hellos.
TRILL Hellos, like all TRILL IS-IS packets, are primarily
distinguished from Layer 3 IS-IS packets on Ethernet by being sent to
the All-IS-IS-RBridges multicast address (01-80-C2-00-00-41). TRILL
IS-IS packets on Ethernet also have the L2-IS-IS Ethertype (0x22F4)
and are Ethertype encoded.
Although future extensions to TRILL may include the use of Level 2
IS-IS, [RFC6325] specifies TRILL using a single Level 1 Area using
the fixed Area Address zero (see Section 4.2 of [RFC7176]).
IS-IS Layer 3 routers are frequently connected to other Layer 3
routers that are part of a different routing domain. In that case,
the externalDomain flag (see [IS-IS]) is normally set for the port
through which such a connection is made. The setting of this flag to
"true" causes no IS-IS PDUs to be sent out of the port and any IS-IS
PDUs received to be discarded, including Hellos. RBridges operate in
a different environment where all neighbor RBridges merge into a
single campus. For loop safety, RBridges do not implement the
externalDomain flag or implement it with the fixed value "false".
They send and can receive TRILL Hellos on every port that is not
disabled.
Eastlake, et al. Standards Track [Page 26]
RFC 7177 TRILL: Adjacency May 2014
8.1. Contents of TRILL Hellos
The table below lists mandatory (M) and optional (O) content TLVs for
TRILL Hellos that are particularly relevant to this document,
distinguishing between TRILL LAN Hellos and TRILL P2P Hellos. A "-"
indicates that an occurrence would be ignored. There are additional
TLVs and sub-TLVs that can occur in TRILL Hellos [RFC7176].
LAN P2P Number Content Item
--- --- ------ ----------------------------------------------
M M 1 Area Addresses TLV with Area Address zero only
M M 1 MT Port Capabilities TLV containing a
VLAN-FLAGs sub-TLV
O O 0-n Other MT Port Capability TLVs
M - 0-n TRILL Neighbor TLV (see Section 8.2.1)
- M 1 Three-Way Handshake TLV
O O 0-n Protocols Supported TLV -- MUST list the
TRILL NLPID (0xC0) [RFC6328]
O O 0-1 BFD-Enabled TLV
O O 0-1 Authentication TLV
- - 0-n Padding TLV -- SHOULD NOT be included
Table 4: TRILL Hello Contents
A TRILL Hello MAY also contain any TLV permitted in a Layer 3 IS-IS
Hello. As with all IS-IS PDUs, TLVs that are unsupported/unknown in
TRILL Hellos are ignored.
8.2. Transmitting TRILL Hellos
TRILL Hellos are sent with the same timing as Layer 3 IS-IS Hellos
[IS-IS]; however, no Hellos are sent if a port is in the Suspended or
Down state or if the port is disabled.
TRILL Hello PDUs SHOULD NOT be padded and MUST NOT be sent if they
exceed 1,470 bytes; however, a received TRILL Hello longer than
1,470 bytes is processed normally.
TRILL Hello PDU headers MUST conform to the following:
o Maximum Area Addresses equal to 1.
o Circuit Type equal to 1.
See Section 8.1 for mandatory Hello TLV contents and some optional
Hello TLV contents.
Eastlake, et al. Standards Track [Page 27]
RFC 7177 TRILL: Adjacency May 2014
8.2.1. TRILL Neighbor TLVs
A TRILL Neighbor TLV SHOULD NOT be included in TRILL point-to-point
Hellos, as it MUST be ignored in that context and wastes space.
TRILL Neighbor TLVs sent in a LAN Hello on an Ethernet link MUST show
the neighbor information, as sensed by the transmitting RBridge, for
the VLAN on which the Hello is sent. Since implementations
conformant to this document maintain such information on a per-VLAN
basis only for the Designated VLAN, such implementations only send
the TRILL Neighbor TLV in TRILL LAN Hellos in the Designated VLAN.
It is RECOMMENDED that, if there is sufficient room, a TRILL Neighbor
TLV or TLVs, as described in Section 4.4.2.1 of [RFC6325], covering
the entire range of MAC addresses and listing all adjacencies with a
non-zero Designated VLAN Hello Holding Time, or an empty list of
neighbors if there are no such adjacencies, be in TRILL Hellos sent
on the Designated VLAN. If this is not possible, then TRILL Neighbor
TLVs covering sub-ranges of MAC addresses should be sent so that the
entire range is covered reasonably promptly. Delays in sending TRILL
Neighbor TLVs will delay the advancement of adjacencies to the Report
state and the discovery of some link failures. Rapid (for example,
sub-second) detection of link or node failures is best addressed with
a protocol designed for that purpose, such as BFD (see Section 6).
To ensure that any RBridge RB2 can definitively determine whether RB1
can hear RB2, RB1's neighbor list MUST eventually cover every
possible range of IDs, that is, within a period that depends on RB1's
policy and not necessarily within any specific period such as its
Holding Time. In other words, if X1 is the smallest ID reported in
one of RB1's neighbor lists, and the "smallest" flag is not set, then
X1 MUST appear in a different neighbor list as well, as the largest
ID reported in that fragment. Or lists may overlap, as long as there
is no gap, such that some range, say, between Xi and Xj, would never
appear in any list.
Eastlake, et al. Standards Track [Page 28]
RFC 7177 TRILL: Adjacency May 2014
8.3. Receiving TRILL Hellos
Assuming that a packet is labeled as TRILL IS-IS -- for example, on
Ethernet it has the L2-IS-IS Ethertype and the All-IS-IS-RBridges
destination multicast address or is so marked by the appropriate code
point on other link types such as PPP [RFC6361] or a pseudowire
[RFC7173] -- it will be examined to see if it appears to be an IS-IS
PDU. If so, and it appears to be a TRILL Hello PDU, the following
tests are performed:
o The type of Hello PDU (LAN or P2P) is compared with the port
configuration. If a LAN Hello is received on a port configured to
be point-to-point or a P2P Hello is received on a port not
configured to be point-to-point, it is discarded.
o If the Circuit Type field is not 1, the PDU is discarded.
o If the PDU does not contain an Area Address TLV or it contains an
Area Address TLV that is not the single Area Address zero, it is
discarded.
o If the Hello includes a Protocols Supported TLV that does not list
the TRILL NLPID (0xC0), it is discarded. It is acceptable if
there is no Protocols Supported TLV present.
o If the Hello does not contain an MT Port Capabilities TLV
containing a VLAN-FLAGS sub-TLV [RFC7176], it is discarded.
o If the maximumAreaAddresses field of the PDU is not 1, it is
discarded.
o If IS-IS authentication is in use on the link and either the PDU
has no Authentication TLV or validation of the PDU's
Authentication TLV fails, it is discarded.
If none of the rules in the list above cause the packet to be
discarded and the packet is parseable, it is assumed to be a
well-formed TRILL Hello received on the link. It is treated as an
event A0, A1, A2, or A3, based on the criteria listed in Section 3.3.
9. Multiple Ports on the Same Broadcast Link
It is possible for an RBridge RB1 to have multiple ports on the same
broadcast (LAN) link that are not in the Suspended state. It is
important for RB1 to recognize which of its ports are on the same
link. RB1 can detect this condition based on receiving TRILL Hello
messages with the same LAN ID on multiple ports.
Eastlake, et al. Standards Track [Page 29]
RFC 7177 TRILL: Adjacency May 2014
The DRB election is port-based (see Section 4), and only the Hellos
from the elected port can perform certain functions such as dictating
the Designated VLAN or whether a pseudonode will be used; however,
the election also designates the RBridge with that port as the DRB
for the link. An RBridge may choose to load split some tasks among
its ports on the link if it has more than one. Section 4.4.4 of
[RFC6325] describes when it is safe to do so.
10. IANA Considerations
This document serves as a reference for 'Fail' (Failed MTU test),
value 0, in the "TRILL Neighbor TLV NEIGHBOR RECORD Flags" registry.
IANA has updated that reference to point to this RFC.
11. Security Considerations
This memo provides improved documentation of some aspects of the
TRILL base protocol standard, particularly five aspects of the TRILL
adjacency establishment and Hello protocol as listed in Section 1.
It does not change the security considerations of the TRILL base
protocol as detailed in Section 6 of [RFC6325].
See [RFC7175] for security considerations for BFD, whose use in
connection with TRILL adjacency is discussed in this document,
particularly Section 6.
Eastlake, et al. Standards Track [Page 30]
RFC 7177 TRILL: Adjacency May 2014
Appendix A. Changes from RFC 6327
This document has the following changes from [RFC6327]. It obsoletes
[RFC6327].
1. This document extends the TRILL Hello size limit, MTU testing, and
state machine to point-to-point links.
2. This document incorporates the updates to [RFC6327] from
[RFC7180].
3. The bulk of [RFC6327] was written from the point of view that
links between TRILL switches would only be Ethernet. In fact,
they could be any technology, such as PPP [RFC6361], pseudowire
[RFC7173], or IP [TrillIP]. This replacement document generalizes
[RFC6327] to cover such link types.
4. This document includes a specification of one-hop BFD session
establishment in connection with adjacency.
5. Numerous editorial changes were incorporated.
Appendix B. Changes to RFC 6325
Section 2 of this document replaces Section 4.4.1 of [RFC6325].
Section 8 of this document replaces Section 4.4.2 of [RFC6325],
except for Section 4.4.2.1. The changes in [RFC6325] made by this
document include
- Prohibiting the sending of TRILL Hellos out of a port while it is
in the Suspended state, and the specification of the Suspended
state. ([RFC6325] specifies that Hellos be sent with the same
timing as [IS-IS].)
- Permitting the inclusion of the Three-Way-Handshake TLV,
BFD-Enabled TLV, and other TLVs in TRILL Hellos when these were
omitted in TRILL Hello contents lists in Section 4.4.2 of
[RFC6325].
- Extending the TRILL Hello protocol to support point-to-point and
non-Ethernet links.
Eastlake, et al. Standards Track [Page 31]
RFC 7177 TRILL: Adjacency May 2014
Normative References
[IS-IS] ISO/IEC 10589:2002, Second Edition, "Information
technology -- Telecommunications and information exchange
between systems -- Intermediate System to Intermediate
System intra-domain routeing information exchange protocol
for use in conjunction with the protocol for providing the
connectionless-mode network service (ISO 8473)", 2002.
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, December 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008.
[RFC6213] Hopps, C. and L. Ginsberg, "IS-IS BFD-Enabled TLV",
RFC 6213, April 2011.
[RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, July 2011.
[RFC6328] Eastlake 3rd, D., "IANA Considerations for Network Layer
Protocol Identifiers", BCP 164, RFC 6328, July 2011.
[RFC7175] Manral, V., Eastlake 3rd, D., Ward, D., and A. Banerjee,
"Transparent Interconnection of Lots of Links (TRILL):
Bidirectional Forwarding Detection (BFD) Support",
RFC 7175, May 2014.
[RFC7176] Eastlake 3rd, D., Senevirathne, T., Ghanwani, A., Dutt,
D., and A. Banerjee, "Transparent Interconnection of Lots
of Links (TRILL) Use of IS-IS", RFC 7176, May 2014.
[RFC7178] Eastlake 3rd, D., Manral, V., Li, Y., Aldrin, S., and D.
Ward, "Transparent Interconnection of Lots of Links
(TRILL): RBridge Channel Support", RFC 7178, May 2014.
[RFC7180] Eastlake 3rd, D., Zhang, M., Ghanwani, A., Manral, V., and
A. Banerjee, "Transparent Interconnection of Lots of Links
(TRILL): Clarifications, Corrections, and Updates",
RFC 7180, May 2014.
Eastlake, et al. Standards Track [Page 32]
RFC 7177 TRILL: Adjacency May 2014
Informative References
[802.1AX] "IEEE Standard for Local and metropolitan area networks--
Link Aggregation", 802.1AX-2008, January 2008.
[802.1Q] "IEEE Standard for Local and metropolitan area networks--
Media Access Control (MAC) Bridges and Virtual Bridged
Local Area Networks", 802.1Q-2011, August 2011.
[ESADI] Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
Stokes, "TRILL: ESADI (End Station Address Distribution
Information) Protocol", Work in Progress, April 2014.
[FCoE] Excerpt of discussion of "FCoE Max Size" generated from
T11/09-251v1, 04/27/2009, "FCoE frame or FCoE PDU",
<www.t11.org>.
[MT] Eastlake, D., Zhang, M., Banerjee, A., and V. Manral,
"TRILL: Multi-Topology", Work in Progress, September 2013.
[RFC3719] Parker, J., Ed., "Recommendations for Interoperable
Networks using Intermediate System to Intermediate System
(IS-IS)", RFC 3719, February 2004.
[RFC6327] Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D., and
V. Manral, "Routing Bridges (RBridges): Adjacency",
RFC 6327, July 2011.
[RFC6361] Carlson, J. and D. Eastlake 3rd, "PPP Transparent
Interconnection of Lots of Links (TRILL) Protocol Control
Protocol", RFC 6361, August 2011.
[RFC6439] Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
Hu, "Routing Bridges (RBridges): Appointed Forwarders",
RFC 6439, November 2011.
[RFC7172] Eastlake 3rd, D., Zhang, M., Agarwal, P., Perlman, R., and
D. Dutt, "Transparent Interconnection of Lots of Links
(TRILL): Fine-Grained Labeling", RFC 7172, May 2014.
[RFC7173] Yong, L., Eastlake 3rd, D., Aldrin, S., and J. Hudson,
"Transparent Interconnection of Lots of Links (TRILL)
Transport Using Pseudowires", RFC 7173, May 2014.
[TrillIP] Wasserman, M., Eastlake, D., and D. Zhang, "Transparent
Interconnection of Lots of Links (TRILL) over IP", Work in
Progress, March 2014.
Eastlake, et al. Standards Track [Page 33]
RFC 7177 TRILL: Adjacency May 2014
Acknowledgements
The suggestions and comments of the following are gratefully
acknowledged:
Stewart Bryant, Elwyn Davies, Adrian Farrel, Brian Haberman, Jon
Hudson, Arnel Lim, and Gayle Noble.
The authors of [RFC6327] included Dinesh Dutt. The acknowledgements
section of [RFC6327] includes the following, listed in alphabetic
order: Jari Arkko, Ayan Banerjee, Les Ginsberg, Sujay Gupta, David
Harrington, Pete McCann, Erik Nordmark, and Mike Shand.
Authors' Addresses
Donald E. Eastlake 3rd
Huawei Technologies
155 Beaver Street
Milford, MA 01757
USA
Phone: +1-508-333-2270
EMail: d3e3e3@gmail.com
Radia Perlman
Intel Labs
2200 Mission College Blvd.
Santa Clara, CA 95054
USA
Phone: +1-408-765-8080
EMail: Radia@alum.mit.edu
Anoop Ghanwani
Dell
5450 Great America Parkway
Santa Clara, CA 95054
USA
EMail: anoop@alumni.duke.edu
Eastlake, et al. Standards Track [Page 34]
RFC 7177 TRILL: Adjacency May 2014
Howard Yang
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: howardy@cisco.com
Vishwas Manral
Ionos Corp.
4100 Moorpark Ave.
San Jose, CA 95117
USA
EMail: vishwas@ionosnetworks.com
Eastlake, et al. Standards Track [Page 35]