Summary

A routing protocol essentially assembles and organizes addressing information according to location within a network. IS-IS was originally designed for routing in ISO CLNS environments where the Layer 3 protocol is CNLP and the network layer addresses are described as NSAPs. Adapting IS-IS to carry IP information didn't eliminate NSAP addressing altogether because of the strong links within the protocol basis. Therefore, using IS-IS on IP routers requires node-based NSAPs to be defined in addition to the link-based IP addresses on the interfaces.

This chapter attempts to demystify NSAP addressing by explaining why it is still featured in Integrated IS-IS, even for IP-only routing, and by discussing the NSAP addressing architecture. The NSAP format is covered and you studied the three major relevant components when using IS-IS for IP routing, namely Area ID, SysID, and NSEL. Even though the IS-IS- related specifications suggest 1 to 8 bytes for the SysID, Cisco routers support a fixed 6-byte length in conformance with the U.S. GOSIP version 2.0 standard. This chapter discusses the significance of the SysID in IS-IS operations and provides guidelines for defining unique NSAPs on routers in an IS-IS area or domain. One method is to base the SysID on a loopback IP address on the router. In general, globally unique NSAPs are not needed for IS-IS routing in the IP domains that form the Internet, even though some service providers obtain and deploy such NSAPs in their networks. The chapter discusses the seven top-level NSAP addressing domains and provides insight into the role of RAs that allocate NSAP addresses to government and private organizations worldwide.

An interesting subject, IS-IS multihoming is discussed and compared with IS-IS multi-area support and IP subnettng. Multihoming configurations are useful for transitions such as merging, splitting, and renumbering of IS-IS areas. Multihoming causes Level 1 LSPs to be exchanged between IS-IS areas instead of using Level 2 routing. However, exchanging Level 1 LSPs between areas effectively merges them. A significant difference between multihoming and the multi-area feature is that the latter uses multiple IS-IS processes to support multiple independent areas, whereas the former merges areas under one process.

An NSAP can be as long as 20 bytes and a greater part of it is represented in hexadecimal, a format with which many people have difficulty working. Network operators who work with IS-IS on a regular basis cherish the convenience of working with symbolic names rather than long NSAP addresses. This chapter discussed the two methods available on Cisco routers for NSAP addresstosymbolic name mapping: static mapping and dynamic mapping. The latter method is specified by the IETF RFC 2763. This chapter concludes with a review of NSAP examples that illustrates how to delineate the three major fields in an NSAP.