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RFC 9857 - BGP-LS Advertisement of SR Policies

Field Value
Title Advertisement of Segment Routing Policies Using BGP - Link State
Authors S. Previdi, K. Talaulikar (Ed.), J. Dong, H. Gredler, J. Tantsura
Date October 2025
Status Proposed Standard
Link datatracker.ietf.org/doc/rfc9857

Summary

RFC 9514 lets a controller learn SRv6 topology (locators, SIDs) through BGP-LS. RFC 9857 adds the other half: the state of SR Policies instantiated on headends. Each SR Policy candidate path — whether configured locally, pushed via PCEP, or pushed via the BGP SR Policy SAFI — can be reported with its Binding SID, state, constraints, and segment lists, for both SR-MPLS and SRv6.

This matters when the headend does not speak PCEP (management-based PCE), when a parent PCE needs a global view from child PCEs, or when a service-placement system or NMS needs to see which policies exist and whether they are up. It closes the loop for the BGP SR Policy SAFI (RFC 9830): controllers push policies with BGP and read their operational state back with BGP-LS.

The BGP-LS producer can be the headend itself or a PCE relaying what its PCCs report.

Key Sections

Section Topic Description
3 SR Policy Candidate Path NLRI New NLRI type 5 with Protocol-ID 9 ("Segment Routing")
4 Candidate Path Descriptor Endpoint, color, originator, discriminator, protocol-origin
5.1-5.2 Binding SID TLVs SR-MPLS BSID and SRv6 BSID with allocation/fallback flags
5.3-5.5 State and Names Candidate path state, policy name, candidate path name
5.6 Constraints Affinity, SRLG, bandwidth, disjointness, bidirectional, metric
5.7 Segment Lists Per-segment encoding (types A-K), metrics, bandwidth, identifiers

New Codepoints

Type Name
NLRI 5 SR Policy Candidate Path NLRI
Protocol-ID 9 Segment Routing
TLV 554 SR Policy Candidate Path Descriptor
TLV 1201 SR Binding SID (SR-MPLS)
TLV 1202 SR Candidate Path State
TLV 1203 / 1213 SR Candidate Path Name / SR Policy Name
TLV 1204 SR Candidate Path Constraints (sub-TLVs 1208-1211, 1214, 1215)
TLV 1205 / 1206 SR Segment List / SR Segment
TLV 1207 / 1216 / 1217 Segment List Metric / Bandwidth / Identifier
TLV 1212 SRv6 Binding SID

SRv6-Specific Pieces

Element Detail
SRv6 Binding SID TLV (1212) 128-bit BSID plus the specified BSID; flags B (allocated), U (specified BSID unavailable), F (fell back to dynamic)
SRv6 segment types Type B (SRv6 SID), Type I (End SID by node address), Types J/K (End.X SID by adjacency, global or link-local)
Behavior and structure The RFC 9514 SRv6 Endpoint Behavior TLV (1250) and SID Structure TLV (1252) can be attached as sub-TLVs to a BSID or a segment

Protocol-Origin Values

Value Meaning
1 / 2 / 3 PCEP / BGP SR Policy / Configuration (CLI, YANG, NETCONF) — reported by the headend
10 / 20 / 30 Same origins, reported by a PCE acting as the BGP-LS producer

Common Pitfalls

  • The NLRI reports state at candidate-path granularity; composite candidate paths are out of scope.
  • Large policy counts produce many BGP-LS updates. Check whether the platform lets you filter which policies are exported.
  • The Local Node Descriptor identifies the headend, even when a PCE is the producer — do not mistake it for the BGP-LS speaker.
  • Other TE path types (e.g., RSVP-TE LSPs) are handled by a separate draft, not this RFC.
Document Relationship Description
draft-ietf-idr-bgp-ls-sr-policy Draft lineage Draft that became RFC 9857
RFC 9830 Complement Advertising Segment Routing Policies in BGP (SR Policy SAFI)
RFC 9831 Complement Segment Type Extensions for BGP SR Policy (incl. SRv6 segment types)
RFC 9552 Base protocol BGP-LS
  • SR Policy - Candidate paths, BSIDs, and controller-driven TE
  • Telemetry - Other ways to collect SRv6 state
  • RFC 9514 - BGP-LS Extensions for SRv6
  • RFC 9603 - PCEP Extensions for SRv6
  • RFC 9256 - SR Policy Architecture

References

  1. RFC 9857 - Advertisement of Segment Routing Policies Using BGP - Link State
  2. RFC 9552 - Distribution of Link-State and TE Information Using BGP
  3. RFC 9256 - Segment Routing Policy Architecture
  4. RFC 9830 - Advertising Segment Routing Policies in BGP