IS-IS Extensions in Support of Inter-Autonomous System (AS) MPLS and GMPLS Traffic Engineering
obsoletes
- rfc-5316 — ISIS Extensions in Support of Inter-Autonomous System (AS) MPLS and GMPLS Traffic Engineering
Extracted elements (29)
RFC 5316 only allowed a 32-bit Router ID in TLV 141's fixed header, which is problematic in IPv6-only deployments lacking an IPv4 address. RFC 9346 adds the IPv6 Local ASBR Identifier sub-TLV and specifies use of 0.0.0.0 as a sentinel value to support IPv6-only operation.
Since there is no IGP peering on inter-AS links, there is no bidirectional TE link information. The ASBR therefore 'proxies' for the remote ASBR by advertising an additional LSP describing the remote device's view of the link, enabling CSPF to perform two-way checks on the inter-AS link.
This document explicitly excludes distribution of TE information from within one AS to another, any TE reachability information for destinations outside the AS, TE aggregation, and exchange of private information between ASes, in conformance with the confidentiality and scaling assumptions surrounding AS use in the Internet.
Legacy routers receiving an advertisement for an inter-AS TE link will ignore it because they do not know TLV 141 and its sub-TLVs. They will continue to flood the LSP but will not attempt to use the information received.
When advertising an IPv4 TE Router ID, if the Traffic Engineering router ID TLV (RFC 5305) is being advertised, the value SHOULD be identical to the address in that TLV. Similarly for IPv6, if the IPv6 TE Router ID TLV (RFC 6119) is advertised, the values SHOULD be identical.
A TE Router ID MUST be an address that is unique within the IS-IS domain and stable, i.e., it can always be referenced in a path reachable from multiple hops away regardless of the state of the node's interfaces.
Hellos MUST NOT be exchanged over the inter-AS link, and consequently, an IS-IS adjacency MUST NOT be formed on the inter-AS link.
If the originating node does not support IPv4, the reserved value 0.0.0.0 MUST be used in the Router ID field of TLV 141, and the IPv6 Router ID sub-TLV MUST be present. Inter-AS Reachability Information TLVs with Router ID 0.0.0.0 and no IPv6 Local ASBR Identifier sub-TLV MUST be ignored.
If the S bit is set(1), the Inter-AS Reachability Information TLV MUST be flooded across the entire routing domain. If the S bit is not set(0), the TLV MUST NOT be leaked between levels. This bit MUST NOT be altered during TLV leaking.
Routers or PCEs capable of 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 AS through another TE link, except as the result of specific policy configurations.
The IPv4 Remote ASBR Identifier sub-TLV MUST be included if the neighboring ASBR has an IPv4 address; if not, the IPv6 Remote ASBR Identifier sub-TLV MUST be included instead.
The Remote AS Number sub-TLV MUST be included when a router advertises an inter-AS TE link. It specifies the AS number of the neighboring AS to which the advertised link connects.
When TE is enabled on an inter-AS link and the link is up, the ASBR SHOULD advertise this link. When the link is down or TE is disabled, the ASBR SHOULD withdraw the advertisement. When TE parameters change, the ASBR SHOULD re-advertise but MUST take precautions against excessive re-advertisements.
When the Inter-AS Reachability Information TLV is leaked from Level 2 to Level 1, the D bit MUST be set. Inter-AS Reachability Information TLVs with the D bit set MUST NOT be leaked from Level 1 to Level 2 to prevent TLV looping.
When the TE Router ID is needed domain-wide, the IS-IS Router CAPABILITY TLV MUST be included in the ASBR's LSP. If the ASBR has an IPv4 TE Router ID, the IPv4 TE Router ID sub-TLV MUST be included; if not, the IPv6 TE Router ID sub-TLV MUST be included instead.
The Inter-AS Reachability Information TLV (type 141) is defined to advertise inter-AS TE links within an IS-IS domain. It contains a 4-octet Router ID, a 3-octet Default Metric, a 1-octet Flags field (S bit, D bit, and reserved), a 1-octet Sub-TLVs Length, and 0-246 octets of sub-TLVs.
IANA has registered four sub-TLV types under TLV 141 in the 'IS-IS Sub-TLVs for TLVs Advertising Neighbor Information' registry: type 24 (Remote AS Number, 4 octets), type 25 (IPv4 Remote ASBR Identifier, 4 octets), type 26 (IPv6 Remote ASBR Identifier, 16 octets), and type 45 (IPv6 Local ASBR Identifier, 16 octets).
IANA has registered TLV type 141 ('Inter-AS Reachability Information') in the 'IS-IS Top-Level TLV Codepoints' registry; valid in LSP only (not IIH, SNP, or Purge).
IANA has registered two sub-TLV types under TLV 242 (IS-IS Router CAPABILITY TLV) in the 'IS-IS Sub-TLVs for IS-IS Router CAPABILITY TLV' registry: 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 as described in Section 4.1, the inter-AS BGP session SHOULD be secured using mechanisms such as RFC 5925 (TCP Authentication Option) to provide authentication and integrity checks.
Remote AS Number and Remote ASBR Identifier are obtained manually from a neighboring administration as part of a commercial relationship; if the Remote AS Number received in a BGP OPEN differs from the locally configured IS-IS TE value, local policy SHOULD be applied to alert the operator or suppress the advertisement to prevent inter-AS TE LSPs traversing unintended ASes.
The protocol extensions can be secured within the AS using existing IS-IS security mechanisms such as cleartext passwords or Hashed Message Authentication Codes (RFC 5304, RFC 5310). There is no exchange of information between ASes and no new IS-IS security relationship between ASes is required.
The Inter-AS Reachability Information TLV encodes: Router ID (4 octets), Default Metric (3 octets), Flags (1 octet with S bit at position 0, D bit at position 1, and 6 reserved bits), Sub-TLVs Length (1 octet), and Sub-TLVs (0-246 octets).
The IPv4 Remote ASBR Identifier sub-TLV (type 25) is 4 octets in length and carries the IPv4 identifier of the remote ASBR to which the inter-AS link connects.
The IPv4 TE Router ID sub-TLV (type 11) for inclusion in the IS-IS Router CAPABILITY TLV is 4 octets in length and carries the IPv4 TE Router ID for domain-wide advertisement.
The IPv6 Local ASBR Identifier sub-TLV (type 45) is 16 octets in length and carries the IPv6 identifier of the local ASBR originating the inter-AS link advertisement; MUST be present when the originating node does not support IPv4.
The IPv6 Remote ASBR Identifier sub-TLV (type 26) is 16 octets in length and carries the IPv6 identifier of the remote ASBR to which the inter-AS link connects.
The IPv6 TE Router ID sub-TLV (type 12) for inclusion in the IS-IS Router CAPABILITY TLV is 16 octets in length and carries the IPv6 TE Router ID for domain-wide advertisement.
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 octets are used for the AS number, the high-order 2 octets MUST be set to 0.