



lsvr                                                          J. Xu, Ed.
Internet-Draft                                               L. Zhu, Ed.
Intended status: Standards Track                             C. Wen, Ed.
Expires: 30 August 2026                                     China Unicom
                                                        26 February 2026


      BGP-LS-SPF Extensions for SRv6 Policy State Synchronization
                draft-xu-bgp-ls-spf-srv6-policy-state-00

Abstract

   This document defines extensions to BGP-LS-SPF (BGP Link State
   Shortest Path First) to support synchronization of Segment Routing
   over IPv6 (SRv6) policy state information.  It introduces a new
   optional Sub-TLV for the SRv6 End.X SID TLV that indicates whether an
   SRv6 SID is currently active in an SR policy.  This enables dynamic
   interaction between SR policies and BGP-LS-SPF route computation,
   improving network responsiveness to policy changes.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 30 August 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.










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   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 Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  SRv6 Policy State Sub-TLV . . . . . . . . . . . . . . . . . .   3
     2.1.  Sub-TLV Format  . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Flags Field Description . . . . . . . . . . . . . . . . .   3
   3.  Operational Considerations  . . . . . . . . . . . . . . . . .   4
     3.1.  Policy State Advertisement  . . . . . . . . . . . . . . .   4
     3.2.  BGP-LS-SPF Route Computation  . . . . . . . . . . . . . .   4
     3.3.  Backward Compatibility  . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   BGP-LS-SPF [RFC9815] extends BGP for Link-State (LS) distribution and
   SPF-based route computation.  [RFC9514] defines BGP-LS extensions for
   Segment Routing over IPv6 (SRv6), including the SRv6 End.X SID TLV
   used to advertise SRv6 SIDs associated with network adjacencies.

   In network deployments using BGP-LS-SPF as the underlay routing
   protocol, there is a need for dynamic interaction between SR policies
   and the SPF computation process.  Currently, BGP-LS-SPF can
   distribute SRv6 SID information, but it lacks the ability to indicate
   whether a particular SID is actively being used in an SR policy.

   This document defines a new optional Sub-TLV for the SRv6 End.X SID
   TLV that carries policy state information.  This enables BGP-LS-SPF
   speakers to synchronize the activation state of SRv6 SIDs relative to
   SR policies, facilitating more intelligent route computation that
   considers current policy utilization, thereby enabling more flexible
   traffic steering, load balancing, and other policy implementations.

   The key benefits of this extension include:





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   *  Improved convergence during policy changes by providing real-time
      visibility into SID activation states.

   *  Enhanced traffic engineering capabilities through policy-aware
      path selection in the SPF computation.

   *  Backward compatibility through optional Sub-TLV implementation.

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.  SRv6 Policy State Sub-TLV

2.1.  Sub-TLV Format

   This document defines a new optional Sub-TLV for the SRv6 End.X SID
   TLV [RFC9514]: the SRv6 Policy State Sub-TLV.  The Sub-TLV provides a
   mechanism to indicate whether an SRv6 SID is currently active in one
   or more SR policies.

    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        |    Length     |P|R|R|R|R|R|R|R|   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 1: SRv6 Policy State Sub-TLV

   where:

   Type: 1 octet.  TBA.

   Length: 1 octet.  Value 2.

   Flags: 1 octet of flags.

   Reserved: 1 octet.  MUST be set to zero on transmission and MUST be
   ignored on receipt [RFC2119].

2.2.  Flags Field Description






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

             Figure 2: Flags Field of SRv6 Policy State Sub-TLV

   The Flags field contains the following bits:

   P flag (Policy Active) : Bit 0.  When set to 1, indicates that the
   SRv6 SID is currently active in one or more SR policies.  When set to
   0, indicates that the SID is not active in any SR policy or the
   policy state is unknown.

   R flags (Reserved) : Bits 1-7.  Reserved for future use.  SHOULD be
   set to zero on transmission and SHOULD be ignored on receipt.

3.  Operational Considerations

3.1.  Policy State Advertisement

   The SRv6 Policy State Sub-TLV is included in the SRv6 End.X SID TLV
   when the advertising node has knowledge of the SR policy state for
   the corresponding SID.  The Sub-TLV is optional and MAY be included
   or omitted based on local policy and implementation capability.

   When the policy state of an SRv6 SID changes (e.g., an SR policy
   activating or deactivating the SID), the advertising node SHOULD
   update the corresponding SRv6 End.X SID TLV with the current policy
   state information.  This update follows normal BGP-LS-SPF
   advertisement procedures.

   The absence of the SRv6 Policy State Sub-TLV in an SRv6 End.X SID TLV
   indicates that policy state information is not available for that
   SID.  Receiving nodes MUST NOT make assumptions about the policy
   state when the Sub-TLV is absent.

3.2.  BGP-LS-SPF Route Computation

   BGP-LS-SPF implementations that support this extension MAY use the
   policy state information during SPF computation to prefer paths
   containing active SR policy SIDs.  The specific algorithm for
   incorporating policy state into route computation is implementation-
   dependent and subject to local policy.

   Example behavior might include:





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   *  Preferring paths with SIDs marked as active (P-flag set) over
      paths with inactive SIDs when multiple equal-cost paths exist.

   *  Triggering SPF recalculation when policy state changes affect
      currently used paths.

   Implementations that do not support this extension will ignore the
   Sub-TLV and continue with normal SPF computation, maintaining
   backward compatibility.

3.3.  Backward Compatibility

   The SRv6 Policy State Sub-TLV is optional.  Implementations that do
   not support this extension will silently ignore the Sub-TLV as per
   the rules for handling unknown TLVs in [RFC9514].  This ensures
   backward compatibility with existing BGP-LS-SPF implementations.

   Nodes that support this extension but do not receive the Sub-TLV for
   a particular SID MUST proceed with normal SPF computation without
   policy state information for that SID.

4.  Security Considerations

   This extension introduces a new Sub-TLV that carries policy state
   information.  The security considerations for BGP-LS [RFC9552] and
   BGP-LS-SPF [RFC9815] apply to this document.

   Additionally, the following considerations apply:

   *  False policy state advertisements could lead to suboptimal routing
      or traffic blackholing.  Implementations SHOULD apply appropriate
      validation to policy state information when possible.

   *  The policy state information may reveal details about network
      operational policies.  In environments where this information is
      sensitive, implementations SHOULD consider filtering policy state
      advertisements to unauthorized peers.

   *  The mechanisms described in [RFC5925] for securing BGP sessions
      SHOULD be used to protect the integrity and authenticity of policy
      state advertisements.

5.  IANA Considerations

   IANA is requested to assign a new value in the "BGP-LS SRv6 End.X SID
   TLV Sub-TLVs" registry under the "Border Gateway Protocol (BGP)
   Parameters" group.




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   The requested assignment is: Value: TBA Description: SRv6 Policy
   State

6.  Normative References

   [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>.

   [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>.

   [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>.

Authors' Addresses

   Jie Xu (editor)
   China Unicom
   Beijing
   China
   Email: xuj556@chinaunicom.cn


   Lin Zhu (editor)
   China Unicom
   Beijing
   China
   Email: zhul14@chinaunicom.cn


   Chenyang Wen (editor)
   China Unicom
   Beijing
   China
   Email: wency15@chinaunicom.cn





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