Advertising Layer 2 Bundle Member Link Attributes in OSPF
updates
- rfc-9085 — Border Gateway Protocol - Link State (BGP-LS) Extensions for Segment Routing
Extracted elements (17)
L2 bundle member link advertisements are intentionally scoped to serve external entities (e.g., PCEs) that wish to control traffic on individual physical links; they do not alter OSPF route computation. The document explicitly excludes defining what attributes are advertised, a minimum set, how they are configured, how they are used, or how they are passed to external entities.
L3-semantic sub-TLVs (e.g., Remote IPv4 Address, Local/Remote Interface ID, OSPFv2 Link MSD) are not applicable to L2 bundle member links, while L2-semantic sub-TLVs (e.g., Adj-SID, bandwidth, delay, administrative group) are applicable. This distinction drives the applicability table in Section 2.
The advertisement of the L2 Bundle Member Attributes sub-TLV may be asymmetric for an OSPF link—advertised by only one end—depending on the underlying L2 connectivity. This is a deliberate allowance reflecting real deployment asymmetry.
The L2 Bundle Member Descriptor is a 4-octet link-local identifier corresponding to the 'link local identifier' defined in RFC 4202 and used as 'Local Interface ID' in RFC 8510, ensuring consistent identification of member links across protocols.
This document updates RFC 9085 by specifying advertisement of OSPF L2 bundle member link attributes via BGP-LS. The BGP-LS mechanism for OSPF follows the same approach already defined for IS-IS in Section 2.2.3 of RFC 9085.
Advertisements of L2 bundle member links MUST NOT be done when the member link becomes operationally down or is no longer a member of the identified L2 bundle. The presence of the advertisement implies the member link is operationally up and part of the bundle.
Implementations MUST NOT enable the advertisement of L2 bundle member links and their attributes in OSPF LSAs by default and MUST provide a configuration option to enable their advertisement on specific links.
Specifications that introduce new sub-TLVs of the OSPFv2 Extended Link TLV or the OSPFv3 Router-Link TLV MUST indicate their applicability (Y/N) to the L2 Bundle Member Attributes sub-TLV.
Sub-TLVs that are not applicable to the L2 Bundle Member Attributes sub-TLV MUST NOT be used as nested sub-TLVs within it. An implementation MUST ignore any sub-TLVs received that are not applicable in the context of the L2 Bundle Member Attributes sub-TLV.
For OSPFv2, applicable sub-TLVs that MAY appear within the L2 Bundle Member Attributes sub-TLV include: Adj-SID (2), LAN Adj-SID/Label (3), Application-Specific Link Attributes (10), Shared Risk Link Group (11), Unidirectional Link Delay (12), Min/Max Unidirectional Link Delay (13), Unidirectional Delay Variation (14), Unidirectional Link Loss (15), Residual/Available/Utilized Bandwidth (16-18), Administrative Group (19), Extended Administrative Group (20), TE Metric (22), and Maximum Link Bandwidth (23).
For OSPFv3, applicable sub-TLVs that MAY appear within the L2 Bundle Member Attributes sub-TLV include: Adj-SID (5), LAN Adj-SID (6), Application-Specific Link Attributes (11), Shared Risk Link Group (12), Unidirectional Link Delay (13), Min/Max Unidirectional Link Delay (14), Unidirectional Delay Variation (15), Unidirectional Link Loss (16), Residual/Available/Utilized Bandwidth (17-19), Administrative Group (20), Extended Administrative Group (21), TE Metric (22), and Maximum Link Bandwidth (23).
The L2 Bundle Member Attributes sub-TLV is a new sub-TLV introduced for both OSPFv2 and OSPFv3 to advertise link attributes of individual L2 bundle member links. In OSPFv2 it is an optional sub-TLV of the Extended Link TLV (type 24); in OSPFv3 it is an optional sub-TLV of the Router-Link TLV of the E-Router-LSA (type 29).
IANA allocated code point 24 in the 'OSPFv2 Extended Link TLV Sub-TLVs' subregistry for 'L2 Bundle Member Attributes', and added an 'L2BM' column to that registry indicating sub-TLV applicability (Y/N) to the L2 Bundle Member Attributes sub-TLV.
IANA allocated code point 29 in the 'OSPFv3 Extended-LSA Sub-TLVs' subregistry for 'L2 Bundle Member Attributes', and added an 'L2BM' column to that registry indicating sub-TLV applicability (Y/N/X) to the L2 Bundle Member Attributes sub-TLV.
If the protocol is used in an environment where unauthorized physical access to links occurs, attacks are possible. Use of OSPF authentication as defined in RFC 5709, RFC 7474, RFC 4552, and RFC 7166 is recommended for preventing such attacks.
The advertisements defined in this document are identical to existing OSPF link attribute advertisements (RFC 3630, RFC 4203, RFC 5329, RFC 7471, RFC 8665, RFC 8666) applied to L2 bundle member links; no new security issues are introduced beyond those already covered by those documents.
The L2 Bundle Member Attributes sub-TLV contains a 2-octet Type field (24 for OSPFv2, 29 for OSPFv3), a 2-octet Length field (total length of value portion including nested sub-TLVs), a 4-octet L2 Bundle Member Descriptor (link-local identifier for the member link), and variable-length Member Link Attribute sub-TLVs.