Internet Engineering Task Force (IETF) Y. Li
Request for Comments: 7968 D. Eastlake 3rd
Category: Standards Track W. Hao
ISSN: 2070-1721 H. Chen
Huawei Technologies
S. Chatterjee
Cisco
August 2016
Transparent Interconnection of Lots of Links (TRILL):
Using Data Labels for Tree Selection for Multi-Destination Data
Abstract
TRILL (Transparent Interconnection of Lots of Links) uses
distribution trees to deliver multi-destination frames. Multiple
trees can be used by an ingress Routing Bridge (RBridge) for flows,
regardless of the VLAN, Fine-Grained Label (FGL), and/or multicast
group of the flow. Different ingress RBridges may choose different
distribution trees for TRILL Data packets in the same VLAN, FGL,
and/or multicast group. To avoid unnecessary link utilization,
distribution trees should be pruned based on one or more of the
following: VLAN, FGL, or multicast destination address. If any VLAN,
FGL, or multicast group can be sent on any tree, for typical fast-
path hardware, the amount of pruning information is multiplied by the
number of trees, but there is limited hardware capacity for such
pruning information.
This document specifies an optional facility to restrict the TRILL
Data packets sent on particular distribution trees by VLAN, FGL,
and/or multicast groups, thus reducing the total amount of pruning
information so that it can more easily be accommodated by fast-path
hardware.
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
http://www.rfc-editor.org/info/rfc7968.
Li, et al. Standards Track [Page 1]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
Copyright Notice
Copyright (c) 2016 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
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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.
Table of Contents
1. Introduction ....................................................3
1.1. Background Description .....................................3
1.2. Terminology Used in This Document ..........................4
2. Motivations .....................................................5
3. Tree Selection Based on Data Labels .............................9
3.1. Overview of the Mechanism ..................................9
3.2. APPsub-TLVs Supporting Tree Selection .....................10
3.2.1. The Tree and VLANs APPsub-TLV ......................11
3.2.2. The Tree and VLANs Used APPsub-TLV .................12
3.2.3. The Tree and FGLs APPsub-TLV .......................12
3.2.4. The Tree and FGLs Used APPsub-TLV ..................13
3.2.5. The Tree and Groups APPsub-TLV .....................13
3.2.6. The Tree and Groups Used APPsub-TLV ................14
3.3. Detailed Processing .......................................14
3.4. Failure Handling ..........................................15
4. Backward Compatibility .........................................17
5. Security Considerations ........................................18
6. IANA Considerations ............................................19
7. References .....................................................19
7.1. Normative References ......................................19
7.2. Informative References ....................................20
Acknowledgments ...................................................21
Authors' Addresses ................................................21
Li, et al. Standards Track [Page 2]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
1. Introduction
1.1. Background Description
One or more distribution trees, identified by their root nicknames,
are used to distribute multi-destination data in a (Transparent
Interconnection of Lots of Links) (TRILL) campus [RFC6325]. The
Routing Bridge (RBridge) having the highest tree root priority
announces the total number of trees that should be computed for the
campus. It may also specify the list of trees that RBridges need to
compute using the Tree Identifiers (TREE-RT-IDs) sub-TLV [RFC7176].
Every RBridge can specify the trees it will use for multi-destination
TRILL Data packets it originates in the Trees Used Identifiers
(TREE-USE-IDs) sub-TLV [RFC7176], and the VLANs or Fine-Grained
Labels (FGLs) [RFC7172] it is interested in are specified in
Interested VLANs and/or Interested Labels sub-TLVs [RFC7176]. It is
suggested that by default the ingress RBridge uses the distribution
tree whose root is the closest [RFC6325]. The TREE-USE-IDs sub-TLV
is used to build the RPF (Reverse Path Forwarding) check table that
is used for RPF checking. Interested VLANs and Interested Labels
sub-TLVs are used for distribution tree pruning, and the
multi-destination forwarding table with pruning information is built
based on that RPF check table. To reduce unnecessary link loads,
each distribution tree should be pruned per VLAN/FGL, eliminating
branches that have no potential receivers downstream as specified in
[RFC6325]. Further pruning based on Layer 2 or Layer 3 multicast
addresses is also possible.
Li, et al. Standards Track [Page 3]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
Defaults are provided, but how many trees are calculated, where the
tree roots are located, and which tree or trees are to be used by an
ingress RBridge are implementation dependent. With the increasing
demand to use TRILL in data center networks, there are some features
we can explore for multi-destination frames in the data center use
case. In order to achieve non-blocking data forwarding, a fat tree
structure is often used. Figure 1 shows a typical data center
network based on the fat tree structure. RB1 and RB2 are aggregation
switches, and RB11 through RB14 are access switches. It is a common
practice to configure the tree roots to be at the aggregation
switches for efficient traffic transportation. All the ingress
RBridges that are access switches will then be equally distant from
all the tree roots.
+-----+ +-----+
| RB1 | | RB2 |
+-----+ +-----+
/ | \\ / /|\
/ | \ \ / / | \
/ | \ \ / | \-----+
/ | \/ \ | |
/ | /\/ \| |
/ /---+---/ /\ |\ |
/ / | / \ | \ |
/ / | / \ | \ |
/ / | / \ | \ |
+-----+ +-----+ +-----+ +-----+
| RB11| | RB12| | RB13| | RB14|
+-----+ +-----+ +-----+ +-----+
Figure 1: TRILL Network Based on Fat Tree Structure
1.2. Terminology Used in This Document
This document uses the terminology from [RFC6325] and [RFC7172], some
of which is repeated below for convenience, along with some
additional terms listed below:
Campus: The name for a network using the TRILL protocol in the same
sense that a "bridged LAN" is the name for a network using
bridging. In TRILL, the word "campus" has no academic
implication.
Data Label: VLAN or FGL.
ECMP: Equal-Cost Multipath [RFC6325].
FGL: Fine-Grained Label [RFC7172].
Li, et al. Standards Track [Page 4]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
Interested Labels sub-TLV: Short for "Interested Labels and Spanning
Tree Roots sub-TLV" [RFC7176].
Interested VLANs sub-TLV: Short for "Interested VLANs and Spanning
Tree Roots sub-TLV" [RFC7176].
IPTV: "Television" (video) over IP.
RBridge: An alternative name for a TRILL switch.
RPF: Reverse Path Forwarding.
TRILL: Transparent Interconnection of Lots of Links (or Tunneled
Routing in the Link Layer).
TRILL switch: A device implementing the TRILL protocol. Sometimes
called 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 RFC 2119 [RFC2119].
2. Motivations
In the structure of Figure 1, if we choose to put the tree roots at
RB1 and RB2, the ingress RBridge (e.g., RB11) would find more than
one equal-cost closest tree root (i.e., RB1 and RB2). An ingress
RBridge has two options to select the tree root for multi-destination
frames: choose one and only one as the distribution tree root, or use
an ECMP-like algorithm to balance the traffic among the multiple
trees whose roots are at the same distance from the RBridge.
- For the former (one distribution tree root), a single tree used by
each ingress RBridge can have the problem of uneven or inefficient
link usage. For example, if RB11 chooses the tree that is rooted
at RB1 as the distribution tree, the link between RB11 and RB2
will not be used for multi-destination frames ingressed by RB11.
- For the latter (an ECMP-like algorithm), ECMP-based tree selection
results in a linear increase in multicast forwarding table size
with the number of trees, as explained in the next paragraph.
A multicast forwarding table at an RBridge is normally used to map
the key of (distribution tree nickname + VLAN) to an index to a list
of ports for multicast packet replication. The key used for mapping
is simply the tree nickname when the RBridge does not prune the tree.
Li, et al. Standards Track [Page 5]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
The key could be the distribution tree nickname augmented by the FGL
and/or Layer 2 or 3 multicast address when the RBridge supports FGL
and/or Layer 2 or 3 pruning information.
For any RBridge RBn, for each VLAN x, if RBn is in a distribution
tree t used by traffic in VLAN x, there will be an entry of (t, x,
port list) in the multicast forwarding table on RBn. Typically, each
entry contains a distinct combination of (tree nickname, VLAN) as the
lookup key. If there are n such trees and m such VLANs, the
multicast forwarding table size on RBn is n*m entries. If an FGL is
used [RFC7172] and/or finer pruning is used (for example, VLAN +
multicast group address is used for pruning), the value of m
increases. In the larger-scale data center, more trees would be
necessary for purposes of better load-balancing; this results in an
increased value for n. In either case, the number of table entries
(i.e., n*m) will increase dramatically.
Li, et al. Standards Track [Page 6]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
The left-hand table in Figure 2 shows an example of the multicast
forwarding table on RB11 in the Figure 1 topology, with two
distribution trees in a campus using typical fast-path hardware.
Before VLAN-Based After VLAN-Based
Tree Selection Tree Selection
+--------------+-----+---------+ +--------------+-----+---------+
|tree nickname |VLAN |port list| |tree nickname |VLAN |port list|
+--------------+-----+---------+ +--------------+-----+---------+
| tree 1 | 1 | | | tree 1 | 1 | |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 1 | 2 | | | tree 1 | 2 | |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 1 | ... | | | tree 1 | ... | |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 1 | ... | | | tree 1 | 1999| |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 1 | ... | | | tree 1 | 2000| |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 1 | 4093| | | tree 2 | 2001| |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 1 | 4094| | | tree 2 | 2002| |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 2 | 1 | | | tree 2 | ... | |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 2 | 2 | | | tree 2 | 4093| |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 2 | ... | | | tree 2 | 4094| |
+--------------+-----+---------+ +--------------+-----+---------+
| tree 2 | ... | |
+--------------+-----+---------+
| tree 2 | ... | |
+--------------+-----+---------+
| tree 2 | ... | |
+--------------+-----+---------+
| tree 2 | 4093| |
+--------------+-----+---------+
| tree 2 | 4094| |
+--------------+-----+---------+
Figure 2: Multicast Forwarding Table
before and after Using VLAN-Based Tree Selection
The number of entries is approximately 2*4K in this case. If four
distribution trees are used in a TRILL campus and RBn has 4K VLANs
with downstream receivers, it consumes 16K table entries. The size
of fast-path TRILL multicast forwarding tables is typically limited
by hardware; therefore, the table entries are a precious resource.
Li, et al. Standards Track [Page 7]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
In some implementations, the table is shared with Layer 3 IP
multicast for a total of 16K or 8K table entries. Therefore, we want
to reduce the table size consumed for TRILL distribution trees as
much as possible and at the same time maintain load-balancing among
the trees.
In cases where blocks of consecutive VLANs or FGLs can be assigned to
a tree, the multicast forwarding table could be greatly compressed if
entries could have a Data Label value and mask, with the fast-path
hardware doing the longest prefix matching. But few, if any,
fast-path implementations provide such logic.
A straightforward way to alleviate the problem of limited table
entries is not to prune the distribution tree. However, this can
only be used in restricted scenarios, for the following reasons:
- Not pruning wastes bandwidth for multi-destination packets. There
is normally broadcast traffic, like ARP and unknown unicast, that
can be pruned on a VLAN (or FGL) so that it is not sent down
branches of a distribution tree where it is not needed. In
addition, if there is a lot of Layer 3 multicast traffic, no
pruning may result in a worst-case scenario where that user data
is unnecessarily flooded all over the campus. The volume of
flooded data could be very large if certain applications such as
IPTV are supported. More precise pruning, such as pruning based
on multicast groups, may be desirable in this case.
- Not pruning is only useful at pure transit nodes. Edge nodes
always need to maintain the multicast forwarding table with the
key of (tree nickname + VLAN (or FGL)), since the edge node needs
to decide whether and how to replicate the frame to local access
ports. It is likely that edge nodes are relatively low-end
switches with a smaller shared table size, say 4K, available.
- Due to security concerns, VLAN-based (or FGL-based) traffic
isolation is a basic requirement in some scenarios. No pruning
may increase the risk of leakage of the traffic. Misbehaving
RBridges may take advantage of this leakage of traffic.
In addition to the concern regarding multicast table size, some
silicon does not currently support hashing-based tree nickname
selection at the ingress RBridge but commonly uses VLAN-based tree
selection. If the control plane of the ingress RBridge maps the
incoming VLAN x to a tree nickname t, the data plane will always use
tree t for VLAN x multi-destination frames. Such an ingress RBridge
may choose multiple trees to be used for load-sharing; it can use one
and only one tree for each VLAN. If we make sure that all ingress
Li, et al. Standards Track [Page 8]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
RBridges campus-wide send VLAN x multi-destination packets only use
tree t, then there would be no need to store the multicast table
entry with the key of (tree-other-than-t, x) on any RBridge.
This document describes the TRILL control-plane support for
distribution tree selection based on a VLAN, FGL, and/or multicast
address to reduce the multicast forwarding table size. It is
compatible with the silicon implementations mentioned in the previous
paragraph.
3. Tree Selection Based on Data Labels
Data Label (VLAN-based or FGL-based) tree selection can be used as a
distribution tree selection mechanism, especially when the multicast
forwarding table size is a concern. This section specifies that
mechanism and how to extend it so that tree selection can be based on
multicast groups.
3.1. Overview of the Mechanism
The RBridge that has the highest priority to be a tree root announces
the tree nicknames and the Data Labels allowed on each tree. Such
announcements of correspondence of tree to Data Label can be based on
static configuration or some predefined algorithm beyond the scope of
this document. An ingress RBridge selects the tree-VLAN
correspondence that it wishes to use from the list announced by the
highest-priority tree root. It SHOULD NOT transmit VLAN x frames on
tree y if the highest-priority tree root does not say that VLAN x is
allowed on tree y.
If we make sure that a particular VLAN is allowed on one and only one
tree, we can keep the number of multicast forwarding table entries on
any RBridge fixed at 4K maximum (or up to 16M in the case of an FGL).
Take Figure 1 as an example, where two trees are rooted at RB1 and
RB2, respectively. The highest-priority tree root appoints tree 1 to
carry VLAN 1-2000 and tree 2 to carry VLAN 2001-4094. With such an
announcement by the highest-priority tree root, every RBridge that
understands the announcement will not send VLAN 2001-4094 traffic on
tree 1 and will not send VLAN 1-2000 traffic on tree 2. That way, no
RBridge would need to store the entries for tree 1 / VLAN 2001-4094
or tree 2 / VLAN 1-2000. Figure 2 shows the multicast forwarding
table on an RBridge before and after we use VLAN-based tree
selection. The number of entries is reduced by a factor f, where f
is the number of trees used in the campus. In this example, it is
reduced from 2*4094 to 4094. This affects both transit nodes and
edge nodes. The data-plane encoding does not change.
Li, et al. Standards Track [Page 9]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
3.2. APPsub-TLVs Supporting Tree Selection
Six new APPsub-TLVs that can be carried in the TRILL GENINFO TLV
[RFC7357] in Extended Level 1 Flooding Scope (E-L1FS) FS-Link State
Protocol Data Units (FS-LSPs) [RFC7780] are defined below. The first
four can be considered analogous to finer-granularity versions of the
TREE-RT-IDs sub-TLV and the TREE-USE-IDs sub-TLV [RFC7176]. Two
APPsub-TLVs supporting VLAN-based tree selection are specified in
Sections 3.2.1 and 3.2.2. They are used by the highest-priority tree
root to announce the allowed VLANs on each tree in the campus and by
an ingress RBridge to announce the tree-VLAN correspondence that it
selects from the list announced by the highest-priority tree root.
Two APPsub-TLVs supporting FGL-based tree selection are specified in
Sections 3.2.3 and 3.2.4 for the same purpose. Sections 3.2.5 and
3.2.6 define two APPsub-TLVs to support finer granularity in
selecting trees based on multicast groups rather than Data Labels.
New APPsub-TLVs Description
======================= =============
Tree and VLANs announcement by the highest-priority
tree root of the VLANs allowed per tree
Tree and VLANs Used tree-VLAN correspondence that an
ingress RBridge selects
Tree and FGLs announcement by the highest-priority
tree root of the FGLs allowed per tree
Tree and FGLs Used tree-FGL correspondence that an
ingress RBridge selects
Tree and Groups announcement by the highest-priority
tree root of the multicast groups
allowed on each tree
Tree and Groups Used tree and multicast group correspondence
that an ingress RBridge selects
Li, et al. Standards Track [Page 10]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
3.2.1. The Tree and VLANs APPsub-TLV
The RBridge that is the highest-priority tree root announces the
VLANs allowed on each tree with the Tree and VLANs (TREE-VLANs)
APPsub-TLV. Multiple instances of this APPsub-TLV may be carried.
The same tree nicknames may occur in multiple Tree-VLAN RECORDs
within the same APPsub-TLV or across multiple APPsub-TLVs. The
APPsub-TLV format is as follows:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 11 | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| Tree-VLAN RECORD (1) | (6 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| ................. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| Tree-VLAN RECORD (N) | (6 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
where each Tree-VLAN RECORD is of the form:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nickname | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | Start.VLAN | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RESV | End.VLAN | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Type: TRILL GENINFO APPsub-TLV type; set to 11 (TREE-VLANs).
o Length: 6*n bytes, where there are n Tree-VLAN RECORDs. Thus, the
value of Length can be used to determine n. If Length is not a
multiple of 6, the APPsub-TLV is corrupt and MUST be ignored.
o Nickname: The nickname identifying the distribution tree by its
root.
o RESV: 4 bits that MUST be sent as zero and ignored on receipt.
o Start.VLAN, End.VLAN: These fields are the VLAN IDs of the allowed
VLAN range on the tree, inclusive. To specify a single VLAN, the
VLAN's ID appears as both the start and end VLAN. If End.VLAN is
less than Start.VLAN, the Tree-VLAN RECORD MUST be ignored.
Li, et al. Standards Track [Page 11]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
3.2.2. The Tree and VLANs Used APPsub-TLV
This APPsub-TLV has the same structure as the TREE-VLANs APPsub-TLV
specified in Section 3.2.1. The differences are that its APPsub-TLV
type is set to 12 (TREE-VLAN-USE) and the tree-VLAN correspondences
in the Tree-VLAN RECORDs listed are those correspondences that the
originating RBridge wants to use for multi-destination packets. This
APPsub-TLV is used by an ingress RBridge to distribute the tree-VLAN
correspondence that it selects from the list announced by the
highest-priority tree root.
3.2.3. The Tree and FGLs APPsub-TLV
The RBridge that is the highest-priority tree root can use the Tree
and FGLs (TREE-FGLs) APPsub-TLV to announce the FGLs allowed on each
tree. Multiple instances of this APPsub-TLV may be carried. The
same tree nicknames may occur in the multiple Tree-FGL RECORDs within
the same APPsub-TLV or across multiple APPsub-TLVs. Its format is as
follows:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 13 | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| Tree-FGL RECORD (1) | (8 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| ................. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| Tree-FGL RECORD (N) | (8 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
where each Tree-FGL RECORD is of the form:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Nickname | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+
| Start.FGL | (3 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+
| End.FGL | (3 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+
Li, et al. Standards Track [Page 12]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
o Type: TRILL GENINFO APPsub-TLV type; set to 13 (TREE-FGLs).
o Length: 8*n bytes, where there are n Tree-FGL RECORDs. Thus, the
value of Length can be used to determine n. If Length is not a
multiple of 8, the APPsub-TLV is corrupt and MUST be ignored.
o Nickname: The nickname identifying the distribution tree by
its root.
o RESV: 4 bits that MUST be sent as zero and ignored on receipt.
o Start.FGL, End.FGL: These fields are the FGL IDs of the allowed
FGL range on the tree, inclusive. To specify a single FGL, the
FGL's ID appears as both the start and end FGL. If End.FGL is
less than Start.FGL, the Tree-FGL RECORD MUST be ignored.
3.2.4. The Tree and FGLs Used APPsub-TLV
This APPsub-TLV has the same structure as the TREE-FGLs APPsub-TLV
specified in Section 3.2.3. The differences are that its APPsub-TLV
type is set to 14 (TREE-FGL-USE) and the Tree-FGL correspondences in
the Tree-FGL RECORDs listed are those that the originating RBridge
wants to use for multi-destination packets. This APPsub-TLV is used
by an ingress RBridge to distribute the tree-FGL correspondence that
it selects from the list announced by the highest-priority tree root.
3.2.5. The Tree and Groups APPsub-TLV
Tree selection based on Data Labels is easily extended to tree
selection based on Data Label + Layer 2 or 3 multicast groups. We
can appoint multicast group 1 in VLAN 10 to tree 1 and appoint
group 2 in VLAN 10 to tree 2 for better load-sharing.
The RBridge that is the highest-priority tree root can announce the
multicast groups allowed on each tree for each Data Label with the
Tree and Groups (TREE-GROUPs) APPsub-TLV. Multiple instances of this
APPsub-TLV may be carried. The APPsub-TLV format is as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 15 | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Tree Nickname | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Sub-Sub-TLVs (variable)
+-+-+-+-+-+-+-+-+-+....
Li, et al. Standards Track [Page 13]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
o Type: TRILL GENINFO APPsub-TLV type; set to 15 (TREE-GROUPs).
o Length: 2 + the length of the Group Sub-Sub TLVs that are
included.
o Nickname: The nickname identifying the distribution tree by its
root.
o Group Sub-Sub-TLVs: Zero or more of the TLV structures that are
allowed as sub-TLVs of the Group Address (GADDR) TLV [RFC7176].
Each such TLV structure specifies a multicast group and either a
VLAN or FGL. Although these TLV structures are considered
sub-TLVs when they appear inside a GADDR TLV, they are technically
sub-sub-TLVs when they appear inside a TREE-GROUPs APPsub-TLV that
is in turn inside a TRILL GENINFO TLV [RFC7357].
3.2.6. The Tree and Groups Used APPsub-TLV
The Tree and Groups Used (TREE-GROUPs-USE) APPsub-TLV has the same
structure as the TREE-GROUPs APPsub-TLV specified in Section 3.2.5.
The differences are that its APPsub-TLV type is set to 16
(TREE-GROUPs-USE) and the Tree Nickname and Group sub-sub-TLVs listed
in this APPsub-TLV are those that the originating RBridge wants to
use for multi-destination packets. This APPsub-TLV is used by an
ingress RBridge to distribute the tree-group correspondence that it
selects from the list announced by the highest-priority tree root.
3.3. Detailed Processing
The highest-priority tree root RBridge MUST include all the necessary
tree-related sub-TLVs defined in [RFC7176] as usual in its E-L1FS
FS-LSP and MAY include the TREE-VLANs APPsub-TLV and/or the TREE-FGLs
APPsub-TLV in its E-L1FS FS-LSP [RFC7780]. In this way, it MAY
indicate that each VLAN and/or FGL is only allowed on one or some
other number of trees less than the number of trees being calculated
in the campus in order to save table space in the fast-path
forwarding hardware.
An ingress RBridge that understands the TREE-VLANs APPsub-TLV SHOULD
select the tree-VLAN correspondences that it wishes to use and put
them in TREE-VLAN-USE APPsub-TLVs. If there are multiple tree
nicknames announced in a TREE-VLANs APPsub-TLV for VLAN x, the
ingress RBridge chooses one of them if it supports this feature. For
example, the ingress RBridge may choose the closest (minimum-cost)
root among them. How to make such a choice is out of scope for this
document. It may be desirable to have some fixed algorithm to
make sure that all ingress RBridges choose the same tree for VLAN x
in this case. Any single Data Label that the ingress RBridge is
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RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
interested in should be related to only one tree ID in a
TREE-VLAN-USE APPsub-TLV to minimize the multicast forwarding table
size on other RBridges, but as long as the Data Label is related to
less than all the trees being calculated, it will reduce the burden
on the forwarding table size.
When an ingress RBridge encapsulates a multi-destination frame for
Data Label x, it SHOULD use a tree nickname that it selected
previously in a TREE-VLAN-USE or TREE-FGL-USE APPsub-TLV for
Data Label x. However, that may not be possible because either
(1) the RBridge may not have advertised such TREE-VLAN-USE or
TREE-FGL-USE APPsub-TLVs, in which case it can use any tree that has
been advertised as permitted for the Data Label by the
highest-priority tree root RBridge, or (2) the tree or trees it
advertised might be unavailable due to failures.
If RBridge RBn does not perform pruning, it builds the multicast
forwarding table as specified in [RFC6325].
If RBn prunes the distribution tree based on VLANs, RBn uses the
information received in TREE-VLAN-USE APPsub-TLVs to mark the set of
VLANs reachable downstream for each adjacency and for each related
tree. If RBn prunes the distribution tree based on FGLs, RBn uses
the information received in TRILL-FGL-USE APPsub-TLVs to mark the
set of FGLs reachable downstream for each adjacency and for each
related tree.
Logically, an ingress RBridge that does not support VLAN-based or
FGL-based tree selection is equivalent to the one that supports it
but uses it in such a way as to gain no advantage; for example, it
announces the use of all trees for all VLANs and FGLs.
3.4. Failure Handling
This section discusses failure scenarios for a distribution tree root
for the case where that tree root is not the highest-priority root
and the case where it is the highest-priority root. This section
also discusses some other transient error conditions.
Failure of a tree root that is not the highest-priority tree root:
It is the responsibility of the highest-priority tree root to
inform other RBridges of any change in the allowed tree-VLAN
correspondence. When the highest-priority tree root learns that
the root of tree t has failed, it should reassign the VLANs
allowed on tree t to other trees or to a tree replacing the
failed one.
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RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
Failure of the highest-priority tree root: It is suggested that the
tree root of second-highest priority be pre-configured with the
proper knowledge of the tree-VLAN correspondence allowed when the
highest-priority tree root fails. The information announced by
the RBridge that has the second-highest priority to be a tree root
would be in the link state of all RBridges but would not take
effect unless the RBridge noticed the failure of the
highest-priority tree root. When the highest-priority tree root
fails, the tree root that formerly had second-highest priority
will become the highest-priority tree root of the campus. When an
RBridge notices the failure of the original highest-priority tree
root, it can immediately use the stored information announced by
the tree root that originally had second-highest priority. It is
suggested that the tree-VLAN correspondence information be
pre-configured on the tree root of second-highest priority to be
the same as that on the highest-priority tree root for the trees
other than the highest-priority tree itself. This can minimize
the change to multicast forwarding tables in the case of
highest-priority tree root failure. For a large campus, it may
make sense to pre-configure this information in a similar way on
the third-priority, fourth-priority, or even lower-priority tree
root RBridges.
In some transient conditions, or in the case of a misbehaving
highest-priority tree root, an ingress RBridge may encounter the
following scenarios:
- No tree has been announced for which VLAN x frames are allowed.
- An ingress RBridge is supposed to transmit VLAN x frames on
tree t, but the root of tree t is no longer reachable.
For the second case, an ingress RBridge may choose another reachable
tree root that allows VLAN x frames according to the highest-priority
tree root announcement. If there is no such tree available, then it
is the same as the first case above. The ingress RBridge should then
be "downgraded" to a conventional RBridge with behavior as specified
in [RFC6325]. A timer should be set to allow the temporary transient
stage to complete before the change of the responsive tree or the
downgrade takes effect. The value of the timer should be set to at
least the LSP flooding time of the campus.
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RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
4. Backward Compatibility
RBridges MUST include the TREE-USE-IDs and INT-VLAN sub-TLVs in their
LSPs when required by [RFC6325] whether or not they support the new
TREE-VLAN-USE or TREE-FGL-USE APPsub-TLVs specified by this document.
RBridges that understand the new TREE-VLAN-USE APPsub-TLV sent from
another RBridge RBn should use it to build the multicast forwarding
table and ignore the TREE-USE-IDs and INT-VLAN sub-TLVs sent from the
same RBridge. TREE-USE-IDs and INT-VLAN sub-TLVs are still useful
for some purposes other than building the multicast forwarding table
(e.g., building an RPF table, spanning tree root notification). If
the RBridge does not receive TREE-VLAN-USE APPsub-TLVs from RBn, it
uses the conventional way described in [RFC6325] to build the
multicast forwarding table.
For example, there are two distribution trees, tree 1 and tree 2, in
the campus. RB1 and RB2 are RBridges that use the new APPsub-TLVs
described in this document. RB3 is an old RBridge that is compatible
with [RFC6325]. Assume that RB2 is interested in VLANs 10 and 11 and
RB3 is interested in VLANs 100 and 101. Hence, RB1 receives
((tree 1, VLAN 10), (tree 2, VLAN 11)) as a TREE-VLAN-USE APPsub-TLV
and (tree 1, tree 2) as a TREE-USE-IDs sub-TLV from RB2 on port x.
Also, RB1 receives (tree 1) as a TREE-USE-IDs sub-TLV and no
TREE-VLAN-USE APPsub-TLV from RB3 on port y. RB2 and RB3 announce
their interested VLANs in an INT-VLAN sub-TLV as usual. RB1 will
then build the entry of (tree 1, VLAN 10, port x) and
(tree 2, VLAN 11, port x) based on RB2's LSP and the mechanism
specified in this document. RB1 also builds entries of
(tree 1, VLAN 100, port y), (tree 1, VLAN 101, port y),
(tree 2, VLAN 100, port y), and (tree 2, VLAN 101, port y) based on
RB3's LSP in the conventional way.
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RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
The multicast forwarding table on RB1 with a merged entry would be
like the following:
+--------------+-----+---------+
|tree nickname |VLAN |port list|
+--------------+-----+---------+
| tree 1 | 10 | x |
+--------------+-----+---------+
| tree 1 | 100 | y |
+--------------+-----+---------+
| tree 1 | 101 | y |
+--------------+-----+---------+
| tree 2 | 11 | x |
+--------------+-----+---------+
| tree 2 | 100 | y |
+--------------+-----+---------+
| tree 2 | 101 | y |
+--------------+-----+---------+
As expected, that table is not as small as the one where every
RBridge supports the new TREE-VLAN-USE APPsub-TLVs. In a hybrid
campus, the worst case would be where the number of entries is equal
to the number of entries required by the current practice that does
not support VLAN-based tree selection. Such an extreme case happens
when the set of interested VLANs from the new RBridges is a subset of
the set of interested VLANs from the old RBridges.
Tree selection based on the Data Label and multicast group is
compatible with the current practice. Its effectiveness increases
with more RBridges supporting this feature in the TRILL campus.
5. Security Considerations
This document does not change the general RBridge security
considerations of the TRILL base protocol. The APPsub-TLVs specified
can be secured using the IS-IS authentication feature [RFC5310]. See
Section 6 of [RFC6325] for general TRILL security considerations.
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RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
6. IANA Considerations
IANA has assigned six new TRILL APPsub-TLV types from the range less
than 255, as specified in Section 3, and updated the "TRILL
APPsub-TLV Types under IS-IS TLV 251 Application Identifier 1"
registry on
<http://www.iana.org/assignments/trill-parameters/>, as shown below.
Type Name of APPsub-TLV Reference
---- ----------------------- -------------------------
11 Tree and VLANs Section 3.2.1 of RFC 7968
12 Tree and VLANs Used Section 3.2.2 of RFC 7968
13 Tree and FGLs Section 3.2.3 of RFC 7968
14 Tree and FGLs Used Section 3.2.4 of RFC 7968
15 Tree and Groups Section 3.2.5 of RFC 7968
16 Tree and Groups Used Section 3.2.6 of RFC 7968
7. References
7.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
<http://www.rfc-editor.org/info/rfc6325>.
[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,
DOI 10.17487/RFC7172, May 2014,
<http://www.rfc-editor.org/info/rfc7172>.
[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,
DOI 10.17487/RFC7176, May 2014,
<http://www.rfc-editor.org/info/rfc7176>.
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RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
[RFC7357] Zhai, H., Hu, F., Perlman, R., Eastlake 3rd, D., and O.
Stokes, "Transparent Interconnection of Lots of Links
(TRILL): End Station Address Distribution Information
(ESADI) Protocol", RFC 7357, DOI 10.17487/RFC7357,
September 2014, <http://www.rfc-editor.org/info/rfc7357>.
[RFC7780] Eastlake 3rd, D., Zhang, M., Perlman, R., Banerjee, A.,
Ghanwani, A., and S. Gupta, "Transparent Interconnection
of Lots of Links (TRILL): Clarifications, Corrections, and
Updates", RFC 7780, DOI 10.17487/RFC7780, February 2016,
<http://www.rfc-editor.org/info/rfc7780>.
7.2. Informative References
[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, DOI 10.17487/RFC5310,
February 2009, <http://www.rfc-editor.org/info/rfc5310>.
Li, et al. Standards Track [Page 20]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
Acknowledgments
The authors wish to thank David M. Bond, Liangliang Ma, Naveen Nimmu,
Radia Perlman, Rakesh Kumar, Robert Sparks, Daniele Ceccarelli, and
Sunny Rajagopalan for their valuable comments and contributions.
Authors' Addresses
Yizhou Li
Huawei Technologies
101 Software Avenue
Nanjing 210012
China
Phone: +86-25-56624629
Email: liyizhou@huawei.com
Donald Eastlake 3rd
Huawei Technologies
155 Beaver Street
Milford, MA 01757
United States of America
Phone: +1-508-333-2270
Email: d3e3e3@gmail.com
Weiguo Hao
Huawei Technologies
101 Software Avenue
Nanjing 210012
China
Phone: +86-25-56623144
Email: haoweiguo@huawei.com
Li, et al. Standards Track [Page 21]
RFC 7968 TRILL: Tree Selection Based on Data Labels August 2016
Hao Chen
Huawei Technologies
101 Software Avenue
Nanjing 210012
China
Email: philips.chenhao@huawei.com
Somnath Chatterjee
Cisco Systems
SEZ Unit, Cessna Business Park
Outer Ring Road
Bangalore 560087
India
Email: somnath.chatterjee01@gmail.com
Li, et al. Standards Track [Page 22]