Chapter 4. Addressing in Integrated IS-IS

This chapter focuses on network addressing concepts in Integrated IS-IS and attempts to demystify the seemingly cumbersome CLNP network addressing, which is mandatory on IP routers, even when using Integrated IS-IS in IP-only environments. As discussed in Chapter 3, "Integrated IS-IS Routing Protocol Concepts," Integrated IS-IS retains most of the concepts of the original IS-IS routing protocol, as specified in ISO 10589, and defines new TLVs (RFC 1195) that enable support for IP routing. The CLNP node-based addressing scheme is one of the key concepts retained in Integrated IS-IS. The node-based addressing scheme requires only a single address for the entire node. In contrast however, link-based addressing applies multiple addresses to a node by assigning a unique address to each connecting link. These addresses are applied to the interfaces where network links connect to the router. The IP addressing scheme is link-based , as mentioned in Chapter 1,"Overview of IP Routing." In this scheme, each link is assigned an IP subnetwork number that defines the corresponding interface address. Consequently, an IP node can have many IP addresses assigned to it, depending on the number of enabled interfaces.

In the node-based scheme, IS-IS needs only a single address per IS-IS node. Multiple addresses can be defined on an IS-IS router, but they are not tied to any specific links. This can be done for special reasons discussed later in this chapter. Even when multiple CLNP addresses are defined on an IS-IS router, the same unique System Identifier is retained in all the addresses. CNLP addresses used in IS-IS are called network service access points (NSAP) and are, therefore, frequently referred to as NSAP addresses. Recall from Chapter 2, "Introduction to the IS-IS Routing Protocol," that in the Open System Interconnection (OSI) reference model, each layer provides special services to the next higher layer. The NSAP defines the appropriate service interfaces, similar to the protocol type used by IP routers for TCP and UDP.

An NSAP address consists of several components , including a unique System Identifier that allows it to identify an entire node in a Connectionless Network Protocol (CLNP) network. When using Integrated IS-IS for routing IP, both CLNP and IP addresses are configured on the routers. However, IP hosts not participating in the dynamic routing process don't need to have NSAP addresses. Frequently, the IP hosts do not need to run a dynamic routing protocol and instead rely on IP services, such as ARP, DHCP, and static default routes to communicate with routers and other IP devices. Some IP hosts support dynamic routing protocols, such as the Routing Information Protocol (RIP) or the Open Shortest Path First (OSPF) Protocol. In a pure ISO or dual network, the ISO hosts (end systems) have NSAPs and rely on the ES-IS protocol to communicate with routers and other end systems. To review the introductory IP material, refer to Chapter 1. This chapter introduces and focuses on only ISO addressing concepts for CLNP. Later, insights into requirements, caveats, and suggested best practices for defining and configuring NSAPs on Cisco routers for Integrated IS-IS routing are discussed.

ISO terminology refers to data link (Layer 2) addresses (LAN MAC addresses, Frame Relay DLCIs, and so on) as subnetwork point of attachments (SNPAs). Because a network device might connect to multiple links, it can have multiple SNPA addresses but requires only one OSI network address. ISO protocols providing network layer services can interpret both NSAP and SNPA addresses. As discussed previously, a primary function of the ES-IS protocol is to provide NSAP-to-SNPA mapping for network devices. Figure 4-1 illustrates the application of SNPAs and NSAPs to various nodes in an IS-IS network.

Figure 4-1. Application of ISO NSAP and SNPAs.

The NSAP addressing scheme, which is a major dependency for IS-IS operation, is specified in ISO 8348/AD2. This specification is also available as RFC 941 from the IETF (www.ietf.org). The parent document, ISO 8348, defines the ISO network layer service requirements for connection-oriented services. ISO 8348/AD1 provides additional specification for connectionless-mode transmission, whereas IS0 8348/AD2 deals with network layer addressing. Subsequent sections of this chapter explain the format of NSAP addresses and how they are configured on Cisco routers to support IS-IS routing. Hopefully, if you feel more comfortable with IP addressing, you will overcome any intimidation that CLNP addressing poses and appreciate its inherent simplicity, especially for IS-IS routing applications in IP environments.