[Idr] draft-lp-idr-bgp-ls-sr-policy-supplement - Interaction between "pure backup path" and the reverse path segment

Susan Hares <shares@ndzh.com> Fri, 07 November 2025 17:28 UTC

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Subject: [Idr] draft-lp-idr-bgp-ls-sr-policy-supplement - Interaction between "pure backup path" and the reverse path segment
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Question:
How does the B-Flag interact on the segment list with
the concepts of the reverse path segment from
draft-ietf-idr-sr-policy-path-segement-14.

See discussions below.

Sue

draft-ietf-idr-sr-policy-path-segment-14 defines

This provides a forward and reverse path.


       SR Policy SAFI NLRI: <Distinguisher, Policy-Color, Endpoint>
           Attributes: Tunnel Encaps Attribute (23)
           Tunnel Type: SR Policy
               Binding SID
               Preference
               Priority
               Policy Name
               Explicit NULL Label Policy (ENLP)
               Segment List
                   Weight
                   Path Segment
                   Segment
                   Segment
                   ...
                   Reverse Segment List
                       Path Segment
                       Segment
                       Segment
                       ...


draft-lp-idr-bgp-ls-sr-policy-supplement-03

Defines new flags for the Segment list:

                       0                   1
                         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        | | | | | | | | | |S|B|         |
                        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 1: New Flags in the Flag Field of SR Segment List TLV

   *  S-Flag: Indicates the segment list is in administrative shut state
      when set.  The segment list may be shut by the administrator via
      CLI or other methods, and it is out of the scope of this document.

   *  B-Flag: Indicates that the segment list is a pure backup path as
      specified in [I-D.ietf-pce-multipath] section 4.4 when set.  When
      B-Flag is clear, it indicates it is the primary path that carries
      normal traffic


>From draft-ietf-pce-multipath, we have two definitions:
MULTIPATH-BACKUP TLV and MULTIPATH-OPPDIR-PATH.

Below is the text from draft-ietf-pce-multipath

   This functionality is not part of the SR Policy Architecture
   [RFC9256], but is something optional that may be implemented for
   certain specialized use cases.  One such use case is the P2MP SR
   Policy [I-D.draft-ietf-pce-sr-p2mp-policy].

      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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Backup Path Count       |             Flags           |B|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID 1                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID 2                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Backup Path ID n                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 3: MULTIPATH-BACKUP TLV format

>From draft-ietf-pce-multipath ha the following opposite direction 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              |             Length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Reserved            |             Flags         |L|N|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Opposite Direction Path ID                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 4: MULTIPATH-OPPDIR-PATH TLV format

[Idr] draft-lp-idr-bgp-ls-sr-policy-supplement - … Susan Hares
[Idr] Re: draft-lp-idr-bgp-ls-sr-policy-supplemen… liu.yao71