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RFC 9513 - OSPFv3 Extensions for SRv6

Field Value
Title OSPFv3 Extensions for Segment Routing over IPv6 (SRv6)
Authors Z. Li, Z. Hu, K. Talaulikar, P. Psenak
Date December 2023
Status Proposed Standard
Link datatracker.ietf.org/doc/rfc9513

Summary

RFC 9513 defines the OSPFv3 extensions required to support Segment Routing over IPv6 (SRv6). While IS-IS SRv6 extensions were defined in RFC 9352, many networks rely on OSPFv3 as their IPv6 link-state routing protocol, and this document provides the equivalent functionality for OSPF-based deployments.

The specification introduces new TLVs and sub-TLVs within OSPFv3 Extended LSAs to advertise SRv6 capabilities, SRv6 Locators, SRv6 End SIDs, and SRv6 Adjacency SIDs. It builds on the OSPFv3 Extended LSA framework defined in RFC 8362, which provides a flexible TLV-based encoding for OSPFv3 advertisements.

The extensions enable OSPFv3 to distribute all the information needed for SRv6 forwarding: node capabilities, locator prefixes, endpoint behaviors (SID functions), and adjacency SIDs for traffic engineering. Combined with Flex-Algo support (RFC 9350), these extensions allow OSPFv3 to fully participate in SRv6 network deployments.

Key Sections

Section Topic Description
3 SRv6 Capabilities TLV Advertising node-level SRv6 support and flags
4 SRv6 Locator TLV Advertising SRv6 locator prefixes in the E-Router-LSA
5 SRv6 End SID sub-TLV Associating endpoint behaviors with locators
6 SRv6 Adjacency SID sub-TLV Advertising per-link SRv6 adjacency SIDs
7 SRv6 SID Structure sub-TLV Describing the LOC:FUNC:ARG bit-length structure of a SID

New TLVs and Sub-TLVs

TLV/Sub-TLV Type Carried In Purpose
SRv6 Capabilities TLV Router Information LSA Advertise SRv6 support and capability flags
SRv6 Locator TLV E-Router-LSA Advertise an SRv6 Locator prefix with algorithm and metric
SRv6 End SID Sub-TLV SRv6 Locator TLV Bind an SRv6 endpoint behavior (e.g., End, End.X) to a SID
SRv6 Adjacency SID Sub-TLV E-Router-LSA link entries Advertise per-adjacency SRv6 SIDs (End.X equivalent)
SRv6 SID Structure Sub-TLV SRv6 End SID / Adj SID Describe the Locator Block, Locator Node, Function, and Argument bit lengths

SRv6 Locator Advertisement

The SRv6 Locator TLV carries:

Field Description
Route Type Intra-area or inter-area locator
Algorithm 0 for SPF, 1 for Strict SPF, 128-255 for Flex-Algo
Locator Prefix The IPv6 prefix representing the SRv6 Locator
Metric Cost associated with reaching this locator
Flags N-flag (node SID), A-flag (no-algorithm)

SRv6 SID Structure Sub-TLV

Field Bits Description
Locator Block Length (LBL) Variable Length of the SRv6 SID Locator Block in bits
Locator Node Length (LNL) Variable Length of the Locator Node identifier in bits
Function Length (FL) Variable Length of the Function field in bits
Argument Length (AL) Variable Length of the Argument field in bits

OSPFv3 SRv6 vs IS-IS SRv6

Aspect OSPFv3 (RFC 9513) IS-IS (RFC 9352)
LSA/LSP type E-Router-LSA (RFC 8362) Standard IS-IS LSP
Capabilities SRv6 Capabilities TLV in RI LSA SRv6 Capabilities sub-TLV in Router Capability TLV
Locator SRv6 Locator TLV in E-Router-LSA SRv6 Locator TLV (top-level)
End SID Sub-TLV of SRv6 Locator TLV SRv6 End SID sub-TLV of Locator TLV
Adjacency SID Sub-TLV of link in E-Router-LSA SRv6 End.X SID sub-TLV in extended IS reachability
Flex-Algo Algorithm field in Locator TLV Algorithm field in Locator TLV

Common Pitfalls

  • OSPFv3 SRv6 extensions require Extended LSAs (RFC 8362). Legacy OSPFv3 LSA types cannot carry the new TLVs. Ensure all routers support and are configured for Extended LSAs.
  • The SRv6 SID Structure sub-TLV is optional but strongly recommended. Without it, remote nodes must infer the LOC:FUNC:ARG boundaries, which may lead to interoperability issues.
  • OSPFv3 area boundaries affect SRv6 locator reachability. Inter-area locator advertisements require proper ABR configuration.
  • Do not confuse OSPFv3 SR extensions (RFC 8666, for SR-MPLS) with OSPFv3 SRv6 extensions (RFC 9513). They serve different data planes.
  • When deploying both IS-IS and OSPFv3 for SRv6, ensure consistent locator and SID advertisements to avoid forwarding inconsistencies.

References

  1. RFC 9513 - OSPFv3 Extensions for Segment Routing over IPv6 (SRv6)
  2. RFC 8362 - OSPFv3 Link State Advertisement (LSA) Extensibility
  3. RFC 9352 - IS-IS Extensions for Segment Routing over IPv6 (SRv6)
  4. RFC 8666 - OSPFv3 Extensions for Segment Routing