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TRILL: Clarifications, Corrections, and Updates

The information below is for an old version of the document.
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 , Ayan Banerjee , Vishwas Manral
Last updated 2012-01-27
Replaces draft-eastlake-trill-rbridge-clear-correct
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
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TRILL Working Group                                      Donald Eastlake
INTERNET-DRAFT                                              Mingui Zhang
Intended status: Proposed Standard                                Huawei
Updates: 6325, 6327, 6439                                 Anoop Ghanwani
                                                           Ayan Banerjee
                                                          Vishwas Manral
Expires: July 26, 2012                                  January 27, 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, 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 a
   few errata in the original RFC 6325 and some cases that could use
   clarifications or updates.

   RFC 6327, RFC 6439, and RFC XXXX, provide clarifications with respect
   to Adjacency, Appointed Forwarders, and the TRILL ESADI protocol.
   This document provide 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."

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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

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Table of Contents

      1. Introduction............................................4
      1.1 Precedence.............................................4
      1.2 Terminology and Acronyms...............................4

      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 Distribution Tree Updates.............................11

      4. Nickname Selection.....................................12

      5. MTU (Maximum Transmission Unit)........................14
      5.1 MTU Related Errata in RFC 6325........................14
      5.1.1 MTU PDU Addressing..................................14
      5.1.2 MTU PDU Processing..................................14
      5.1.3 MTU Testing.........................................15
      5.2 Ethernet MTU Values...................................15

      6. Port Modes.............................................17
      7. The CFI / DEI Bit......................................18
      8. Graceful Restart.......................................19
      9. Some Updates to RFC 6327...............................20
      10. Updates on Appointed Forwarders and Inhibition........21
      10.1 Optional TRILL Hello Reduction.......................21
      10.2 Overload and Appointed Forwarders....................23

      11. IANA Considerations...................................24
      12. Security Considerations...............................25
      Normative References......................................26
      Informative References....................................26

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,
   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.

   Since the TRILL base protocol [RFC6325] was approved, the active
   development of TRILL has revealed a few errors in the original
   specification document [RFC6325] and cases that could use
   clarifications or updates.

   [RFC6327], [RFC6439], and [RFCXXXX], provide clarifications with
   respect to Adjacency, Appointed Forwarders, and the TRILL ESADI
   protocol. 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 Terminology and Acronyms

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

D. Eastlake, et al                                              [Page 4]
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      CFI - Canonical Format Indicator [802]

      DEI - Drop Eligibility Indicator [802.1Q-2011]

      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

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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. 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 on. 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 normally be routed through it.
   (Also, in some cases, TRILL provides for setting the overload bit in
   the pseudo node 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 reasonable 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 psuedo nodes.

2.1 Reachability

   Frames are not least cost routed through an overloaded TRILL Switch
   if any other path is available, 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; such links are
   reserved for traffic engineered frames the handling of which is
   beyond the scope of this document.

   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. 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

   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. 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 seconds or longer. 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.

2.2 Distribution Trees

   A RBridge in overload cannot be trusted to correctly calculate
   distribution trees or correctly perform the 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 Reverse Path Forwarding Checks for multi-destination
   frames, an RBridge RB1 can similarly 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
   overload, RB2 MUST NOT discard the frame because the egress is an

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   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 a Reverse Path Forwarding Check 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 Forwarder).

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
   tree of which RB4 is the root and which is represented by double

D. Eastlake, et al                                              [Page 8]
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   lines. Only RBridge RB2 is overloaded.

                  +-----+   +-----+     +-----+
                  | RB7 +===+ RB5 +=====+ RB3 |
                  +-----+   +--+--+     +-++--+
                               |          ||
                           +---+---+      ||
                      |    +-------+      ||
                      |                   ||
                  +---+-+    +-----+  ++==++=++
                  | RB8 +====+ RB6 +==++ RB4 ||
                  +-----+    +-----+  ++=====++

   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 signal 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 (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 OOMF service. If RB2 does not have OOMF
   service available to it, RB2 can still receive multi-destination

D. Eastlake, et al                                              [Page 9]
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   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 it
   might use so that the Reverse Path Forwarding Check 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 RB, such as supporting DHCP and ARP/ND.
   (Everyone sometimes needs a little OOMF (pronounced oompf) to get off
   the ground.)

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3. Distribution Trees

   A correction and a clarification related to distribution trees appear
   in the subsections below. See also Section 2.2.

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. It should
   read "(up to k if j is zero or the minimum of ( j, k) if j is non-

3.2 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.

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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 and is corrected as follows:

      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. (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 nicknames (0x0001 through 0xFFBF
      inclusive) are in use with higher priority by IS-IS reachable

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      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 an
      RBridge Channel message to such a TRILL Switch [Channel].

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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 properly by
   IS-IS and thus that IS-IS can operate properly.

   If nothing is known about the 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. However, to avoid having to
   refragment LSPs, originatingL1LSPBufferSize SHOULD be configured to a
   smaller value if it is known that other RBridges will be announcing
   such smaller value or that the campus will partition due to a
   significant number of links with an MTU of such smaller value. In a
   well configured campus, to minimize any LSP 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. As TRILL
   IS-IS PDUs, when multicast on an Ethernet link, they MUST be sent to
   the All-IS-IS-RBridges multicast address.

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. It is

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   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.
   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
   after the eight byte fixed IS-IS PDU header. This IS-IS PDU header
   starts with the 0x83 Intradomain Routeing Protocol Discriminator byte
   and ends with the Maximum Area Addresses byte, inclusive.  In layer 3
   IS-IS, originatingL1LSPBufferSize defaults to 1492 bytes and thus the
   default Layer 3 LSP size, including this header, is 1500 bytes.  In
   TRILL, originatingL1LSPBufferSize defaults to 1470 bytes, allowing 22
   bytes of additional headroom or safety margin to accommodate legacy
   devices with, for example, the classic Ethernet maximum MTU, and
   headers such as an Outer.VLAN. We will call this safety margin
   "Margin" below.

   Assuming the campus wide minimum link MTU is Sz, RBridges on Ethernet

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   links MUST limit most TRILL IS-IS PDUs so that PDUz (the length of
   the PDU starting just before and including the L2-IS-IS Ethertype and
   ending just before the Ethernet frame FCS) does not to exceed

         PDUz = ( Sz + 32 - Margin ) bytes

   The PDU exceptions are TRILL Hello PDUs, which MUST NOT exceed this
   limit assuming an Sz of 1470 bytes, and MTU-probe and MTU-ack PDUs
   which are padded, depending on the size Tz being tested, to ( Tz + 32
   - Margin ) bytes.

   Sz does not limit TRILL Data frames. They are only limited by the MTU
   of the RBridges and links that they actually pass through; however,
   links that can accommodate IS-IS PDUs up to Sz should accommodate,
   with a reasonable safety margin, TRILL Data frame payloads, starting
   after the Inner.VLAN and ending just before the FCS, of ( Sz + 10 -
   Margin ) bytes. Most modern Ethernet equipment has ample headroom for
   frames with extensive headers and is sometimes engineered to
   accommodate 9K byte jumbo frames.

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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 (a 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  |

D. Eastlake, et al                                             [Page 17]
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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 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 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 Switch 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 bridges and ports would.

D. Eastlake, et al                                             [Page 18]
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 19]
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9. Some 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 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 paragraph immediately before the 3.2 section header is
         modified by adding "TRILL-PORT-VER sub-TLV [RFC6326bis] if
         included" to those items which MUST be the same in all TRILL
         Hellos sent out the same RBridge port regardless of the VLAN on
         which they are sent but can occasionally change.

      2. In Section 3.2, the state entry for each adjacency is expanded
         to include the 5 bytes of data from the TRILL-PORT-VER received
         in the most recent TRILL Hello from the remote RBridge.

      3. In Section 3.3, a bullet item as follows is added to the bullet
         items after the event descriptions: "The five bytes of TRILL-
         PORT-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. In the first part of Section 4, a bullet item is added to the
         list as follows: "The five bytes of TRILL-PORT-VER sub-TLV data
         used in TRILL Hellos sent on the port."

D. Eastlake, et al                                             [Page 20]
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 partitioned
   VLANs or disagreement over the Designated VLAN or the like in a link;
   however, as long as all RBridge ports on the link are configured for
   the same desired Designated VLAN, can see each others 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 4.6.3 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
   TRILL-PORT-VER sub-TLV (see Section 9 above and [RFC6326bis]). Unless
   all adjacencies that are not in the Down state out a port indicate
   support of these mechanisms and the mechanisms are used, it may be
   unsafe to reduce the VLANs on which TRILL Hellos are sent to fewer
   VLANs than recommended in [RFC6325].

D. Eastlake, et al                                             [Page 21]
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 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 encoding 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 encoding
   may be mixed in the same Hello and 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 22]
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 knowldge 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 appointed an RBridge in overload as
   Appointed Forwarder for an VLAN unless there is no alternative.
   Furthermore, if an Appointed Forwarder becomes overloaded, the DRB
   SHOULD re-assign VLANs from the overloaded RBridged to another
   RBridge on the link that is not overloaded, if one is available.

   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
   link even if RB1 is overloaded.

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

11. IANA Considerations

   The following IANA actions are required:

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

   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 TRILL-PORT-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 24]
INTERNET-DRAFT                     TRILL: Clarifications and Corrections

12. Security Considerations

   This memo improves the documentation of the TRILL protocol, corrects
   some errors 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

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

D. Eastlake, et al                                             [Page 25]
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.

   [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.

   [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.

   [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] - draft-ietf-trill-rbridge-channel, work in progress.

   [RFCXXXX] - H. Zhai, F. Hu, R. Perlman, D. Eastlake, "RBridges: The
         ESADI Protocol", draft-hu-trill-rbridge-esadi, work in

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

Authors' Addresses

   Donald Eastlake
   Huawei Technologies
   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

   Ayan Banerjee
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134 USA

   Tel.:  +1-408-527-0539

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

   Tel:   +1-408-477-0000

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

Copyright and IPR Provisions

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   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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D. Eastlake, et al                                             [Page 28]