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.