Section 10.2. Extending IS-IS

10.2. Extending IS-IS

New capabilities are added to IS-IS through the addition of new TLVs. This section introduces one such added TLV. Chapters 11 through 13 discuss other TLVs that have been added to IS-IS to support optional capabilities.

10.2.1. The IS-IS Extensibility Advantage

IS-IS tends to be easier to extend than OSPF, and this can be observed historically by the timing of vendors' rollouts of new features supported by IS-IS and OSPF. Both MPLS TE and IPv6, for example, were supported in IS-IS production implementations six months to one year before they were supported in OSPF.

The reason for this easier extensibility has to do with the fact that new features are introduced by adding new TLVs to an LSP, while the structure of the LSP itself remains unchanged. This is typically simpler than defining a new LSA for OSPF.

An additional advantage when extending IS-IS is the way the protocol handles unknown TLVs. Unlike OSPF, which drops unrecognized LSAs, IS-IS ignores unknown TLVs in the LSP and passes them on to other neighbors unchanged. This behavior simplifies the introduction of new capabilities into your network, particularly when the capability is to be added only to a subset of all routers in the IS-IS domain.

Finally, the addition of some IP capabilitiesparticularly support for IPv6is simpler because IS-IS is not itself an IP protocol. As a result, there are no functional dependencies on IP addressing semantics.

10.2.2. The Protocols Supported TLV

The Protocols Supported TLV shown in Figure 10.9 specifies, as the name implies, what protocols the originating router supports. It lists one or more Network Layer Protocol Identifiers (NLPIDs), which are defined in ISO/TR 9577 and in several extension documents. Because IS-IS was originally designed to route just CLNP, this TLV was added when the protocol was extended to support IP. With it, the originator can advertise whether it supports CLNP only, IPv4 only, or both. With the subsequent extension of IS-IS to support IPv6, as described in Chapter 12, that protocol is also listed in the Protocols Supported TLV when the originator uses IS-IS to route IPv6. The NLPID of IPv4 is 204 (0xcc), and the NLPID of IPv6 is 142 (0x8e).

Figure 10.9. The Protocols Supported TLV.