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Transparent Interconnection of Lots of Links (TRILL): Clarifications, Corrections, and Updates

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7180.
Authors Donald E. Eastlake 3rd , Mingui Zhang , Anoop Ghanwani , Vishwas Manral , Ayan Banerjee
Last updated 2018-12-20 (Latest revision 2012-07-30)
Replaces draft-eastlake-trill-rbridge-clear-correct
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd (None)
IESG IESG state Became RFC 7180 (Proposed Standard)
Action Holders
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Ralph Droms
IESG note
Send notices to (None)
TRILL Working Group                                      Donald Eastlake
INTERNET-DRAFT                                              Mingui Zhang
Intended status: Proposed Standard                                Huawei
Updates: 6325, 6327, 6439                                 Anoop Ghanwani
                                                          Vishwas Manral
                                                           Ayan Banerjee
                                                        Cumulus Networks
Expires: January 29, 2013                                  July 30, 2012

            TRILL: Clarifications, Corrections, and Updates


   The IETF TRILL (TRansparent Interconnection of Lots of Links)
   protocol provides least cost pair-wise data forwarding without
   configuration in multi-hop networks with arbitrary topology and link
   technology, safe forwarding even during periods of temporary loops,
   and support for multipathing of both unicast and multicast traffic.
   TRILL accomplishes this by using IS-IS (Intermediate System to
   Intermediate System) link state routing and by encapsulating traffic
   using a header that includes a hop count. Since the TRILL base
   protocol was approved in March 2010, active development of TRILL has
   revealed errata in the original RFC 6325 and some cases that could
   use clarifications or updates.

   RFC 6327 and RFC 6439 provide clarifications and updates with respect
   to Adjacency and Appointed Forwarders. This document provides other
   known clarifications, corrections, and updates to RFC 6325, RFC 6327,
   and RFC 6439.

Status of This Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.  Distribution of this document is
   unlimited.  Comments should be sent to the TRILL working group
   mailing list <>.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

D. Eastlake, et al                                              [Page 1]
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   The list of current Internet-Drafts can be accessed at The list of Internet-Draft
   Shadow Directories can be accessed at

D. Eastlake, et al                                              [Page 2]
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Table of Contents

      1. Introduction............................................4
      1.1 Precedence.............................................4
      1.2 Changes That Are Not Backwards Compatible..............4
      1.3 Terminology and Acronyms...............................5

      2. Overloaded and/or Unreachable RBridges..................6
      2.1 Reachability...........................................6
      2.2 Distribution Trees.....................................7
      2.3 Overloaded Receipt of TRILL Data Frames................7
      2.3.1 Known Unicast Receipt................................7
      2.3.2 Multi-Destination Receipt............................8
      2.4 Overloaded Origination of TRILL Data Frames............8
      2.4.1 Known Unicast Origination............................8
      2.4.2 Multi-Destination Origination........................8 An Example Network.................................8 Indicating OOMF Support............................9 Using OOMF Service................................10

      3. Distribution Trees.....................................11
      3.1 Number of Distribution Trees..........................11
      3.2 Clarification of Distribution Tree Updates............11
      3.3 IP Address Based Multicast Pruning....................11
      3.4 Numbering of Distribution Trees.......................12

      4. Nickname Selection.....................................13

      5. MTU (Maximum Transmission Unit)........................15
      5.1 MTU Related Errata in RFC 6325........................15
      5.1.1 MTU PDU Addressing..................................15
      5.1.2 MTU PDU Processing..................................16
      5.1.3 MTU Testing.........................................16
      5.2 Ethernet MTU Values...................................16

      6. Port Modes.............................................18
      7. The CFI / DEI Bit......................................19
      8. Graceful Restart.......................................20
      9. Updates to RFC 6327....................................21

      10. Updates on Appointed Forwarders and Inhibition........22
      10.1 Optional TRILL Hello Reduction.......................22
      10.2 Overload and Appointed Forwarders....................24

      11. IANA Considerations...................................25
      12. Security Considerations...............................26
      Change History............................................26
      Normative References......................................28
      Informative References....................................29

D. Eastlake, et al                                              [Page 3]
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1. Introduction

   The IETF TRILL (Transparent Interconnection of Lots of Links)
   protocol [RFC6325] provides optimal pair-wise data frame forwarding
   without configuration in multi-hop networks with arbitrary topology
   and link technology, safe forwarding even during periods of temporary
   loops, and support for multipathing of both unicast and multicast
   traffic. TRILL accomplishes this by using IS-IS (Intermediate System
   to Intermediate System) [IS-IS] [RFC1195] [RFC6326bis] link state
   routing and encapsulating traffic using a header that includes a hop
   count. The design supports VLANs (Virtual Local Area Networks) and
   optimization of the distribution of multi-destination frames based on
   VLANs and IP derived multicast groups.

   In the more than two years since the TRILL base protocol [RFC6325]
   was approved, the active development of TRILL has revealed five
   errors in the original specification document [RFC6325] and cases
   that could use clarifications or updates.

   [RFC6327] and [RFC6439] provide clarifications with respect to
   Adjacency and Appointed Forwarders. This document provides other
   known clarifications, corrections, and updates to [RFC6325],
   [RFC6327], and [RFC6439].

1.1 Precedence

   In case of conflict between this document and any of [RFC6325],
   [RFC6327], or [RFC6439], this document takes precedence. In addition,
   Section 1.2 (Normative Content and Precedence) of [RFC6325] is
   updated to provide a more complete precedence ordering of the
   sections of [RFC6325] as following, where sections to the left take
   precedence over sections to their right:

                         4 > 3 > 7 > 5 > 2 > 6 > 1

1.2 Changes That Are Not Backwards Compatible

   The change made by Section 3.4 below is not backward compatible with
   [RFC6325] but has nevertheless been adopted to reduce distribution
   tree changes resulting from topology changes.

   The several other changes herein that are fixes to previously posted
   [RFC6325] Errata [Err3002] [Err3003] [Err3004] [Err3052] [Err3053]
   may not be backward compatible with previous implementations that
   conformed to errors in the specification.

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1.3 Terminology and Acronyms

   This document uses the acronyms defined in [RFC6325] and the
   following additional acronyms:

      CFI - Canonical Format Indicator [802]

      DEI - Drop Eligibility Indicator [802.1Q-2011]

      EISS - Enhanced Internal Sublayer Service

      OOMF - Overload Originated Multi-destination Frame

      TRILL Switch - An alternative name for an RBridge

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

D. Eastlake, et al                                              [Page 5]
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2. Overloaded and/or Unreachable RBridges

   RBridges may be in overload as indicated by the [IS-IS] overload flag
   in their LSPs (Link State PDUs (Protocol Data Units)). This means
   that either (1) they are incapable of holding the entire link state
   database and thus do not have a view of the entire topology or (2)
   they have been configured to have the overload bit set. Although
   networks should be engineered to avoid actual link state overload, it
   might occur under various circumstances.  For example, if a large
   campus included one or more low-end TRILL Switches.

   It is a common operational practice to set the overload bit in an
   [IS-IS] router (such as an RBridge) when performing maintenance on
   that router that might affect its ability to correctly forward
   frames; this will usually leave the router reachable for maintenance
   traffic but transit traffic will not be routed through it. (Also, in
   some cases, TRILL provides for setting the overload bit in the
   pseudonode of a link to stop TRILL Data traffic on an access link
   (see Section 4.9.1 of [RFC6325]).)

   [IS-IS] and TRILL make a reasonable effort to do what they can even
   if some RBridges/routers are in overload. They can do reasonably well
   if a few scattered nodes are in overload. However, actual least cost
   paths are no longer assured if any RBridges are in overload.

   For the effect of overload on the appointment of forwarders, see
   Section 10.2.

   In this Section 2, the term "neighbor" refers only to actual RBridges
   and ignores pseudonodes.

2.1 Reachability

   Frames are not least cost routed through an overloaded TRILL Switch,
   although they may originate or terminate at an overloaded TRILL
   Switch. In addition, frames will not be least cost routed over links
   with cost 2**24 - 1 [RFC5305]; such links are reserved for traffic
   engineered frames, the handling of which is beyond the scope of this

   As a result, a portion of the campus may be unreachable for least
   cost routed TRILL Data because all paths to it would be through a
   link with cost 2**24 - 1 or through an overloaded RBridge. For
   example, an RBridge RB1 is not reachable by TRILL Data if all of its
   neighbors are connected to RB1 by links with cost 2**24 - 1. Such
   RBridges are called "data unreachable".

   The link state database at an RBridge RB1 can also contain

D. Eastlake, et al                                              [Page 6]
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   information on TRILL Switches that are unreachable by IS-IS link
   state flooding due to link or RBridge failures. When such failures
   partition the campus, the TRILL Switches adjacent to the failure and
   on the same side of the failure as RB1 will update their LSPs to show
   the lack of connectivity and RB1 will receive those updates.  As a
   result, RB1 will be aware of the partition.  Nodes on the far side of
   the partition are both "IS-IS unreachable" and data unreachable.
   However, LSPs held by RB1 for TRILL Switches on the far side of the
   failure will not be updated and may stay around until they time out,
   which could be tens of minutes or longer. (The default in [IS-IS] is
   twenty minutes.)

2.2 Distribution Trees

   An RBridge in overload cannot be trusted to correctly calculate
   distribution trees or correctly perform the RPFC (Reverse Path
   Forwarding Check). Therefore, it cannot be trusted to forward multi-
   destination TRILL Data frames. It can only appear as a leaf node in a
   TRILL multi-destination distribution tree. Furthermore, if all the
   immediate neighbors of an RBridge are overloaded, then it is omitted
   from all trees in the campus and is unreachable by multi-destination

   When an RBridge determines what nicknames to use as the roots of the
   distribution trees it calculates, it MUST ignore all nicknames held
   by TRILL Switches that are in overload or are data unreachable.  When
   calculating RPFCs for multi-destination frames, an RBridge RB1 MAY,
   to avoid calculating unnecessary RPF check state, ignore any trees
   that cannot reach to RB1 even if other RBridges list those trees as
   trees those other TRILL Switches might use. (But see Section 3.)

2.3 Overloaded Receipt of TRILL Data Frames

   The receipt of TRILL Data frames by overloaded RBridge RB2 is
   discussed in the subsections below.  In all cases, the normal Hop
   Count decrement is performed and the TRILL Data frame is discarded if
   the result is less than one or if the egress nickname is illegal.

2.3.1 Known Unicast Receipt

   RB2 will not usually receive unicast TRILL Data frames unless it is
   the egress, in which case it decapsulates and delivers the frames
   normally. If RB2 receives a unicast TRILL Data frame for which it is
   not the egress, perhaps because a neighbor does not yet know it is in

D. Eastlake, et al                                              [Page 7]
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   overload, RB2 MUST NOT discard the frame because the egress is an
   unknown nickname as it might not know about all nicknames due to its
   overloaded condition. If any neighbor, other than the neighbor from
   which it received the frame, is not overloaded it MUST attempt to
   forward the frame to one of those neighbors. If there is no such
   neighbor, the frame is discarded.

2.3.2 Multi-Destination Receipt

   If RB2 in overload receives a multi-destination TRILL Data frame, RB2
   MUST NOT apply an RPFC since, due to overload, it might not do so
   correctly. RB2 decapsulates and delivers the frame locally where it
   is Appointed Forwarder for the frame's VLAN, subject to any multicast
   pruning.  But since, as stated above, RB2 can only be the leaf of a
   distribution tree, it MUST NOT forward a multi-destination TRILL Data
   frame (except as an egressed native frame where RB2 is Appointed

2.4 Overloaded Origination of TRILL Data Frames

   Overloaded origination of unicast frames with known egress and of
   multi-destination frames are discussed in the subsections below.

2.4.1 Known Unicast Origination

   When an overloaded RBridge RB2 ingresses or creates a known
   destination unicast TRILL Data frame, it delivers it locally if the
   destination MAC is local.  Otherwise RB2 unicasts it to any neighbor
   TRILL Switch that is not overloaded. It MAY use what routing
   information it has to help select the neighbor.

2.4.2 Multi-Destination Origination

   Overloaded RBridge RB2 ingressing or creating a multi-destination
   TRILL Data frame is more complex than for a known unicast frame. An Example Network

   For example, consider the network below in which, for simplicity, end
   stations and any bridges are not shown. There is one distribution

D. Eastlake, et al                                              [Page 8]
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   tree of which RB4 is the root and which is represented by double
   lines. Only RBridge RB2 is overloaded.

            +-----+    +-----+     +-----+     +-----+
            | RB7 +====+ RB5 +=====+ RB3 +=====+ RB1 |
            +-----+    +--+--+     +-++--+     +--+--|
                          |          ||           |
                      +---+---+      ||           |
               +------+RB2(ov)|======++           |
               |      +-------+      ||           |
               |                     ||           |
            +--+--+     +-----+  ++==++=++     +--+--+
            | RB8 +=====+ RB6 +==++ RB4 ++=====+ RB9 |
            +-----+     +-----+  ++=====++     +-----+

   Since RB2 is overloaded it does not know what the distribution tree
   or trees are for the network. Thus there is no way it can provide
   normal TRILL Data encapsulation for multi-destination native frames.
   So RB2 tunnels the frame to a neighbor that is not overloaded if it
   has such a neighbor that has signaled it is willing to offer this
   service. RBridges indicate this in their Hellos as described below.
   This service is called OOMF (Overloaded Origination of Multi-
   destination Frame) service.

   -  The multi-destination frame MUST NOT be locally distributed in
      native form at RB2 before tunneling to a neighbor because this
      would cause the frame to be delivered twice. For example, if RB2
      locally distributed a multicast native frame and then tunneled it
      to RB5, RB2 would get a copy of the frame when RB3 transmitted it
      as a TRILL Data frame on the multi-access RB2-RB3-RB4 link. Since
      RB2 would, in general, not be able to tell that this was a frame
      it had tunneled for distribution, RB2 would decapsulate it and
      locally distribute it a second time.

   -  On the other hand, if there is no neighbor of RB2 offering RB2 the
      OOMF service, RB2 cannot tunnel the frame to a neighbor. In this
      case RB2 MUST locally distribute the frame where it is Appointed
      Forwarder for the frame's VLAN and optionally subject to multicast
      pruning. Indicating OOMF Support

   A RBridge RB3 indicates its willingness to offer the OOMF service to
   RB2 in the TRILL Neighbor TLV in RB3's TRILL Hellos by setting a bit
   associated with the SNPA (SubNetwork Point of Attachment, also known
   as MAC address) of RB2 on the link. (See Section 11.) Overloaded
   RBridge RB2 can only distribute multi-destination TRILL Data frames
   to the campus if a neighbor of RB2 not in overload offers RB2 the

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   OOMF service. If RB2 does not have OOMF service available to it, RB2
   can still receive multi-destination frames from non-overloaded
   neighbors and, if RB2 should originate or ingress such a frame, it
   distributes it locally in native form. Using OOMF Service

   If RB2 sees this OOMF (Overloaded Origination of Multi-destination
   Frame) service advertised for it by any of its neighbors on any link
   to which RB2 connects, it selects one such neighbor by a means beyond
   the scope of this document. Assuming RB2 selects RB3 to handle multi-
   destination frames it originates. RB2 MUST advertise in its LSP that
   it might use any of the distribution trees that RB3 advertises so
   that the RPFC will work in the rest of the campus. Thus,
   notwithstanding its overloaded state, RB2 MUST retain this
   information from RB3 LSPs, which it will receive as it is directly
   connected to RB3.

   RB2 then encapsulates such frames as TRILL Data frames to RB3 as
   follows: M bit = 0, Hop Count = 2, ingress nickname = a nickname held
   by RB2, and, since RB2 cannot tell what distribution tree RB3 will
   use, egress nickname = a special nickname indicating an OOMF frame
   (see Section 11). RB2 then unicasts this TRILL Data frame to RB3.
   (Implementation of Item 4 in Section 4 below provides reasonable
   assurance that, notwithstanding its overloaded state, the ingress
   nickname used by RB2 will be unique within at least the portion of
   the campus that is IS-IS reachable from RB2.)

   On receipt of such a frame, RB3 does the following:

   -  change the egress nickname field to designate a distribution tree
      that RB3 normally uses,
   -  set the M bit to one,
   -  change the Hop Count to the value it would normally use if it were
      the ingress, and
   -  forward the frame on that tree.

   RB3 MAY rate limit the number of frames for which it is providing
   this service by discarding some such frames from RB2. The provision
   of even limited bandwidth for OOMFs by RB3, perhaps via the slow
   path, may be important to the bootstrapping of services at RB2 or at
   end stations connected to RB2, such as supporting DHCP and ARP/ND.
   (Everyone sometimes needs a little OOMF (pronounced oomph) to get off
   the ground.)

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3. Distribution Trees

   A correction, a clarification, and two updates related to
   distribution trees appear in the subsections below. See also Section

3.1 Number of Distribution Trees

   In [RFC6325], Section 4.5.2, page 56, Point 2, 4th paragraph, the
   parenthetical "(up to the maximum of {j,k})" is incorrect [Err3052].
   It should read "(up to k if j is zero or the minimum of (j, k) if j
   is non-zero)".

3.2 Clarification of Distribution Tree Updates

   When a link state database change causes a change in the distribution
   tree(s), there are several possibilities. If a tree root remains a
   tree root but the tree changes, then local forwarding and RPFC
   entries for that tree should be updated as soon as practical.
   Similarly, if a new nickname becomes a tree root, forwarding and RPFC
   entries for the new tree should be installed as soon as practical.
   However, if a nickname ceases to be a tree root and there is
   sufficient room in local tables, the forwarding and RPFC entries for
   the former tree MAY be retained so that any multi-destination TRILL
   Data frames already in flight on that tree have a higher probability
   of being delivered.

3.3 IP Address Based Multicast Pruning

   The TRILL base protocol specification [RFC6325] provides for and
   recommends the pruning of multi-destination frame distribution trees
   based on the location of IP multicast routers and listeners; however,
   multicast listening is identified by derived MAC addresses as
   communicated in the Group MAC Address sub-TLV [RFC6326bis].

   TRILL Switches MAY communicate multicast listeners and prune
   distribution trees based on the actual IPv4 or IPv6 multicast
   addresses involved. Additional Group Address sub-TLVs are provided in
   [RFC6326bis] to carry this information. A TRILL Switch that is only
   capable of pruning based on derived MAC address SHOULD calculate and
   use such derived MAC addresses from multicast listener IPv4/IPv6
   address information it receives.

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3.4 Numbering of Distribution Trees

   Section 4.5.1 of [RFC6325] specifies that, when building distribution
   tree number j, node (RBridge) N that has multiple possible parents in
   the tree is attached to possible parent number j mod p. Trees are
   numbered starting with 1 but possible parents are numbered starting
   with 0. As a result, if there are two trees and two possible parents,
   in tree 1 parent 1 will be selected and in tree 2 parent 0 will be

   This is changed so that the selected parent MUST be (j-1) mod p. As a
   result, in the case above, tree 1 will select parent 0 and tree 2
   will select parent 1. This change is not backwards compatible with
   [RFC6325]. If all RBridges in a campus do not determine distribution
   trees in the same way then, for most topologies, the reverse path
   forwarding checking will drop many multi-destination frames before
   they have been properly delivered.

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4. Nickname Selection

   Nickname selection is covered by Section 3.7.3 of [RFC6325]. However,
   the following should be noted:

   1. The second sentence in the second bullet item in Section 3.7.3 of
      [RFC6325] on page 25 is erroneous [Err3002] and is corrected as

      1.a The occurrence of "IS-IS ID (LAN ID)" is replaced with

       1.b The occurrence of "IS-IS System ID" is replaced with "seven
          byte IS-IS ID (LAN ID)".

      The resulting corrected [RFC6325] sentence reads as follows: "If
      RB1 chooses nickname x, and RB1 discovers, through receipt of an
      LSP for RB2 at any later time, that RB2 has also chosen x, then
      the RBridge or pseudonode with the numerically higher priority
      keeps the nickname, or if there is a tie in priority, the RBridge
      with the numerically higher seven byte IS-IS ID (LAN ID) keeps the
      nickname, and the other RBridge MUST select a new nickname."

   2. In examining the link state database for nickname conflicts,
      nicknames held by IS-IS unreachable TRILL Switches MUST be ignored
      but nicknames held by IS-IS reachable TRILL Switches MUST NOT be
      ignored even if they are data unreachable.

   3. An RBridge may need to select a new nickname, either initially
      because it has none or because of a conflict. When doing so, the
      RBridge MUST consider as available all nicknames that do not
      appear in its link state database or that appear to be held by IS-
      IS unreachable TRILL Switches; however, it SHOULD give preference
      to selecting new nicknames that do not appear to be held by any
      TRILL Switch in the campus, reachable or unreachable, so as to
      minimize conflicts if IS-IS unreachable TRILL Switches later
      become reachable.

   4. An RBridge, even after it has acquired a nickname for which there
      appears to be no conflicting claimant, MUST continue to monitor
      for conflicts with the nickname or nicknames it holds. It does so
      by checking in LSPs it receives that should update its link state
      database for any of its nicknames held with higher priority by
      another TRILL Switch that is IS-IS reachable. If it finds such a
      conflict, it MUST select a new nickname, even when in overloaded
      state. (It is possible to receive an LSP that should update the
      link state database but does not due to overload.)

   5. In the very unlikely case that an RBridge is unable to obtain a
      nickname because all valid RBridge nicknames (0x0001 through

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      0xFFBF inclusive) are in use with higher priority by IS-IS
      reachable TRILL Switches, it will be unable to act as an ingress,
      egress, or tree root but will still be able to function as a
      transit TRILL Switch. Although it cannot be a tree root, such an
      RBridge is included in distribution trees computed for the campus
      unless all its neighbors are overloaded. It would not be possible
      to send a unicast RBridge Channel message specifically to such a
      TRILL Switch [Channel]; however, it will receive unicast Channel
      messages sent by a neighbor to the Any-RBridge egress nickname and
      will receive appropriate multi-destination Channel messages.

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5. MTU (Maximum Transmission Unit)

   MTU values in TRILL key off the originatingL1LSPBufferSize value
   communicated in the IS-IS originatingLSPBufferSize TLV [IS-IS]. The
   campus-wide value Sz, as described in [RFC6325] Section 4.3.1, is the
   minimum value of originatingL1LSPBufferSize for the RBridges in a
   campus, but not less than 1470. The MTU testing mechanism and
   limiting LSPs to Sz assures that the LSPs can be flooded by IS-IS and
   thus that IS-IS can operate properly.

   If nothing is known about the MTU of the links or the
   originatingL1LSPBufferSize of other RBridges in a campus, the
   originatingL1LSPBufferSize for an RBridge should default to the
   minimum of the LSP size that its TRILL IS-IS software can handle and
   the minimum MTU of the ports that it might use to receive or transmit
   LSPs. If an RBridge does have knowledge of link MTUs or other RBridge
   originatingL1LSPBufferSize then, to avoid the necessity to regenerate
   the local LSPs using a different maximum size, the RBridge's
   originatingL1LSPBufferSize SHOULD be configured to the minimum of (1)
   the smallest value that other RBridges are or will be announcing as
   their originatingL1LSPBufferSize and (2) a value small enough that
   the campus will not partition due to a significant number of links
   with limited MTU. However, as provided in [RFC6325], in no case can
   originatingL1LSPBufferSize be less than 1470. In a well configured
   campus, to minimize any LSP regeneration due to re-sizing, it is
   desirable for all RBridges to be configured with the same

   Section 5.1 below corrects errata in [RFC6325] and Section 5.2
   clarifies the meaning of various MTU (Maximum Transmission Unit)
   limits for TRILL Ethernet links.

5.1 MTU Related Errata in RFC 6325

   Three MTU related errata in [RFC6325] are corrected in the
   subsections below.

5.1.1 MTU PDU Addressing

   Section 4.3.2 of [RFC6325] incorrectly states that multi-destination
   MTU-probe and MTU-ack TRILL IS-IS PDUs are sent on Ethernet links
   with the All-RBridges multicast address as the Outer.MacDA [Err3004].
   As TRILL IS-IS PDUs, when multicast on an Ethernet link, they MUST be
   sent to the All-IS-IS-RBridges multicast address.

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5.1.2 MTU PDU Processing

   As discussed in [RFC6325] and, in more detail, in [RFC6327], MTU-
   probe and MTU-ack PDUs MAY be unicast; however, Section 4.6 of
   [RFC6325] erroneously does not allow for this possibility [Err3003].
   It is corrected by replacing Item numbered "1" in Section 4.6.2 of
   [RFC6325] with the following quoted text to which TRILL Switches MUST

   "1. If the Ethertype is L2-IS-IS and the Outer.MacDA is either All-
       IS-IS-RBridges or the unicast MAC address of the receiving
       RBridge port, the frame is handled as described in Section"

   The reference to "Section" in the above quoted text is to
   that Section in [RFC6325].

5.1.3 MTU Testing

   The last two sentences of Section 4.3.2 of [RFC6325] have errors
   [Err3053]. They currently read:

      If X is not greater than Sz, then RB1 sets the "failed minimum MTU
      test" flag for RB2 in RB1's Hello. If size X succeeds, and X > Sz,
      then RB1 advertises the largest tested X for each adjacency in the
      TRILL Hellos RB1 sends on that link, and RB1 MAY advertise X as an
      attribute of the link to RB2 in RB1's LSP.

   They should read:

      If X is not greater than or equal to Sz, then RB1 sets the "failed
      minimum MTU test" flag for RB2 in RB1's Hello. If size X succeeds,
      and X >= Sz, then RB1 advertises the largest tested X for each
      adjacency in the TRILL Hellos RB1 sends on that link, and RB1 MAY
      advertise X as an attribute of the link to RB2 in RB1's LSP.

5.2 Ethernet MTU Values

   originatingL1LSPBufferSize is the maximum permitted size of LSPs
   starting with the 0x83 Intradomain Routeing Protocol Discriminator
   byte.  In layer 3 IS-IS, originatingL1LSPBufferSize defaults to 1492
   bytes. (This is because, in its previous life as DECnet Phase V, IS-
   IS was encoded using the SNAP SAP [RFC5342] format which takes 8
   bytes of overhead and 1492 + 8 = 1500, the classic Ethernet maximum.
   When standardized by ISO/IEC [IS-IS] to use LLC encoding, this
   default could have been increased by a few bytes but was not.)

D. Eastlake, et al                                             [Page 16]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

   In TRILL, originatingL1LSPBufferSize defaults to 1470 bytes. This
   allows 27 bytes of headroom or safety margin to accommodate legacy
   devices with the classic Ethernet maximum MTU despite headers such as
   an Outer.VLAN. This safety margin is called "Margin" below.

   Assuming the campus wide minimum link MTU is Sz, RBridges on Ethernet
   links MUST limit most TRILL IS-IS PDUs so that PDUz (the length of
   the PDU starting just after the L2-IS-IS Ethertype and ending just
   before the Ethernet frame FCS) does not to exceed Sz. The PDU
   exceptions are TRILL Hello PDUs, which MUST NOT exceed 1470 bytes,
   and MTU-probe and MTU-ack PDUs that are padded, depending on the size
   being tested (which may exceed Sz).

   Sz does not limit TRILL Data frames. They are only limited by the MTU
   of the devices and links that they actually pass through; however,
   links that can accommodate IS-IS PDUs up to Sz would accommodate,
   with a generous safety margin, TRILL Data frame payloads, starting
   after the Inner.VLAN and ending just before the FCS, of Sz - 24
   bytes. Most modern Ethernet equipment has ample headroom for frames
   with extensive headers and is sometimes engineered to accommodate 9K
   byte jumbo frames.

D. Eastlake, et al                                             [Page 17]
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6. Port Modes

   Section 4.9.1 of [RFC6325] specifies four mode bits for RBridge ports
   but may not be completely clear on the effects of various
   combinations of bits.

   The table below explicitly indicates the effect of all possible
   combinations of the TRILL port mode bits. "*" in one of the first
   four columns indicates that the bit can be either zero or one. The
   following columns indicate allowed frame types. The Disable bit
   normally disables all frames but, as an implementation choice, some
   or all low level Layer 2 control frames (as specified in [RFC6325]
   Section 1.4) can still be sent or received.

               |D| | | |        |       |     |     |     |
               |i| |A| |        |       |TRILL|     |     |
               |s| |c|T|        |       |Data |     |     |
               |a| |c|r|        |       |     |     |     |
               |b|P|e|u|        |native | LSP |     |     |
               |l|2|s|n|Layer 2 |ingress| SNP |TRILL| P2P |
               |e|P|s|k|Control |egress | MTU |Hello|Hello|
               |0|0|0|0|  Yes   |  Yes  | Yes | Yes | No  |
               |0|0|0|1|  Yes   |  No   | Yes | Yes | No  |
               |0|0|1|0|  Yes   |  Yes  | No  | Yes | No  |
               |0|0|1|1|  Yes   |  No   | No  | Yes | No  |
               |0|1|0|*|  Yes   |  No   | Yes | No  | Yes |
               |0|1|1|*|  Yes   |  No   | No  | No  | Yes |
               |1|*|*|*|Optional|  No   | No  | No  | No  |

   (The formal name of the "access bit" is the "TRILL traffic disable
   bit" and the formal name of the "trunk bit" is the "end-station
   service disable bit" [RFC6325].)

D. Eastlake, et al                                             [Page 18]
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7. The CFI / DEI Bit

   In May 2011, the IEEE promulgated [802.1Q-2011] which changes the
   meaning of the bit between the priority and VLAN ID bits in the
   payload of C-VLAN tags. Previously this bit was called the CFI
   (Canonical Format Indicator) bit [802] and had a special meaning in
   connection with IEEE 802.5 (Token Ring) frames. Now, under
   [802.1Q-2011], it is a DEI (Drop Eligibility Indicator) bit, similar
   to that bit in S-VLAN / B-VLAN tags where this bit has always been a
   DEI bit.

   The TRILL base protocol specification [RFC6325] assumed, in effect,
   that the link by which end stations are connected to TRILL Switches
   and the restricted virtual link provided by the TRILL Data frame are
   IEEE 802.3 Ethernet links on which the CFI bit is always zero. Should
   an end station be attached by some other type of link, such as a
   Token Ring link, [RFC6325] implicitly assumed that such frames would
   be canonicalized to 802.3 frames before being ingressed and
   similarly, on egress, such frames would be converted from 802.3 to
   the appropriate frame type for the link. Thus, [RFC6325] required
   that the CFI bit in the Inner.VLAN always be zero.

   However, for TRILL Switches with ports conforming to the change
   incorporated in the IEEE 802.1Q-2011 standard, the bit in the
   Inner.VLAN, now a DEI bit, MUST be set to the DEI value provided by
   the EISS (Enhanced Internal Sublayer Service) interface on ingressing
   a native frame.  Similarly, this bit MUST be provided to the EISS
   when transiting or egressing a TRILL Data frame. As with the 3-bit
   priority field, the DEI bit to use in forwarding a transit frame MUST
   be taken from the Inner.VLAN.  The exact effect on the Outer.VLAN DEI
   and priority bits and whether or not an Outer.VLAN appears at all on
   the wire for output frames may depend on output port configuration.

   TRILL campuses with a mixture of ports, some compliant with
   [802.1Q-2011] and some compliant with pre-802.1Q-2011 standards,
   especially if they have actual Token Ring links, may operate
   incorrectly and may corrupt data, just as a bridged LAN with such
   mixed ports and links would.

D. Eastlake, et al                                             [Page 19]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

8. Graceful Restart

   TRILL Switches SHOULD support the features specified in [RFC5306]
   which describes a mechanism for a restarting IS-IS router to signal
   to its neighbors that it is restarting, allowing them to reestablish
   their adjacencies without cycling through the down state, while still
   correctly initiating link state database synchronization.

D. Eastlake, et al                                             [Page 20]
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9. Updates to RFC 6327

   [RFC6327] provides for multiple states of the potential adjacency
   between two TRILL Switches. It makes clear that only an adjacency in
   the "Report" state is reported in LSPs.  LSP synchronization (LSP and
   SNP transmission and receipt), however, is performed if and only if
   there is at least one adjacency on the link in either the "Two-Way"
   or "Report" state.

   To support the PORT-TRILL-VER sub-TLV specified in [RFC6326bis], the
   following updates are made to [RFC6327]:

      1. The first sentence of the last paragraph in [RFC6327] Section
         3.1 is modified from
            "All TRILL LAN Hellos issued by an RBridge on a particular
            port MUST have the same source MAC address, priority,
            desired Designated VLAN, and Port ID, regardless of the VLAN
            in which the Hello is sent."
            "All TRILL LAN Hellos issued by an RBridge on a particular
            port MUST have the same source MAC address, priority,
            desired Designated VLAN, Port ID, and PORT-TRILL-VER sub-TLV
            [RFC6326bis] if included, regardless of the VLAN in which
            the Hello is sent."

      2. An additional bullet item is added to the end of [RFC6327]
         Section 3.2 as follows:

            o  The 5 bytes of data from the PORT-TRILL-VER received in
               the most recent TRILL Hello from the neighbor RBridge.

      3. In [RFC6327] Section 3.3, near the bottom of page 12, a bullet
         item as follows is added:

            o  The five 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.

      4. At the beginning of [RFC6327] Section 4, a bullet item is added
         to the list as follows:

            o  The five bytes of PORT-TRILL-VER sub-TLV data used in
               TRILL Hellos sent on the port.

D. Eastlake, et al                                             [Page 21]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

10. Updates on Appointed Forwarders and Inhibition

   An optional method of Hello reduction is specified in Section 10.1
   below and a recommendation on forwarder appointments in the face of
   overload is given in Section 10.2.

10.1 Optional TRILL Hello Reduction

   If a network manager has sufficient confidence that they know the
   configuration of bridges, ports, and the like, within a link, they
   may be able to reduce the number of TRILL Hellos sent on that link;
   for example, if all RBridges on the link will see all Hellos
   regardless of VLAN constraints, Hellos could be sent on fewer VLANs.
   However, because adjacencies are established in the Designated VLAN,
   an RBridge MUST always attempt to send Hellos in the Designated VLAN.
   Hello reduction makes TRILL less robust in the face of decreased VLAN
   connectivity in a link such as partitioned VLANs, many VLANs disabled
   on ports, or disagreement over the Designated VLAN; however, as long
   as all RBridge ports on the link are configured for the same desired
   Designated VLAN, can see each other's frames in that VLAN, and
   utilize the mechanisms specified below to update VLAN inhibition
   timers, operations will be safe. (These considerations do not arise
   on links between RBridges that are configured as point-to-point
   since, in that case, each RBridge sends point-to-point Hellos, other
   TRILL IS-IS PDUs, and TRILL Data frames only in what it believes to
   be the Designated VLAN of the link and no native frame end-station
   service is provided.)

   The provision for a configurable set of "Announcing VLANs", as
   described in Section 4.4.3 of [RFC6325] provides a mechanism in the
   TRILL base protocol for a reduction in TRILL Hellos.

   To maintain loop safety in the face of occasional lost frames,
   RBridge failures, link failures, new RBridges coming up on a link,
   and the like, the inhibition mechanism specified in [RFC6439] is
   still required. Under Section 3 of [RFC6439], a VLAN inhibition timer
   can only be set by the receipt of a Hello sent or received in that
   VLAN. Thus, to safely send a reduced number of TRILL Hellos on a
   reduced number of VLANs requires additional mechanisms to set the
   VLAN inhibition timers at an RBridge, thus extending Section 3, Item
   4, of [RFC6439]. Two such mechanisms are specified below. Support for
   both of these mechanisms is indicated by a capability bit in the
   PORT-TRILL-VER sub-TLV (see Section 9 above and [RFC6326bis]). It may
   be unsafe for an RBridge to send TRILL Hellos on fewer VLANs than the
   set of VLANs recommended in [RFC6325] on a link unless all its
   adjacencies on that link (excluding those in the Down state
   [RFC6327]) indicate support of these mechanisms and these mechanisms
   are in use.

D. Eastlake, et al                                             [Page 22]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

   1. An RBridge RB2 MAY include in any TRILL Hello an Appointed
      Forwarders sub-TLV [RFC6326bis] appointing itself for one or more
      ranges of VLANs. The Appointee Nickname field(s) in the Appointed
      Forwarder sub-TLV MUST be the same as the Sender Nickname in the
      Special VLANs and Flags sub-TLV in the TRILL Hello. This indicates
      the sending RBridge believes it is Appointed Forwarder for those
      VLANs. An RBridge receiving such a sub-TLV sets each of its VLAN
      inhibition timers for every VLAN in the block or blocks listed in
      the Appointed Forwarders sub-TLV to the maximum of its current
      value and the Holding Time of the Hello containing the sub-TLV.
      This is backwards compatible because such sub-TLVs will have no
      effect on any receiving RBridge not implementing this mechanism
      unless RB2 is the DRB (Designated RBridge) sending Hello on the
      Designated VLAN in which case, as specified in [RFC6439], RB2 MUST
      include in the Hello all forwarder appointments, if any, for
      RBridges other than itself on the link.

   2. An RBridge MAY use the new VLANs Appointed sub-TLV [RFC6326bis].
      When RB1 receives a VLANs Appointed sub-TLV in a TRILL Hello from
      RB2 on any VLAN, RB1 updates the VLAN inhibition timers for all
      the VLANs that RB2 lists in that sub-TLV as VLANs for which RB2 is
      Appointed Forwarder. Each such timer is updated to the maximum of
      its current value and the Holding Time of the TRILL Hello
      containing the VLANs Appointed sub-TLV. This sub-TLV will be an
      unknown sub-TLV to RBridge not implementing it and such RBridges
      will ignore it. Even if a TRILL Hello send by the DRB on the
      Designated VLAN includes one or more VLANs Appointed sub-TLVs, as
      long as no Appointed Forwarders sub-TLVs appear, the Hello is not
      required to indicate all forwarder appointments.

   Two different encodings are providing above to optimize the listing
   of VLANs. Large blocks of contiguous VLANs are more efficiently
   encoded with the Appointed Forwarders sub-TLV and scattered VLANs are
   more efficiently encoded with the VLANs Appointed sub-TLV. These
   encodings may be mixed in the same Hello. The use of these sub-TLVs
   does not affect the requirement that the "AF" bit in the Special
   VLANs and Flags sub-TLV MUST be set if the originating RBridge
   believes it is Appointed Forwarder for the VLAN in which the Hello is
   sent. If the above mechanisms are used on a link, then each RBridge
   on the link MUST send Hellos in one or more VLANs with such VLANs
   Appointed sub-TLV(s) and/or self-appointment Appointed Forwarders
   sub-TLV(s) and the "AF" bit appropriately set such that no VLAN
   inhibition timer will improperly expire unless three or more Hellos
   are lost. For example, an RBridge could announce all VLANs for which
   it believes it is Appointed Forwarder in a Hello sent on the
   Designated VLAN three times per Holding Time.

D. Eastlake, et al                                             [Page 23]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

10.2 Overload and Appointed Forwarders

   An RBridge in overload (see Section 2) will, in general, do a poorer
   job of ingressing and forwarding frames than an RBridge not in
   overload that has full knowledge of the campus topology. For example,
   an overloaded RBridge may not be able to distribute multi-destination
   TRILL Data frames at all.

   Therefore, the DRB SHOULD NOT appoint an RBridge in overload as an
   Appointed Forwarder unless there is no alternative.  Furthermore, if
   an Appointed Forwarder becomes overloaded, the DRB SHOULD re-assign
   VLANs from the overloaded RBridge to another RBridge on the link that
   is not overloaded, if one is available. DRB election is not affected
   by overload.

   A counter-example would be if all campus end stations in VLAN-x were
   on links attached to RB1 via ports where VLAN-x was enabled. In such
   a case, RB1 SHOULD be made the VLAN-x Appointed Forwarder on all such
   links even if RB1 is overloaded.

D. Eastlake, et al                                             [Page 24]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

11. IANA Considerations

   The following IANA actions are required:

   1. The nickname 0xTBD [0xFFC1 suggested], which was reserved by
      [RFC6325], is allocated for use in the TRILL Header egress
      nickname field to indicate an OOMF (Overload Originated Multi-
      destination Frame).

   2. Bit 1 from the seven previously reserved (RESV) bits in the per
      neighbor "Neighbor RECORD" in the TRILL Neighbor TLV [RFC6326bis]
      is allocated to indicate that the RBridge sending the TRILL Hello
      volunteers to provide the OOMF forwarding service described in
      Section 2.4.2 to such frames originated by the TRILL Switch whose
      SNPA (MAC address) appears in that Neighbor RECORD.

   3. Bit 0 is allocated from the Capability bits in the PORT-TRILL-VER
      sub-TLV [RFC6326bis] to indicate support of the VLANs Appointed
      sub-TLV [RFC6326bis] and the VLAN inhibition setting mechanisms
      specified in Section 10.1.

D. Eastlake, et al                                             [Page 25]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

12. Security Considerations

   This memo improves the documentation of the TRILL protocol, corrects
   five errata in [RFC6325], and updates [RFC6325], [RFC6327], and
   [RFC6439]. It does not change the security considerations of these


   The contributions of the following persons are gratefully

      Somnath Chatterjee, Weiguo Hao, Rakesh Kumar, Yizhou Li, Radia
      Perlman, Mike Shand, Meral Shirazipour, and Varun Varshah.

   This document was produced with raw nroff. All macros used were
   defined in the source file.

Change History

   RFC Editor Note: Please delete this Change History section before

   Changes from -01 to -02:
      Add Section 3.4. Minor editorial changes and update of the version
      and dates.

   Changes from -02 to -03:
      Emphasize in Section 3.4 that the change is not backwards
      compatible and that all the RBridges in a campus must calculate
      the distribution trees the same way or RPFC will discard too much.
      Provide additional detail at the end of Item 5, Section 4, as to
      what Channel messages can be received by an RBridge without a
      nickname. Add Section 1.2 on incompatible changes, renumbering the
      former Section 1.2 as 1.3.

   Changes from -03 to -04:
      Update author information.  Numerous typos fixes and editorial
      changes based on the GENART review and various Area Director
      reviews. The update from -03 to -04 is not intended to make any
      technical change.

   Changes from -04 to -05:
      Change text to make it clear that frames are not least cost routed
      through overloaded RBridges. Minor editorial changes, some to
      resolve second-pass GENART comments.

D. Eastlake, et al                                             [Page 26]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

   Changes from -05 to -06:
      Improve wording of Section 4, bullet 4, and Section 5, paragraph
      2, as per suggestions by Mike Shand. Fix typos in Change History.

D. Eastlake, et al                                             [Page 27]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

Normative References

   [802.1Q-2011] - IEEE 802.1, "IEEE Standard for Local and metropolitan
         area networks - Virtual Bridged Local Area Networks", IEEE Std
         802.1Q-2011, May 2011.

   [Err3002] - RFC Errata, Errata ID 3002, RFC 6325, http://www.rfc-

   [Err3003] - RFC Errata, Errata ID 3003, RFC 6325, http://www.rfc-

   [Err3004] - RFC Errata, Errata ID 3004, RFC 6325, http://www.rfc-

   [Err3052] - RFC Errata, Errata ID 3052, RFC 6325, http://www.rfc-

   [Err3053] - RFC Errata, Errata ID 3053, RFC 6325, http://www.rfc-

   [IS-IS] - ISO/IEC 10589:2002, Second Edition, "Intermediate System to
         Intermediate System Intra-Domain Routeing 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.

   [RFC5305] - Li, T. and H. Smit, "IS-IS Extensions for Traffic
         Engineering", RFC 5305, October 2008.

   [RFC5306] - Shand, M. and L. Ginsberg, "Restart Signaling for IS-IS",
         RFC 5306, October 2008.

   [RFC6325] - Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
         Ghanwani, "Routing Bridges (RBridges): Base Protocol
         Specification", RFC 6325, July 2011.

   [RFC6327] - Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D.,
         and V. Manral, "Routing Bridges (RBridges): Adjacency", RFC
         6327, July 2011.

   [RFC6439] - Perlman, R., Eastlake, D., Li, Y., Banerjee, A., and F.
         Hu, "Routing Bridges (RBridges): Appointed Forwarders", RFC
         6439, November 2011.

D. Eastlake, et al                                             [Page 28]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

   [RFC6326bis] - Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and
         A. Ghanwani, draft-eastlake-isis-rfc6326bis, work in progress.

Informative References

   [802] - IEEE 802, "IEEE Standard for Local and metropolitan area
         networks: Overview and Architecture", IEEE Std 802.1-2001, 8
         March 2002.

   [Channel] - Eastlake, E., Manral, V., Li, Y., Ward, D., draft-ietf-
         trill-rbridge-channel, work in progress.

   [RFC5342] - Eastlake 3rd, D., "IANA Considerations and IETF Protocol
         Usage for IEEE 802 Parameters", BCP 141, RFC 5342, September

D. Eastlake, et al                                             [Page 29]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

Authors' Addresses

   Donald Eastlake
   Huawei R&D USA
   155 Beaver Street
   Milford, MA 01757 USA

   Phone: +1-508-333-2270

   Mingui Zhang
   Huawei Technologies Co., Ltd
   Huawei Building, No.156 Beiqing Rd.
   Z-park, Shi-Chuang-Ke-Ji-Shi-Fan-Yuan, Hai-Dian District,
   Beijing 100095 P.R. China


   Anoop Ghanwani
   350 Holger Way
   San Jose, CA 95134 USA

   Phone: +1-408-571-3500

   Vishwas Manral
   HP Networking
   19111 Pruneridge Avenue
   Cupertino, CA 95014 USA

   Tel:   +1-408-477-0000

   Ayan Banerjee
   Cumulus Networks
   1089 West Evelyn Avenue
   Sunnyvale, CA 94086 USA


D. Eastlake, et al                                             [Page 30]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

Copyright and IPR Provisions

   Copyright (c) 2012 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
   ( in effect on the date of
   publication of this document. Please review these documents
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   include Simplified BSD License text as described in Section 4.e of
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   described in the Simplified BSD License.  The definitive version of
   an IETF Document is that published by, or under the auspices of, the
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D. Eastlake, et al                                             [Page 31]