A span of interconnected routers operated and managed by the same administrative group is referred to as an autonomous system of routers or a routing domain. Such a system of routers allows forwarding of data traffic from one location to the other. The current IS-IS specification, ISO 10589, refers to network nodes as intermediate systems, but this book uses the equivalent terminology of routers more frequently because it is more popular in current networking literature.
Individual routing domains are interconnected to form larger networks, such as the Internet, allowing transfer of data from one routing domain to the other over a large geographic span. Routers use routing protocols to learn about various locations within local or remote network domains. The two basic types of routing protocols follow:
Interior Gateway Protocols (IGPs)—Optimized only for operation within a single network domain. IGPs are also known as intradomain routing protocols.
Exterior Gateway Protocols (EGPs)—Optimized for exchange of routing information between domains. EGPs are also referred to as interdomain routing protocols.
IS-IS is designed and optimized to provide IGP functionality. The Border Gateway Protocol (BGP) is a well-known routing protocol with extensive capabilities for interdomain routing.
Typically, routing protocols support only one network layer protocol (Layer 3 in the OSI reference model). Therefore, when you use routers to provide connectivity for multiple Layer 3 protocols concurrently, they are usually configured with different routing protocols for each type of Layer 3 protocol supported. This approach is referred to as ships in the night.
As mentioned in the preceding chapter, Integrated IS-IS supports two network layer protocols: ISO CLNP and IP. Another routing protocol, which supports multiple Layer 3 protocols, is the Cisco proprietary Enhanced Interior Gateway Routing Protocol (EIGRP). EIGRP can be used to route IP, the Internet Packet Exchange Protocol (IPX), and AppleTalk all at the same time. Popular routing protocols that support only one network layer protocol include the NetWare Link Services Protocol (NLSP), which is based on the IS-IS protocol and supports only IPX; the Open Shortest Path First (OSPF) Protocol supports only IP. Versions 1 and 2 of the Routing Information Protocol (RIP) are also IP-only routing protocols. IS-IS and OSPF are similar in many regards and are the two most popular IGPs that are widely deployed in Internet service provider, IP-based enterprise networks.
IS-IS Routing Domain
An IS-IS routing domain is a network in which all the routers run the Integrated IS-IS routing protocol to support intradomain exchange of routing information. The network environment can be IP-only, ISO CLNP-only, or both. The IS-IS protocol was originally intended to support only CLNP. RFC 1195 adapts the original IS-IS specification (ISO 10589) to support IP, in what is referred to as Integrated IS-IS. The following implementation requirements are specified by RFC 1195:
Pure IP domains route only IP traffic but support forwarding and processing of OSI packets required for IS-IS operation.
Pure ISO domains carry only ISO traffic including those required for IS-IS operation.
A dual domain routes both IP and OSI CLNP traffic simultaneously.
It is also possible to design a dual domain so that some areas route IP only, whereas others route CLNP only, and yet others route both IP and CLNP. RFC 1195 imposes restrictions on the manner in which IP and CLNP can be mixed within an area. The underlying goal is to achieve consistent routing information within an area by having identical Level 1 link-state databases on all routers in that area. Hence, all routers in an area are required to be configured in the same way, either for IP-only or CLNP-only or both. To clarify further, a router is not allowed to have a set of links dedicated to IP only and another set to CLNP and yet another set to both protocols. At the domain level, there is no restriction on mixing areas that are uniformly IP-only with other areas that are uniformly CLNP-only or uniformly configured for both IP and CLNP. In order words, all links in an area must be configured the same way, but links in the backbone can have the attached routers configured differently.