ISIS Extensions in Support of Inter-Autonomous System (AS) MPLS and GMPLS Traffic Engineering
obsoleted by
- rfc-9346 — IS-IS Extensions in Support of Inter-Autonomous System (AS) MPLS and GMPLS Traffic Engineering
Extracted elements (27)
Existing sub-TLVs from ISIS-TE and ISIS-TE-V3 for describing TE properties are reused in the Inter-AS Reachability TLV to avoid redundant definitions, carrying all TE information needed within the AS without relying on external protocol state.
The ASBR advertises both directions of an inter-AS TE link internally within its AS, acting as a proxy for the remote ASBR. This is necessary because CSPF requires bidirectional TE link information for path computation, but no IGP peering exists on inter-AS links.
The Inter-AS Reachability TLV replaces the 7-octet System ID/Pseudonode Number field of the extended IS reachability TLV with a 4-octet Router ID and adds a control byte (S bit, D bit), because inter-AS links have no ISIS adjacency and require explicit flooding scope control.
The source of proxied remote ASBR TE information (remote AS number, remote ASBR TE Router ID) is outside the scope of this document; it may come from manual configuration or optionally from BGP, as an implementation choice.
Legacy routers receiving an Inter-AS Reachability TLV will ignore it because they do not recognize TLV type 141 or its sub-TLVs; they will continue to flood the LSP but will not use the information.
Hellos MUST NOT be exchanged over the inter-AS link, and consequently, an ISIS adjacency MUST NOT be formed on the inter-AS link.
Routers or PCEs processing inter-AS TE link advertisements SHOULD NOT use such links to compute paths that exit an AS to a remote ASBR and then immediately re-enter the same AS, except as the result of specific policy configuration.
Sub-TLVs 24, 25, and 26 (remote AS number and remote ASBR identifiers) SHOULD NOT appear in TLV 22 (or TLV 222) and MUST be ignored if received in TLV 22 (or TLV 222).
The flooding-scope S bit SHOULD be set to 0 for single IGP area scope, and MAY be set to 1 for domain-wide flooding. Configuration control SHOULD be provided in ASBR implementations to select the flooding scope.
The IPv4 Remote ASBR ID sub-TLV MUST be included if the neighboring ASBR has an IPv4 address. If the neighboring ASBR does not have an IPv4 address, the IPv6 Remote ASBR ID sub-TLV MUST be included instead.
The IPv6 Remote ASBR ID sub-TLV MUST be included if the neighboring ASBR has an IPv6 address. If the neighboring ASBR does not have an IPv6 address, the IPv4 Remote ASBR ID sub-TLV MUST be included instead.
The Remote AS Number sub-TLV MUST be included when a router advertises an inter-AS TE link. When only 2-octet AS numbers are used, the left (high-order) 2 octets of the 4-octet field MUST be set to 0.
The Router ID in the Inter-AS Reachability TLV MUST be unique within the ISIS area. If the TLV is flooded with domain-wide scope (S bit=1), the Router ID MUST also be unique within the entire ISIS routing domain.
When TE is enabled on an inter-AS link and the link is up, the ASBR SHOULD advertise the link using normal ISIS-TE procedures. When the link is down or TE is disabled, the ASBR SHOULD withdraw the advertisement.
When TE parameters change for an inter-AS link (e.g., available bandwidth), the ASBR SHOULD re-advertise the link but MUST take precautions against excessive re-advertisements.
When the TE Router ID is needed to reach all routers within an entire ISIS routing domain, the IS-IS Router Capability TLV MUST be included in the ASBR's LSP, with the appropriate IPv4 or IPv6 TE Router ID sub-TLV.
A new Inter-AS Reachability TLV (type 141) is defined to advertise inter-AS TE link information within an ISIS routing domain. It contains a 4-octet Router ID, 3-octet default metric, 1-octet control field (S bit, D bit, 6 reserved bits), 1-octet sub-TLV length, and 0-246 octets of sub-TLVs.
This document registers ISIS TLV type 141 (Inter-AS Reachability Information) in the ISIS TLV codepoint registry, valid in LSPs only (not IIH or SNP).
This document registers three new sub-TLV types for TLV 141 in the ISIS sub-TLV registry: type 24 (remote AS number, 4 octets), type 25 (IPv4 remote ASBR identifier, 4 octets), and type 26 (IPv6 remote ASBR identifier, 16 octets). The registry for TLV 22 sub-TLVs is renamed to cover TLVs 22, 141, and 222.
This document registers two new sub-TLV types for the IS-IS Router Capability TLV (TLV 242): type 11 (IPv4 TE Router ID, 4 octets) and type 12 (IPv6 TE Router ID, 16 octets).
If BGP is used to exchange TE information between ASBRs as described in Section 4.1, the inter-AS BGP session SHOULD be secured using BGP authentication mechanisms to provide authentication and integrity checks.
Manually configured inter-AS TE information (remote AS number, remote ASBR ID) should be carefully verified before entry. If a BGP OPEN reveals a different remote AS number than locally configured, local policy SHOULD determine whether to alert the operator or suppress the ISIS advertisement.
The inter-AS TE extensions can be secured within each AS using existing ISIS security mechanisms (cleartext passwords or HMAC-MD5 per RFC 5304). No ISIS security is required between ASes since no ISIS adjacency is formed on inter-AS links and no information is exchanged between ASes.
The Inter-AS Reachability TLV (type 141) encodes: 4 octets Router ID, 3 octets default metric, 1 octet control (S bit for flooding scope, D bit for up/down, 6 reserved bits), 1 octet sub-TLV length, followed by sub-TLVs each with 1-octet type, 1-octet length, and 0-244 octets of value.
The IPv4 Remote ASBR ID sub-TLV (type 25) is 4 octets in length and carries the IPv4 identifier of the remote ASBR. The IPv6 Remote ASBR ID sub-TLV (type 26) is 16 octets and carries the IPv6 identifier of the remote ASBR.
The IPv4 TE Router ID sub-TLV (type 11, for IS-IS Router Capability TLV) is 4 octets. The IPv6 TE Router ID sub-TLV (type 12) is 16 octets. Both are defined for inclusion in the IS-IS router capability TLV (TLV 242) to carry domain-wide TE Router IDs.
The Remote AS Number sub-TLV (type 24) is 4 octets in length and carries the AS number of the neighboring AS. When only 2-octet AS numbers are in use, the high-order 2 octets MUST be set to 0.