Segment Routing BGP Egress Peer Engineering over Layer 2 Bundle Members
draft-ietf-idr-sr-epe-over-l2bundle-00

IDR Working Group                                                C. Lin
Internet Draft                                     New H3C Technologies
Updates: 9085, 9086 (if approved)                                 Z. Li
Intended status: Standards Track                           China Mobile
Expires: March 02, 2025                                         R. Pang
                                                           China Unicom
                                                          K. Talaulikar
                                                          Cisco Systems
                                                                M. Chen
                                                   New H3C Technologies
                                                     September 02, 2024

      Segment Routing BGP Egress Peer Engineering over Layer 2 Bundle
                                 Members
                  draft-ietf-idr-sr-epe-over-l2bundle-00

Abstract

   There are deployments where the Layer 3 interface on which a BGP
   peer session is established is a Layer 2 interface bundle. In order
   to allow BGP-EPE to control traffic flows on individual member links
   of the underlying Layer 2 bundle, BGP Peering SIDs need to be
   allocated to individual bundle member links, and advertisement of
   such BGP Peering SIDs in BGP-LS is required. This document describes
   how to support Segment Routing BGP Egress Peer Engineering over
   Layer 2 bundle members. This document updates [RFC9085] to allow the
   L2 Bundle Member Attributes TLV to be added to the BGP-LS Attribute
   associated with the Link NLRI of BGP peering link. This document
   updates [RFC9085] and [RFC9086] to allow the PeerAdj SID TLV to be
   included as a sub-TLV of the L2 Bundle Member Attributes TLV.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on March 02, 2025.

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Copyright Notice

   Copyright (c) 2024 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
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   warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction...................................................3
      1.1. Requirements Language.....................................4
   2. Problem Statement..............................................4
   3. Advertising Peer Adjacency Segment for L2 Bundle Member in BGP-LS
   ..................................................................5
      3.1. SR-MPLS...................................................5
      3.2. SRv6......................................................6
   4. Manageability Considerations...................................7
   5. MC-LAG Bundles Considerations..................................7
   6. Security Considerations........................................8
   7. IANA Considerations............................................8
   8. References.....................................................8
      8.1. Normative References......................................8
      8.2. Informative References....................................9
   Appendix A. Example...............................................9
   Acknowledgements.................................................11
   Authors' Addresses...............................................11

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1. Introduction

   Segment Routing (SR) leverages the source routing paradigm.  A node
   steers a packet through an ordered list of instructions called
   "segments". Segment Routing can be instantiated on both MPLS and
   IPv6 data planes, which are referred to as SR-MPLS and SRv6.

   BGP Egress Peer Engineering (BGP-EPE) allows an ingress Provider
   Edge (PE) router within the domain to use a specific egress PE and a
   specific external interface/neighbor to reach a particular
   destination.

   The SR architecture [RFC8402] defines three types of BGP Peering
   Segments that may be instantiated at a BGP node:

   o Peer Node Segment (PeerNode SID): instruction to steer to a
      specific peer node

   o Peer Adjacency Segment (PeerAdj SID): instruction to steer over a
      specific local interface towards a specific peer node

   o Peer Set Segment (PeerSet SID): instruction to load-balance to a
      set of specific peer nodes

   [RFC9087] illustrates a centralized controller-based BGP-EPE
   solution involving SR path computation using the BGP Peering
   Segments. A centralized controller learns the BGP Peering SIDs via
   Border Gateway Protocol - Link State (BGP-LS) and then uses this
   information to program a BGP-EPE policy. [RFC9086] defines the
   extension to BGP-LS for advertisement of BGP Peering Segments along
   with their BGP peering node information.

   There are deployments where the Layer 3 interface on which a BGP
   peer session is established is a Layer 2 interface bundle (L2
   Bundle), for instance, a Link Aggregation Group (LAG) [IEEE802.1AX].
   BGP-EPE may wish to control traffic flows on individual member links
   of the underlying Layer 2 bundle. In order to do so, BGP Peering
   SIDs need to be allocated to individual bundle member links, and
   advertisement of such BGP Peering SIDs in BGP-LS is required.

   This document describes how to support Segment Routing BGP Egress
   Peer Engineering over Layer 2 bundle members.

   This document updates [RFC9085] to allow the L2 Bundle Member
   Attributes TLV to be added to the BGP-LS Attribute associated with
   the Link NLRI of BGP peering link. This document updates [RFC9085]
   and [RFC9086] to allow the PeerAdj SID TLV to be included as a sub-
   TLV of the L2 Bundle Member Attributes TLV.

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1.1. 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.

2. Problem Statement

   In the network depicted in Figure 1, B and C establish BGP peer
   session on a Layer 2 bundle. Assume that, the member link 1 has the
   largest available bandwidth. The operator of AS1 wishes to apply a
   BGP-EPE policy to steer certain flows from AS1 to AS2 via member
   link 1 of the Layer 2 bundle to ensure there is no over-
   subscription.

                    L2 Bundle      +--------+
                 /---member 1---\  |        |
               --+---member 2---+--C   AS2  |
   +--------+ /  \---member 3---/  |        |
   |        |/                     +--------+
   A   AS1  B
   |        |\                     +--------+
   +--------+ \                    |        |
               --------------------D   AS3  |
                                   |        |
                                   +--------+

   Figure 1: BGP-EPE over L2 Bundle

   The existing Peer Adjacency SID can be allocated to the Layer 3
   interface between B and C, which is a Layer 2 interface bundle. If
   steered by that Peer Adjacency SID, the traffic will be forwarded by
   load balancing among all the bundle member links. So, the existing
   mechanism cannot meet the requirement of steering traffic flows via
   individual member link.

   In order to support BGP Egress Peer Engineering over Layer 2 bundle
   members, a BGP router needs to have the ability to assign Peer
   Adjacency Segments for member links. And, the Peer Adjacency
   Segments of bundle members need to be advertised in BGP-LS, which
   will be specified in this document.

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3. Advertising Peer Adjacency Segment for L2 Bundle Member in BGP-LS

   BGP peering segments are generally advertised in BGP-LS from a BGP
   node along with its peering topology information, in order to enable
   computation of BGP-EPE policies.

   When a BGP peer session is established over a Layer 2 interface
   bundle, an implementation MAY allocate one or more Peer Adjacency
   Segments for each member link. If so, it SHOULD advertise the Peer
   Adjacency Segments of bundle members in BGP-LS, using the method
   defined in this section.

   In order to advertise the EPE Peer Adjacency SIDs for L2 bundle
   members in BGP-LS, the L2 Bundle Member Attributes TLVs [RFC9085]
   MUST also be included in the Link Attributes for the BGP-LS Link
   NLRI corresponding to the BGP peering session.

   Section 2.2 of [RFC9085] restricted that the L2 Bundle Member
   Attributes TLV "should only be added to the BGP-LS Attribute
   associated with the Link NLRI that describes the link of the IGP
   node". This document updates [RFC9085] to allow the L2 Bundle Member
   Attributes TLV to be added to the BGP-LS Attribute associated with
   the Link NLRI of BGP peering link.

   Each L2 Bundle Member Attributes TLV identifies an L2 bundle member,
   and includes the EPE Peer Adjacency SID for the associated L2 bundle
   member.

   Note that the inclusion of a L2 Bundle Member Attributes TLV implies
   that the identified link is a member of the L2 bundle and that the
   member link is operationally up. If any member link fails, an
   implementation MUST withdraw the L2 Bundle Member Attributes TLV in
   BGP-LS, along with the Peer Adjacency Segments for the failed member
   link.

3.1. SR-MPLS

   For SR-MPLS, Section 5 of [RFC9086] defined the PeerAdj SID TLV and
   its usage for the BGP-LS advertisement of the BGP-EPE PeerAdj SID
   for L3 link. When advertising the SR-MPLS BGP-EPE Peer Adjacency
   SIDs for L2 bundle members, the PeerAdj SID TLV [RFC9086] MUST be
   carried in the L2 Bundle Member Attributes TLV to advertise the SR-
   MPLS Peer Adjacency SID for the associated L2 bundle member. This
   document updates [RFC9085] and [RFC9086] to allow the PeerAdj SID
   TLV to be included as a sub-TLV of the L2 Bundle Member Attributes
   TLV.

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   When advertising SR-MPLS BGP-EPE Peer Adjacency SIDs for L2 bundle
   members, since L2 bundle information is considered a Layer 3 link
   attribute, it must be advertised in the BGP-LS Link NLRI. The
   details for LINK NLRI are the same as those for the PeerAdj SID, as
   described in Section 5.2 of [RFC9086]. This information mustnot be
   included in the BGP-LS Link NLRI that corresponds to the PeerNode
   SID, as defined in Section 5.1 of [RFC9086].

   Note that for directly connected EBGP neighbors, if a BGP neighbor
   is established over an L2 Bundle, an additional BGP-LS Link NLRI(as
   described in Section 5.2 of [RFC9086]) must be generated to
   advertise Peer Link information when generating the BGP-LS Link NLRI
   (as described in Section 5.1 of [RFC9086]) corresponding to the
   PeerNode SID. The L2 Bundle Member Attributes TLV should be included
   under the BGP-LS Link Attribute TLVs.

   The SR-MPLS BGP-EPE Peer Adjacency SIDs for L2 bundle members are
   advertised with a BGP-LS Link NLRI, where:

   *  BGP-LS Link NLRI: as described in Section 5.2 of [RFC9086].

   *  Link Attribute TLVs:

       - include the PeerAdj SID TLV [RFC9086] for Peer Link(Optional)

       - include the L2 Bundle Member Attributes TLV.

        o include the PeerAdj SID TLV [RFC9086] for each L2 Bundle

          Member.

3.2. SRv6

   For SRv6, according to Section 4.1 of [RFC9514], the SRv6 End.X SID
   TLV is used for the advertisement of L3 link BGP EPE Peer Adjacency
   SID. When advertising the SRv6 BGP-EPE Peer Adjacency SIDs for L2
   bundle members, the SRv6 End.X SID TLV [RFC9514] MUST be carried in
   the L2 Bundle Member Attributes TLV to advertise the SRv6 Peer
   Adjacency SID for the associated L2 bundle member.

   Note Appendix A of [RFC 9514], SRv6 BGP PeerNode is no longer
   advertised as BGP LINK NLRI. When advertising SRv6 BGP-EPE Peer
   Adjacency SIDs for L2 bundle members, since L2 bundle information is
   considered a Layer 3 link attribute, it must be advertised in the
   BGP-LS Link NLRI. The details for LINK NLRI are the same as those
   for the Peer Adjacency SID, as described in Section 5.2 of
   [RFC9086].

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   The SRv6 BGP-EPE Peer Adjacency SIDs for L2 bundle members are
   advertised with a BGP-LS Link NLRI, where:

   *  BGP-LS Link NLRI: as described in Section 5.2 of [RFC9086].

   *  Link Attribute TLV:

       - include the SRv6 End.X SID TLV [RFC9514] for Peer

        Link (Optional).

       - include the L2 Bundle Member Attributes TLV.

        o include the SRv6 End.X SID TLV [RFC9514] for each L2 Bundle

         Member.

4. Manageability Considerations

   The manageability considerations described in [RFC9552] and
   [RFC9086] also apply to this document.

   The operator MUST be provided with the options of configuring,
   enabling, and disabling the advertisement of Peer Adjacency Segment
   for L2 Bundle member links, as well as control of which information
   is advertised to which internal or external peer.

5. MC-LAG Bundles Considerations

   In environments where MC-LAG (Multi-Chassis Link Aggregation Group)
   bundles are deployed across multiple devices, it is critical to
   implement mechanisms to prevent Broadcast, Unknown Unicast, and
   Multicast (BUM) traffic from looping and ensure a loop-free network.
   The following loop prevention mechanisms are included:

   o Split Horizon Forwarding: Each MC-LAG device maintains a split
      horizon rule where it does not forward BUM traffic received from
      one MC-LAG member port to another MC-LAG member port. This
      prevents BUM frames from being forwarded back into the MC-LAG,
      creating loops.

   o Designated Forwarder Election: In a typical MC-LAG configuration,
      one device is elected as the designated forwarder for BUM
      traffic. This ensures that only one device is responsible for
      forwarding BUM frames, preventing the possibility of multiple
      devices forwarding the same frame simultaneously and causing a
      loop.

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   o Consistent Hashing Algorithms: MC-LAG devices employ consistent
      hashing algorithms to ensure that traffic distribution across
      member links is stable and predictable. This minimizes the risk
      of reordering and helps in effective loop prevention.

   By incorporating these mechanisms, MC-LAG deployments can
   effectively prevent BUM traffic from looping and ensure a stable,
   loop-free network.

6. Security Considerations

   The security considerations described in [RFC9552] and [RFC9086]
   also apply to this document.

   This document does not introduce any new security consideration.

7. IANA Considerations

   This document has no IANA actions.

8. References

8.1. Normative References

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

   [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>.

   [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>.

   [RFC9085] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
             H., and M. Chen, "Border Gateway Protocol - Link State
             (BGP-LS) Extensions for Segment Routing", RFC 9085, DOI
             10.17487/RFC9085, August 2021, <https://www.rfc-
             editor.org/info/rfc9085>.

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   [RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
             Ray, S., and J. Dong, "Border Gateway Protocol - Link
             State (BGP-LS) Extensions for Segment Routing BGP Egress
             Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
             2021, <https://www.rfc-editor.org/info/rfc9086>.

   [RFC9514] Dawra, G., Filsfils, C., Talaulikar, K., Ed., Chen, M.,
             Bernier, D., and B. Decraene, "Border Gateway Protocol -
             Link State (BGP-LS) Extensions for Segment Routing over
             IPv6 (SRv6)", RFC 9514, DOI 10.17487/RFC9514, December
             2023, <https://www.rfc-editor.org/info/rfc9514>.

   [RFC9552] K. Talaulikar, "Distribution of Link-State and Traffic
             Engineering Information Using BGP", RFC 9552, DOI
             10.17487/RFC9552, December 2023, <https://www.rfc-
             editor.org/info/rfc9552>.

8.2. Informative References

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

   [RFC8668] Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri,
             M., and E. Aries, "Advertising Layer 2 Bundle Member Link
             Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
             December 2019, <https://www.rfc-editor.org/info/rfc8668>.

   [RFC9087] Filsfils, C., Ed., Previdi, S., Dawra, G., Ed., Aries, E.,
             and D. Afanasiev, "Segment Routing Centralized BGP Egress
             Peer Engineering", RFC 9087, DOI 10.17487/RFC9087, August
             2021, <https://www.rfc-editor.org/info/rfc9087>.

Appendix A. Example

   This section shows an example of how Node B in Figure 1 allocates
   and advertises Peer Adjacency Segments for L2 bundle members.

   B allocates a PeerAdj SID for the Layer 2 interface bundle to peer
   C, along with a PeerAdj SID for each member link. B programs its
   forwarding table accordingly:

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   +===============================+====================+
   |          PeerAdj SID          | Outgoing Interface |
   +---------------+---------------+                    |
   | IF on SR-MPLS |  IF on SRv6   |                    |
   |   Data Plane  |  Data Plane   |                    |
   +===============+===============+====================+
   |     1010      |     A::A0     | L2 Bundle to C     |
   +---------------+---------------+--------------------+
   |     1011      |     A::A1     | Member link 1 to C |
   +---------------+---------------+--------------------+
   |     1012      |     A::A2     | Member link 2 to C |
   +---------------+---------------+--------------------+
   |     1013      |     A::A3     | Member link 3 to C |
   +---------------+---------------+--------------------+

   B signals the related BGP-LS Link NLRI and Link Attributes including
   the PeerAdj SID for L3 parent link to the BGP-EPE controller, as
   specified in Section 5.2 of [RFC9086]. In addition, B also
   advertises L2 Bundle Member Attribute TLVs carrying the PeerAdj SIDs
   for L2 bundle members.

   For SR-MPLS, the Link Attributes are as follows:

   o PeerAdj SID TLV (Label-1010)

   o L2 Bundle Member Attribute TLV (Link Local Identifier describing
      the member link 1)

      * PeerAdj SID TLV (Label-1011)

   o L2 Bundle Member Attribute TLV (Link Local Identifier describing
      the member link 2)

      * PeerAdj SID TLV (Label-1012)

   o L2 Bundle Member Attribute TLV (Link Local Identifier describing
      the member link 3)

      * PeerAdj SID TLV (Label-1013)

   For SRv6, the Link Attributes are as follows:

   o SRv6 End.X SID TLV (SID-A::A0)

   o L2 Bundle Member Attribute TLV (Link Local Identifier describing
      the member link 1)

      * SRv6 End.X SID TLV (SID-A::A1)

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   o L2 Bundle Member Attribute TLV (Link Local Identifier describing
      the member link 2)

      * SRv6 End.X SID TLV (SID-A::A2)

   o L2 Bundle Member Attribute TLV (Link Local Identifier describing
      the member link 3)

      * SRv6 End.X SID TLV (SID-A::A3)

Acknowledgements

   Many thanks to Sasha Vainshtein, Acee Lindem, Chen Ran, Liyan Gong,
   Yongqing Zhu, Lan cheng, Wisdom Tan, Yisong Liu, Libin Liu, Liu Yao,
   Hongwei Li, Allan Michael, Huo Pengfei, Gyan Mishra, Dong Jie, Meng
   Liu, etc. for their valuable comments on this document.

Authors' Addresses

   Changwang Lin
   New H3C Technologies
   China
   Email: linchangwang.04414@h3c.com

   Zhenqiang Li
   China Mobile
   China
   Email: lizhenqiang@chinamobile.com

   Ran Pang
   China Unicom
   China
   Email: pangran@chinaunicom.cn

   Ketan Talaulikar
   Cisco Systems
   India
   Email: ketant.ietf@gmail.com

   Mengxiao Chen
   New H3C Technologies
   China
   Email: chen.mengxiao@h3c.com

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