OSPF Prefix Originator Extensions
Extracted elements (21)
For inter-area prefixes, the Advertising Router field in an ABR's area-scoped LSA is set to the ABR itself, causing the original prefix originator information to be lost during propagation across OSPF areas. These extensions explicitly carry originator identity to survive area boundary crossings.
The OSPF Router ID is a 32-bit identifier unique within the domain but not necessarily a reachable address; for OSPFv2, loopback addresses are commonly used as Router IDs, but this assumption does not hold for IPv6 with OSPFv3. The separate Prefix Source Router Address Sub-TLV provides a confirmed reachable address via the Router Address TLV mechanism.
When an NSSA ABR translates NSSA LSAs to AS-external LSAs, information about the NSSA ASBR or the router outside the OSPF domain is not propagated. The extensions enable this information to be preserved and follow the same ABR inter-area procedures for NSSA translation.
Similar intra-area validation (Router ID matching Advertising Router) cannot be reliably performed for inter-area and external prefix advertisements, so the sub-TLV is accepted without that check in those contexts.
The prefix originator information carried by these sub-TLVs is particularly useful when advertised as a prefix attribute to an external controller via BGP-LS (RFC 7752, RFC 9085), enabling topology analysis and traffic engineering in SDN deployments.
These extensions do not change core OSPF route computation; routers that do not understand the new sub-TLVs will ignore them without any impact on forwarding. The sub-TLVs are optional attributes of prefix advertisements.
A prefix advertisement's parent TLV MAY include more than one Prefix Source OSPF Router-ID Sub-TLV, one per ECMP node that originated the advertised prefix. The same applies to the Prefix Source Router Address Sub-TLV.
A received Prefix Source OSPF Router-ID Sub-TLV with the OSPF Router ID field set to 0 MUST be considered invalid and ignored; reception of such sub-TLVs SHOULD be logged as an error subject to rate limiting.
A received Prefix Source Router Address Sub-TLV with a length inconsistent with the prefix's address family MUST be considered invalid and ignored; reception of such sub-TLVs SHOULD be logged as an error subject to rate limiting.
An ABR MUST NOT include the Prefix Source OSPF Router-ID or Prefix Source Router Address Sub-TLVs when it is unable to determine the originating node information for the ECMP-contributing nodes.
For intra-area prefix advertisements, the Prefix Source OSPF Router-ID Sub-TLV MUST be considered invalid and ignored if the OSPF Router ID field does not match the Advertising Router field in the containing LSA.
If the originating node is advertising an OSPFv2 Router Address TLV (RFC 3630) or an OSPFv3 Router IPv6 Address TLV (RFC 5329), that same address MUST be used in the Router Address field of the Prefix Source Router Address Sub-TLV.
When an ABR generates inter-area prefix advertisements, it MUST use prefix originator information only from the set of nodes that contribute to ECMP paths for the prefix, as determined by its prefix calculation.
The Prefix Source OSPF Router-ID Sub-TLV is an optional sub-TLV of the OSPFv2 Extended Prefix TLV (RFC 7684) and of the OSPFv3 Intra-Area-Prefix, Inter-Area-Prefix, and External-Prefix TLVs (RFC 8362) for both IPv4 and IPv6 prefix advertisements.
The Prefix Source Router Address Sub-TLV is an optional sub-TLV of the OSPFv2 Extended Prefix TLV and of the OSPFv3 Intra-Area-Prefix, Inter-Area-Prefix, and External-Prefix TLVs, providing a reachable address (IPv4 or IPv6) for the prefix-originating router.
IANA allocated codepoints 27 (Prefix Source OSPF Router-ID) and 28 (Prefix Source Router Address) in the 'OSPFv3 Extended-LSA Sub-TLVs' registry under 'Open Shortest Path First v3 (OSPFv3) Parameters'.
IANA allocated codepoints 4 (Prefix Source OSPF Router-ID) and 5 (Prefix Source Router Address) in the 'OSPFv2 Extended Prefix TLV Sub-TLVs' registry under 'Open Shortest Path First v2 (OSPFv2) Parameters'.
Because the propagation of prefix source node information crosses OSPF area or domain boundaries, it exposes router identity outside areas where it would normally be hidden or abstracted by the base OSPF protocol. Operators may limit propagation to a subset of prefixes at ABRs or ASBRs.
The security considerations of RFC 7684 (OSPFv2 Extended Prefix LSA) and RFC 8362 (OSPFv3 LSA Extensibility) apply. A rogue node capable of injecting prefix advertisements may use these extensions to advertise bogus prefix source information, but the new sub-TLVs do not affect OSPF route computation itself.
The Prefix Source OSPF Router-ID Sub-TLV carries a 2-octet Type (4 for OSPFv2, 27 for OSPFv3), a 2-octet Length fixed at 4, and a 4-octet OSPF Router ID field identifying the router that originated the prefix in the OSPF domain.
The Prefix Source Router Address Sub-TLV carries a 2-octet Type (5 for OSPFv2, 28 for OSPFv3), a 2-octet Length of 4 (IPv4) or 16 (IPv6), and a Router Address field holding a reachable IPv4 or IPv6 address of the prefix-originating router.