11.4. IS-IS Extensions for Traffic EngineeringThe 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.
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.
Figure 11.10. IS-IS communicates TE link parameters in the Extended IS Reachability TLV.
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. |