



Link State Vector Routing Working Group                          J. Dong
Internet-Draft                                                   J. Chen
Intended status: Standards Track                     Huawei Technologies
Expires: 3 September 2026                                        S. Fang
                                                              Individual
                                                            2 March 2026


       Proposed Update to BGP Link-State SPF NLRI Selection Rules
                  draft-dong-lsvr-bgp-spf-selection-03

Abstract

   For network scenarios such as Massively Scaled Data Centers (MSDCs),
   BGP is extended for Link-State (LS) distribution and the Shortest
   Path First (SPF) algorithm based calculation.  BGP-LS-SPF leverages
   the mechanisms of both BGP protocol and BGP-LS protocol extensions,
   with new selection rules defined for BGP-LS-SPF NLRI.  This document
   proposes some updates to the BGP-LS-SPF NLRI selection rules, so as
   to improve the route updates and convergence, while consistent SPF
   computation result can still be achieved.  This document updates the
   NLRI selection rules in I-D.ietf-lsvr-bgp-spf.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 3 September 2026.

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   Copyright (c) 2026 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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   license-info) in effect on the date of publication of this document.



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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Network Scenarios Which Triggered This Update . . . . . . . .   2
     2.1.  Unnecessary Redundant Advertisement . . . . . . . . . . .   3
     2.2.  Parallal BGP-LS-SPF Peers . . . . . . . . . . . . . . . .   3
   3.  Update to BGP-LS-SPF Selection Rules  . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   For network scenarios such as Massively Scaled Data Centers (MSDCs),
   BGP is extended for Link-State (LS) distribution and the Shortest
   Path First (SPF) algorithm based calculation.  BGP-LS-SPF leverages
   the mechanisms of both BGP protocol and BGP-LS protocol extensions.
   For BGP-LS-SPF NLRIs, the selection rules for Phase 1 of the BGP
   Decision Process (see Section 9.1.1 of [RFC4271]) no longer apply.
   The selection rules for BGP-LS-SPF NLRI are defined in section 6.1 of
   [RFC9815].

   In some network scenarios, the selection rules defined in [RFC9815]
   for BGP-LS-SPF NLRI may not be enough to provide optimized route
   advertisement and convergence.  This document proposes some updates
   to the BGP-LS-SPF NLRI selection rules, so as to improve the route
   convergence while consistent SPF computation result can still be
   achieved.

   This document firstly describes the network scenarios in which the
   existing NLRI selection rules are considered not enough.  Then it
   provides suggested updates to the BGP-LS-SPF NLRI selection rules.

2.  Network Scenarios Which Triggered This Update







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2.1.  Unnecessary Redundant Advertisement

   According to the rules in [RFC9815], for the BGP-LS-SPF NLRIs with
   the same sequence number, the NLRI received from the numerically
   larger BGP ID is preferred.  While in some cases, this may cause
   unnecessary redundant advertisement of the same NLRI.

               +----+  new  +----+         +----+       +----+
               | R6 +-------+ R1 +---------+ R2 +-------+ R5 |
               +----+       +-+--+         +-+--+       +----+
                              |              |
                              |              |
                              |              |
                              |              |
                              |              |
                            +-+--+         +-+--+
                            | R3 +---------+ R4 |
                            +----+         +----+

   As shown in the example in Figure 2, a new BGP session is established
   between R1 and R6, and R1 advertise the link NLRI of R1-R6 to its
   neighboring nodes (R2 and R3).  R2 firstly receives the link NLRI
   R1-R6 from R1 directly, and advertise it further to its neighbors (R4
   and R5).  R4 receives the link NLRI of R1-R6 with the same sequence
   number from both R3 and R2, and according to the NLRI selection
   rules, R4 would prefer the NLRI received from R3 according to the
   rule of numerically larger BGP ID, then R4 advertises this link NLRI
   of R1-R6 to R2.  R2 would also prefer the NLRI received from R4
   according to the rule of numerically larger BGP ID, and further
   advertises this link NLRI to R5, which is a redundant advertisement
   of its previous advertisement of the same link NLRI.

2.2.  Parallal BGP-LS-SPF Peers

   In some scenarios, BGP single-hop peering model is used between
   directly connected BGP nodes.  When two or more parallel links exists
   between the BGP nodes, multiple BGP sessions are established between
   the peering nodes, and each session will be used for the distribution
   of BGP-LS-SPF NLRIs.

                            parallel BGP sessions

               +----+       +----+         +----+       +----+
               |    |       |    +---------+    |       |    |
               | R3 +-------+ R1 +---------+ R2 +-------+ R4 |
               +----+       +-+--+         +-+--+       +----+





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   As shown in the example of Figure 3, there are two parallel links
   between R1 and R2, and a separate BGP session is established on each
   link.  Based on the existing BGP-LS-SPF NLRI selection rules, from
   R2's perspective, for the same NLRI with the same sequence number,
   either the route received from peer R1.1, or the route received from
   peer R1.2 may be selected as the best.  No matter what tie-breaking
   rule is used, depending on the order of the routes received from R1,
   R2 may need to advertise duplicated NLRIs to R4.

3.  Update to BGP-LS-SPF Selection Rules

   As the BGP-LS-SPF NLRIs are used to distribute the link-state
   information, which are then used for the SPF computation, BGP
   attributes which are used in BGP best path selection (e.g. AS_PATH)
   for BGP address families other than BGP-LS are not considered in the
   computation of BGP-LS-SPF.  The consistency of BGP-LS-SPF computation
   result only relies on the sequence number associated with the BGP-LS-
   SPF NLRIs.  For network scenarios where optimized route convergence
   is more desirable, route updates due to the changes in BGP attributes
   which are not considered in the SPF computation, although may help to
   achieve deterministic NLRI selection, is considered not quite
   necessary.

   Thus this document proposes to update the selection rules for BGP-LS-
   SPF NLRI by adding the below rule 3 (b) in front of the rule 4 in
   section 6.1 of [RFC9815]:

      1.  NLRIs self-originated from directly connected BGP SPF peers
      are preferred.  This condition can be determined by comparing the
      BGP Identifiers in the received Local Node Descriptor and the BGP
      OPEN message for an active BGP session.  This rule assures that a
      stale NLRI is updated even if a BGP SPF router loses its sequence
      number state due to a cold start.  Note that once the BGP session
      goes down, the NLRI received is no longer considered as being from
      a directly connected BGP SPF peer.

      2.  Consistent with base BGP [RFC4271], an NLRI received from a
      peer will always replace the same NLRI received from that peer.
      Coupled with rule #1, this will ensure that any stale NLRI in the
      BGP SPF routing domain will be updated.

      3.  The NLRI with the most recent Sequence Number TLV, i.e., the
      highest sequence number is selected.

      3 (b).  The NLRI received from a BGP-LS-SPF peer with the same
      sequence number as the one of the current selected NLRI SHOULD not
      be selected.




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      4.The NLRI received from the BGP speaker with the numerically
      larger BGP Identifier is preferred.

   The rule 3 (b) can help to solve the duplicated advertisement problem
   as described in section 2.

4.  IANA Considerations

   This document makes no request of IANA.

5.  Security Considerations

   The mechanism described in this document provide updates to the NLRI
   selection rules for BGP-LS-SPF.  It does not introduce any additional
   security considerations than those described in [RFC4271] and
   [RFC4272].

6.  Acknowledgements

   The authors would like to thank Haibo Wang, Jun Ge and Li Zhang for
   the valuable discussion and suggestions.

7.  References

7.1.  Normative References

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC9815]  Patel, K., Lindem, A., Zandi, S., and W. Henderickx, "BGP
              Link State (BGP-LS) Shortest Path First (SPF) Routing",
              RFC 9815, DOI 10.17487/RFC9815, July 2025,
              <https://www.rfc-editor.org/info/rfc9815>.

7.2.  Informative References

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,
              <https://www.rfc-editor.org/info/rfc4272>.

Authors' Addresses

   Jie Dong
   Huawei Technologies
   China
   Email: jie.dong@huawei.com



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   Jinqiang Chen
   Huawei Technologies
   China
   Email: chenjinqiang@huawei.com


   Sheng Fang
   Individual
   China
   Email: fang_sheng@163.com









































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