



Internet Engineering Task Force (IETF)                  L. Ginsberg, Ed.
Request for Comments: 8668                           Cisco Systems, Inc.
Category: Standards Track                                    A. Bashandy
ISSN: 2070-1721                                             Unaffiliated
                                                             C. Filsfils
                                                     Cisco Systems, Inc.
                                                              M. Nanduri
                                                                  Oracle
                                                                E. Aries
                                                             Arrcus Inc.
                                                           December 2019


       Advertising Layer 2 Bundle Member Link Attributes in IS-IS

Abstract

   There are deployments where the Layer 3 interface on which IS-IS
   operates is a Layer 2 interface bundle.  Existing IS-IS
   advertisements only support advertising link attributes of the Layer
   3 interface.  If entities external to IS-IS wish to control traffic
   flows on the individual physical links that comprise the Layer 2
   interface bundle, link attribute information about the bundle members
   is required.

   This document introduces the ability for IS-IS to advertise the link
   attributes of Layer 2 (L2) Bundle Members.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8668.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  L2 Bundle Member Attributes TLV
     3.1.  Parallel L3 Adjacencies
     3.2.  Shared Attribute Sub-TLVs
   4.  Advertising L2 Bundle Member Adj-SIDs
     4.1.  L2 Bundle Member Adjacency Segment Identifier Sub-TLV
     4.2.  L2 Bundle Member LAN Adjacency SID Sub-TLV
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Example Encoding
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   There are deployments where the Layer 3 interface on which an IS-IS
   adjacency is established is a Layer 2 interface bundle, for instance,
   a Link Aggregation Group (LAG) [IEEE802.1AX].  This reduces the
   number of adjacencies that need to be maintained by the routing
   protocol in cases where there are parallel links between the
   neighbors.  Entities external to IS-IS such as Path Computation
   Elements (PCEs) [RFC4655] may wish to control traffic flows on
   individual members of the underlying Layer 2 bundle.  In order to do
   so, link attribute information about individual bundle members is
   required.  The protocol extensions defined in this document provide
   the means to advertise this information.

   This document introduces a new TLV to advertise link attribute
   information for each of the L2 Bundle Members that comprise the Layer
   3 interface on which IS-IS operates.

   [RFC8667] introduces a new link attribute, adjacency segment
   identifier (Adj-SID), which can be used as an instruction to
   forwarding to send traffic over a specific link.  This document
   introduces additional sub-TLVs to advertise Adj-SIDs for L2 Bundle
   Members.

   Note that the new advertisements defined in this document are
   intended to be provided to external (to IS-IS) entities.  The
   following items are intentionally not defined and/or are outside the
   scope of this document:

   *  What link attributes will be advertised.  This is determined by
      the needs of the external entities.

   *  A minimum or default set of link attributes.

   *  How these attributes are configured.

   *  How the advertisements are used.

   *  What impact the use of these advertisements may have on traffic
      flow in the network.

   *  How the advertisements are passed to external entities.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  L2 Bundle Member Attributes TLV

   A new TLV is introduced to advertise L2 Bundle Member attributes.
   Although much of the information is identical to and uses the same
   sub-TLVs included in Extended IS Neighbor advertisements (TLVs 22 and
   222), a new TLV is used so that changes to the advertisement of the
   L2 Bundle Member link attributes do not trigger unnecessary action by
   the [ISO10589] Decision Process.

   Advertisement of this information implies that the identified link is
   a member of the L2 Bundle associated with the identified Parent L3
   Neighbor and that the member link is operationally up.  Therefore,
   advertisements MUST be withdrawn if the link becomes operationally
   down or it is no longer a member of the identified L2 Bundle.

   This new TLV utilizes the sub-TLV space defined for TLVs 22, 23, 141,
   222, and 223.

   The following new TLV is introduced:

      L2 Bundle Member Attributes

      Type: 25

      Length: Number of octets to follow

      Parent L3 Neighbor Descriptor

         L3 Neighbor System ID + pseudonode ID (7 octets)

         Flags:  1-octet field of the following flags:

             0 1 2 3 4 5 6 7
            +-+-+-+-+-+-+-+-+
            |P|             |
            +-+-+-+-+-+-+-+-+

            where:

            P-Flag:  When set to 1, one of the sub-TLVs described in
                     Section 3.1 immediately follows the flags field.
                     If the P-Flag is set to 0, then none of the sub-
                     TLVs described in Section 3.1 are present.

            Other bits:  MUST be zero when originated and ignored when
                     received.

      One or more L2 Bundle Attribute Descriptors (as defined below).

         Length of L2 Bundle Attribute Descriptor (1 octet)

            NOTE: This includes all fields described below.

         Number of L2 Bundle Member Descriptors (1 octet)

         L2 Bundle Member Link Local Identifiers
         (4 * Number of L2 Bundle Member Descriptors octets)

            NOTE: An L2 Bundle Member Descriptor is a Link Local
            Identifier as defined in [RFC4202].

         Sub-TLV(s)
         A sub-TLV may define an attribute common to all of the bundle
         members listed, or it may define an attribute unique to each
         bundle member.  Use of these two classes of sub-TLVs is
         described in the following sections.

      NOTE: Only one Parent L3 Neighbor Descriptor is present in a given
      TLV.  Multiple L2 Bundle Attribute Descriptors may be present in a
      single TLV.

3.1.  Parallel L3 Adjacencies

   When there exist multiple L3 adjacencies to the same neighbor,
   additional information is required to uniquely identify the L3
   Neighbor.  One and only one of the following three sub-TLVs is used
   to uniquely identify the L3 adjacency:

   *  IPv4 Interface Address (sub-TLV 6 defined in [RFC5305])

   *  IPv6 Interface Address (sub-TLV 12 defined in [RFC6119])

   *  Link Local/Remote Identifiers (sub-TLV 4 defined in [RFC5307])

   When the P-Flag is set in the flags field in the Parent L3 Neighbor
   Descriptor, one and only one of the above sub-TLVs MUST be present.
   The chosen sub-TLV MUST immediately follow the flags field described
   in Section 3.

   These sub-TLVs MAY be omitted if no parallel adjacencies to the
   neighbor exist.

3.2.  Shared Attribute Sub-TLVs

   These sub-TLVs advertise a single copy of an attribute (e.g., link
   bandwidth).  The attribute applies to all of the L2 Bundle Members in
   the set advertised under the preceding L2 Bundle Member Attribute
   Descriptor.  No more than one copy of a given sub-TLV in this
   category may appear in the set of sub-TLVs under the preceding L2
   Bundle Member Attribute Descriptor.  If multiple copies of a given
   sub-TLV are present, all copies MUST be ignored.

   The set of L2 Bundle Member Descriptors that may be advertised under
   a single L2 Bundle Member Attribute Descriptor is therefore limited
   to bundle members that share the set of attributes advertised in the
   shared attribute sub-TLVs.

   All existing sub-TLVs defined in the IANA registry for Sub-TLVs for
   TLVs 22, 23, 141, 222, and 223 are in the category of shared
   attribute sub-TLVs unless otherwise specified in this document.

4.  Advertising L2 Bundle Member Adj-SIDs

   [RFC8667] defines sub-TLVs to advertise Adj-SIDs for L3 adjacencies.
   However, these sub-TLVs only support the advertisement of a single
   Adj-SID.  As it is expected that each L2 Bundle Member will have
   unique Adj-SIDs in many deployments, it is desirable to define a new
   sub-TLV that allows more efficient encoding of a set of Adj-SIDs in a
   single sub-TLV.  Two new sub-TLVs are therefore introduced to support
   advertising Adj-SIDs for L2 Bundle Members.  The format of the new
   sub-TLVs is similar to that used for L3 adjacencies, but it is
   optimized to allow advertisement of a set of Adj-SIDs (one per L2
   Bundle Member) in a single sub-TLV.

   The two new sub-TLVs defined in the following sections do not fall
   into the category of shared attribute sub-TLVs.

4.1.  L2 Bundle Member Adjacency Segment Identifier Sub-TLV

   This sub-TLV is used to advertise Adj-SIDs for L2 Bundle Members
   associated with a parent L3 adjacency that is point-to-point.  The
   following format is defined for this sub-TLV:

      Type:  41 (1 octet)

      Length:  variable (1 octet)

      Flags:  1-octet field of the following flags:

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |F|*|V|L|S|P|   |
         +-+-+-+-+-+-+-+-+

         where:

         F-Flag:  Address-Family Flag.  If unset, then the Adj-SID
                  refers to an L2 Bundle Member with outgoing IPv4
                  encapsulation.  If set, then the Adj-SID refers to an
                  L2 Bundle Member with outgoing IPv6 encapsulation.

         V-Flag:  Value Flag.  If set, then the Adj-SID carries a value.
                  By default, the flag is SET.

         L-Flag:  Local Flag.  If set, then the value/index carried by
                  the Adj-SID has local significance.  By default, the
                  flag is SET.

         S-Flag:  Set Flag.  When set, the S-Flag indicates that the
                  Adj-SID refers to a set of L2 Bundle Members (and
                  therefore MAY be assigned to other L2 Bundle Members
                  as well).

         P-Flag:  Persistent Flag.  When set, the P-Flag indicates that
                  the Adj-SID is persistently allocated, i.e., the Adj-
                  SID value remains consistent across router restart
                  and/or interface flap.

         Other bits:  MUST be zero when originated and ignored when
                  received.

         NOTE: The flags are deliberately kept congruent to the flags in
         the L3 ADJ-SID defined in [RFC8667].  * indicates a flag used
         in the L3 Adj-SID sub-TLV, but one that is NOT used in this
         sub-TLV.  These bits SHOULD be sent as 0 and MUST be ignored on
         receipt.

      Weight:  1 octet.  The value represents the weight of the Adj-SID
               for the purpose of load balancing.  The use of the weight
               is defined in [RFC8402].

      NOTE: Flags and weight are shared by all L2 Bundle Members listed
      in the L2 Bundle Attribute Descriptor.

      L2 Bundle Member Adj-SID Descriptors:
         There MUST be one descriptor for each of the L2 Bundle Members
         advertised under the preceding L2 Bundle Member Attribute
         Descriptor.  Each descriptor consists of one of the following
         fields:

      SID/Index/Label:  According to the V- and L-Flags, it contains
         either:

         o  A 3-octet local label where the 20 rightmost bits are used
            for encoding the label value.  In this case, the V- and
            L-Flags MUST be set.

         o  A 4-octet index defining the offset in the SID/Label space
            advertised by this router.  See [RFC8667].  In this case, V-
            and L-Flags MUST be unset.

4.2.  L2 Bundle Member LAN Adjacency SID Sub-TLV

   This sub-TLV is used to advertise Adj-SIDs for L2 Bundle Members
   associated with a parent L3 adjacency that is a LAN adjacency.  In
   LAN subnetworks, the Designated Intermediate System (DIS) is elected
   and originates the Pseudonode-LSP (PN-LSP) including all neighbors of
   the DIS.  When Segment Routing is used, each router in the LAN MAY
   advertise the Adj-SID of each of its neighbors on the LAN.
   Similarly, for each L2 Bundle Member, a router MAY advertise an Adj-
   SID to each neighbor on the LAN.

   The following format is defined for this sub-TLV:

      Type:  42 (1 octet)

      Length:  variable (1 octet)

      Neighbor System ID:  6 octets

      Flags:  1-octet field of the following flags:

          0 1 2 3 4 5 6 7
         +-+-+-+-+-+-+-+-+
         |F|*|V|L|S|P|   |
         +-+-+-+-+-+-+-+-+

         where:

         F-Flag:  Address-Family Flag.  If unset, then the Adj-SID
                  refers to an L2 Bundle Member with outgoing IPv4
                  encapsulation.  If set, then the Adj-SID refers to an
                  L2 Bundle Member with outgoing IPv6 encapsulation.

         V-Flag:  Value Flag.  If set, then the Adj-SID carries a value.
                  By default, the flag is SET.

         L-Flag:  Local Flag.  If set, then the value/index carried by
                  the Adj-SID has local significance.  By default, the
                  flag is SET.

         S-Flag:  Set Flag.  When set, the S-Flag indicates that the
                  Adj-SID refers to a set of L2 Bundle Members (and
                  therefore MAY be assigned to other L2 Bundle Members
                  as well).

         P-Flag:  Persistent Flag.  When set, the P-Flag indicates that
                  the Adj-SID is persistently allocated, i.e., the Adj-
                  SID value remains consistent across router restart
                  and/or interface flap.

         Other bits:  MUST be zero when originated and ignored when
                  received.

         NOTE: The flags are deliberately kept congruent to the flags in
         the L3 LAN Adjacency SID defined in [RFC8667].  * indicates a
         flag used in the L3 Adj-SID sub-TLV, but one that is NOT used
         in this sub-TLV.  These bits SHOULD be sent as 0 and MUST be
         ignored on receipt.

      Weight:  1 octet.  The value represents the weight of the Adj-SID
               for the purpose of load balancing.  The use of the weight
               is defined in [RFC8402].

      NOTE: Flags and weight are shared by all L2 Bundle Members listed
      in the L2 Bundle Attribute Descriptor.

      L2 Bundle Member LAN Adjacency SID Descriptors:
         There MUST be one descriptor for each of the L2 Bundle Members
         advertised under the preceding L2 Bundle Member Attribute
         Descriptor.  Each descriptor consists of one of the following
         fields:

      SID/Index/Label:  According to the V- and L-Flags, it contains
         either:

         o  A 3-octet local label where the 20 rightmost bits are used
            for encoding the label value.  In this case, the V- and
            L-Flags MUST be set.

         o  A 4-octet index defining the offset in the SID/Label space
            advertised by this router.  See [RFC8667].  In this case, V-
            and L-Flags MUST be unset.

5.  IANA Considerations

   This document adds the following new TLV to the IS-IS "TLV Codepoints
   Registry".

   Value:  25

   Name:  L2 Bundle Member Attributes

   The name of the IANA registry for Sub-TLVs for TLVs 22, 23, 141, 222,
   and 223 has been changed to include sub-TLV 25.  An additional column
   has been added to the registry to indicate which sub-TLVs may appear
   in the new L2 Bundle Member Attributes TLV.  The column for TLV 25
   has one of the following three values:

   y     sub-TLV may appear in TLV 25 but MUST NOT be shared by multiple
         L2 Bundle Members

   y(s)  sub-TLV may appear in TLV 25 and MAY be shared by multiple L2
         Bundle Members

   n     sub-TLV MUST NOT appear in TLV 25

   The following table indicates the appropriate settings for all
   currently defined sub-TLVs with regard to their use in the new L2
   Bundle Member Attributes TLV.

      +-------+-------------------------------------------+--------+
      | Value | Description                               | TLV 25 |
      +=======+===========================================+========+
      | 3     | Administrative group (color)              | y(s)   |
      +-------+-------------------------------------------+--------+
      | 4     | Link Local/Remote Identifiers             | y(s)   |
      +-------+-------------------------------------------+--------+
      | 6     | IPv4 interface address                    | y(s)   |
      +-------+-------------------------------------------+--------+
      | 8     | IPv4 neighbor address                     | y(s)   |
      +-------+-------------------------------------------+--------+
      | 9     | Maximum link bandwidth                    | y(s)   |
      +-------+-------------------------------------------+--------+
      | 10    | Maximum reservable link bandwidth         | y(s)   |
      +-------+-------------------------------------------+--------+
      | 11    | Unreserved bandwidth                      | y(s)   |
      +-------+-------------------------------------------+--------+
      | 12    | IPv6 Interface Address                    | y(s)   |
      +-------+-------------------------------------------+--------+
      | 13    | IPv6 Neighbor Address                     | y(s)   |
      +-------+-------------------------------------------+--------+
      | 14    | Extended Administrative Group             | y(s)   |
      +-------+-------------------------------------------+--------+
      | 18    | TE Default metric                         | y(s)   |
      +-------+-------------------------------------------+--------+
      | 19    | Link-attributes                           | y(s)   |
      +-------+-------------------------------------------+--------+
      | 20    | Link Protection Type                      | y(s)   |
      +-------+-------------------------------------------+--------+
      | 21    | Interface Switching Capability Descriptor | y(s)   |
      +-------+-------------------------------------------+--------+
      | 22    | Bandwidth Constraints                     | y(s)   |
      +-------+-------------------------------------------+--------+
      | 23    | Unconstrained TE LSP Count (sub-)TLV      | y(s)   |
      +-------+-------------------------------------------+--------+
      | 24    | remote AS number                          | n      |
      +-------+-------------------------------------------+--------+
      | 25    | IPv4 remote ASBR Identifier               | n      |
      +-------+-------------------------------------------+--------+
      | 26    | IPv6 remote ASBR Identifier               | n      |
      +-------+-------------------------------------------+--------+
      | 27    | Interface Adjustment Capability           | y(s)   |
      |       | Descriptor (IACD)                         |        |
      +-------+-------------------------------------------+--------+
      | 28    | MTU                                       | n      |
      +-------+-------------------------------------------+--------+
      | 29    | SPB-Metric                                | y(s)   |
      +-------+-------------------------------------------+--------+
      | 30    | SPB-A-OALG                                | y(s)   |
      +-------+-------------------------------------------+--------+
      | 33    | Unidirectional Link Delay                 | y      |
      +-------+-------------------------------------------+--------+
      | 34    | Min/Max Unidirectional Link Delay         | y      |
      +-------+-------------------------------------------+--------+
      | 35    | Unidirectional Delay Variation            | y      |
      +-------+-------------------------------------------+--------+
      | 36    | Unidirectional Link Loss                  | y      |
      +-------+-------------------------------------------+--------+
      | 37    | Unidirectional Residual Bandwidth         | y      |
      +-------+-------------------------------------------+--------+
      | 38    | Unidirectional Available Bandwidth        | y      |
      +-------+-------------------------------------------+--------+
      | 39    | Unidirectional Utilized Bandwidth         | y      |
      +-------+-------------------------------------------+--------+
      | 40    | RTM Capability                            | n      |
      +-------+-------------------------------------------+--------+

                                 Table 1

   This document adds the following new sub-TLVs to the above registry.

   +------+--------------------------+----+----+----+-----+-----+-----+
   | Type | Description              | 22 | 23 | 25 | 141 | 222 | 223 |
   +======+==========================+====+====+====+=====+=====+=====+
   |  41  | L2 Bundle Member Adj-SID | n  | n  | y  |  n  |  n  |  n  |
   +------+--------------------------+----+----+----+-----+-----+-----+
   |  42  | L2 Bundle Member LAN     | n  | n  | y  |  n  |  n  |  n  |
   |      | Adj-SID                  |    |    |    |     |     |     |
   +------+--------------------------+----+----+----+-----+-----+-----+

                                 Table 2

6.  Security Considerations

   The IS-IS protocol has supported the advertisement of link attribute
   information, including link identifiers, for many years.  The
   advertisements defined in this document are identical to existing
   advertisements defined in [RFC4202], [RFC5305], [RFC8570], and
   [RFC8667], but are associated with L2 links that are part of a bundle
   interface on which the IS-IS protocol operates.  There are therefore
   no new security issues introduced by the extensions in this document.

   As always, if the protocol is used in an environment where
   unauthorized access to the physical links on which IS-IS Protocol
   Data Units (PDUs) are sent occurs, then attacks are possible.  The
   use of authentication as defined in [RFC5304] and [RFC5310] is
   recommended to prevent such attacks.

7.  References

7.1.  Normative References

   [IEEE802.1AX]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks -- Link Aggregation", IEEE 802.1AX,
              <https://ieeexplore.ieee.org/document/7055197>.

   [ISO10589] International Organization for Standardization,
              "Information technology -- Telecommunications and
              information exchange between systems -- Intermediate
              System to Intermediate System intra-domain routeing
              information exchange protocol for use in conjunction with
              the protocol for providing the connectionless-mode network
              service (ISO 8473)", ISO/IEC 10589:2002, Second Edition,
              November 2002.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4202]  Kompella, K., Ed. and Y. Rekhter, Ed., "Routing Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 4202, DOI 10.17487/RFC4202, October 2005,
              <https://www.rfc-editor.org/info/rfc4202>.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC5307]  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
              in Support of Generalized Multi-Protocol Label Switching
              (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
              <https://www.rfc-editor.org/info/rfc5307>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC6119]  Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
              Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
              February 2011, <https://www.rfc-editor.org/info/rfc6119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8570]  Ginsberg, L., Ed., Previdi, S., Ed., Giacalone, S., Ward,
              D., Drake, J., and Q. Wu, "IS-IS Traffic Engineering (TE)
              Metric Extensions", RFC 8570, DOI 10.17487/RFC8570, March
              2019, <https://www.rfc-editor.org/info/rfc8570>.

   [RFC8667]  Previdi, S., Ed., Ginsburg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

7.2.  Informative References

   [RFC4655]  Farrel, A., Vasseur, JP., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

Appendix A.  Example Encoding

   Below is an example encoding of L2 Bundle advertisements in a case
   where we have two parallel adjacencies to the same neighbor whose
   system-id is 1234.1234.1234.00.  The two L2 bundles have the
   following sets of attributes:

   L3 Adjacency #1

   L3 IPv4 local link address: 192.0.2.1

   Four bundle members with the following attributes:

           +-----+---------------+-----------+----------------+
           | Num | Link Local ID | Bandwidth | Adj-SID/Weight |
           +=====+===============+===========+================+
           |  1  |   0x11111111  |     1G    |   0x11111/1    |
           +-----+---------------+-----------+----------------+
           |  2  |   0x11112222  |     1G    |   0x11112/1    |
           +-----+---------------+-----------+----------------+
           |  3  |   0x11113333  |    10G    |   0x11113/1    |
           +-----+---------------+-----------+----------------+
           |  4  |   0x11114444  |    10G    |   0x11114/1    |
           +-----+---------------+-----------+----------------+

                                 Table 3

   L3 Adjacency #2

   L3 IPv4 local link address: 192.0.2.2

   Three bundle members with the following attributes:

           +-----+---------------+-----------+----------------+
           | Num | Link Local ID | Bandwidth | Adj-SID/Weight |
           +=====+===============+===========+================+
           |  1  |   0x22221111  |    10G    |    22221/1     |
           +-----+---------------+-----------+----------------+
           |  2  |   0x22222222  |    10G    |    22222/1     |
           +-----+---------------+-----------+----------------+
           |  3  |   0x22223333  |    10G    |    22223/1     |
           +-----+---------------+-----------+----------------+

                                 Table 4

   This requires two TLVs, one for each L3 adjacency.

   TLV for Adjacency #1:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(25)     | Length(64)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Parent L3 Neighbor Descriptor

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Neighbor System-ID octets 1-4: 1234.1234                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | System-ID octets 5-6: 1234    | P-node: 00    |1|0|0|0|0|0|0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv4 Interface Address Sub-TLV

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(6)      | Length(4)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | IPv4 address: 192.0.2.1                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L2 Bundle Attribute Descriptors

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Len:9+6+10 = 25| # Desc: 2     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Link Local Identifier Bundle Member #1: 0x11111111            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Link Local Identifier Bundle Member #2: 0x11112222            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Maximum Link Bandwidth Sub-TLV

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(9)       | Length(4)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Bandwidth Value: 1G/8                                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L2 Bundle Member Adj-SID Sub-TLV

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(41)     | Length(8)     |0|0|1|1|0|0|0|0| Weight: 1     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Local Label Bundle Member #1: 0x11111         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Local Label Bundle Member #2: 0x11112         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L2 Bundle Attribute Descriptors

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Len:9+6+10 = 25| # Desc: 2     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Link Local Identifier Bundle Member #3: 0x11113333            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Link Local Identifier Bundle Member #4: 0x11114444            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Maximum Link Bandwidth Sub-TLV

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(9)       | Length(4)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Bandwidth Value: 10G/8                                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L2 Bundle Member Adj-SID Sub-TLV

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(41)     | Length(8)     |0|0|1|1|0|0|0|0| Weight: 1     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Local Label Bundle Member #3: 0x11113         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Local Label Bundle Member #4: 0x11114         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   TLV for Adjacency #2:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(25)     | Length(46)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Parent L3 Neighbor Descriptor

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Neighbor System-ID octets 1-4: 1234.1234                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | System-ID octets 5-6: 1234    | P-node: 00    |1|0|0|0|0|0|0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   IPv4 Interface Address Sub-TLV

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(6)      | Length(4)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | IPv4 address: 192.0.2.2                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L2 Bundle Attribute Descriptors

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Len:13+6+13=32 | # Desc: 3     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Link Local Identifier Bundle Member #1: 0x22221111            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Link Local Identifier Bundle Member #2: 0x22222222            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Link Local Identifier Bundle Member #3: 0x22223333            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Maximum Link Bandwidth Sub-TLV

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(9)       | Length(4)    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Bandwidth Value: 10G/8                                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   L2 Bundle Member Adj-SID Sub-TLV

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Type(41)     | Length(11)    |0|0|1|1|0|0|0|0| Weight: 1     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Local Label Bundle Member #1: 0x22221         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Local Label Bundle Member #2: 0x22222         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Local Label Bundle Member #3: 0x22223         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Acknowledgements

   The authors would like to thank Jon Mitchell for his careful review.

Contributors

   The following people gave a substantial contribution to the content
   of this document and should be considered coauthors:

   Stefano Previdi
   Huawei Technologies
   Italy

   Email: stefano@previdi.net

Authors' Addresses

   Les Ginsberg (editor)
   Cisco Systems, Inc.

   Email: ginsberg@cisco.com


   Ahmed Bashandy
   Unaffiliated
   United States of America

   Email: abashandy.ietf@gmail.com


   Clarence Filsfils
   Cisco Systems, Inc.

   Email: cf@cisco.com


   Mohan Nanduri
   Oracle

   Email: mohan.nanduri@oracle.com


   Ebben Aries
   Arrcus Inc.
   2077 Gateway Place, Suite #400
   San Jose, CA 95119
   United States of America

   Email: exa@arrcus.com
