Section 11.4. IS-IS Extensions for Traffic Engineering

11.4. IS-IS Extensions for Traffic Engineering

The IS-IS extensions for traffic engineering support are specified in RFC 3784.^[6] Semantically, the extensions are the same as those for OSPF: The same TE parameters are communicated, the same value ranges are used (such as 4 bytes for the maximum bandwidth parameter), and the values are represented the same (such as bandwidth values in bytes per second). And, as with OSPF, sub-TLVsTLVs nested within the value fields of other TLVsare used to carry the TE parameters.

^[6] Henk Smit and Tony Li, "Intermediate System to Intermediate System (IS-IS) Extensions for Traffic Engineering (TE)," RFC 3784, June 2004.

The sub-TLVs carrying TE parameters are carried in the Extended IS Reachability (type 22) TLV, which was introduced in Section 5.5.7^[7] and is shown again in Figure 11.10. Of particular note is that in another similarity to OSPF TE extensions, the TE parameters are carried in sub-TLVs. For each of the sub-TLVs supported by the Link TLV in OSPF TE LSAs, an analogous sub-TLV is supported by the IS-IS type 22 TLV. So rather than repeat their identical functions, they are listed in Table 11.1 next to their OSPF counterparts.

^[7] The relevance of the Extended IS Reachability TLV in Section 5.5.7 is its support for wide metrics. The implication here is that when IS-IS support for traffic engineering is enabled, support for wide metrics is enabled because both use the same TLV.

Figure 11.10. IS-IS communicates TE link parameters in the Extended IS Reachability TLV.

Table 11.1. Sub-TLVs Carried in IS-IS Extended IS Reachability TLVs or TE Support

Sub-TLV	Type Number	Length (Bytes)	Corresponding OSPF Sub-TLV
Administrative Group (Color)	3	4	Administrative Group
IPv4 Interface Address	6	4	Local Interface IP Address
IPv4 Neighbor Address	8	4	Remote Interface IP Address
Maximum Link Bandwidth	9	4	Maximum Bandwidth
Reservable Link Bandwidth	10	4	Maximum Reservable Bandwidth
Unreserved Bandwidth	11	32	Unreserved Bandwidth
TE Default Metric	18	3	TE Metric

IS-IS TE extensions also have a TLV that is analogous to the OSPF Router Address TLV described in the previous section: the Traffic Engineering Router ID TLV. This TLV type is 134, and it carries the 4-byte RID of the originating router. For both OSPF and IS-IS, this address is configured on a loopback interface and serves as the endpoint for egress LSPs. Because it exists on the loopback interface, it is independent of any instability on a single physical interface. It is a stable endpoint as long as it is reachable through some physical interface.

The OSPF Router Address TLV and the IS-IS Router ID TLV also serve a purpose in TE domains where both OSPF and IS-IS are running. If a router advertises its link TE parameters in both OSPF TE LSAs and IS-IS Extended IS Reachability TLVs, it should also originate both an OSPF Router Address TLV and an IS-IS Router ID TLV containing the same address.

A router building a single TED from both OSPF and IS-IS TE TLVs can identify, based on the identical address in the Router Address TLV and the Router ID TLV, that the OSPF and IS-IS information come from the same router rather than separate routers.