Definition of an IS-IS Link Attribute Sub-TLV
updated by
- rfc-9650 — Revision to Registration Procedures for IS-IS Neighbor Link-Attribute Bit Values
Extracted elements (10)
The triggers for setting the 'Link Excluded from Local Protection Path' bit (0x02) are explicitly left out of scope of this document, indicating the decision of when to exclude a link from protection paths is an implementation or policy matter rather than a protocol matter.
A router not supporting the link-attribute sub-TLV will silently ignore this sub-TLV, ensuring backward compatibility with older implementations.
If a received LSP contains more than one Link-Attribute Sub-TLV, an implementation SHOULD consider only the first encountered instance and ignore subsequent ones.
The Link-attribute sub-TLV is OPTIONAL and MUST appear at most once for a single IS neighbor within a Link State Packet.
When the 'Link Excluded from Local Protection Path' bit (0x02) is set, this link SHOULD NOT be included in any computation of a repair path by any other router in the routing area.
The 'Link Excluded from Local Protection Path' flag (bit 0x02) indicates that when set, the link SHOULD NOT be included in any computation of a repair path by any other router in the routing area.
The 'Local Protection Available' flag (bit 0x01) in the link-attribute sub-TLV indicates that the link is protected by some local protection mechanism (e.g., RSVP-TE Fast Reroute per RFC 4090).
IANA assigned codepoint 19 for the link-attribute sub-TLV within TLV 22, and created a registry for bit values inside the link-attributes sub-TLV. Initial values are 0x1 (Local Protection Available) and 0x2 (Link Excluded from Local Protection). Further values require Standards Action with Early Allocation permitted.
The link-attribute sub-TLV expands the information available in LSPs, making additional router capability data visible to potential attackers who can snoop LSPs. Specifications using this mechanism must address security around disclosure and modification of this information, and an integrity mechanism such as IS-IS Cryptographic Authentication (RFC 3567) should be applied when there is high risk from modification of capability information.
The Link-attribute sub-TLV is carried within TLV 22 and uses the standard IS-IS sub-TLV format: 1 octet sub-type, 1 octet length, followed by a 16-bit flags field. The sub-type is 19 and the length is 2 octets.