IS-IS is one of the first link-state routing protocols. In the previous sections our discussion has focused on RIP, which is a distance-vector routing protocol. As you know from our discussion, distance-vector routing protocols use the distance, or number of hops, to determine the metric for reaching a given destination. Link-state routing protocols, such as IS-IS, take a much more overall picture of the network and base their metrics on a more accurate set of metrics and calculations. We ll discuss some of the overall aspects that all link-state routing protocols share here.

Link-state routing protocols offer much more scalability and many more features than their distance-vector predecessors. Link-state protocols also offer a system of hierarchy within a routed domain that affords the user much greater control and scalability than would be possible using distance-vector routing protocols. These protocols also offer support for VLSM, enabling a network to support multiple subnet masks, making much better utilization of available IP address space. It is for these reasons, along with several others, that link-state protocols are the protocols of choice within large enterprise organizations as well as within the networks of major service providers.

Link-state protocols derive their name from the manner in which they view the network. They take the state, or conditions, of the path into account when making a routing determination. Routers examine not only the state of the specific link, but also the link and its relation to the neighboring router. Information such as...