Intermediate System to Intermediate System(IS-IS) is a router protocol in the OSI environment that routers use among themselves to exchange routing information, error messages, status messages, etc.. The IS-IS protocol works with the link-state algorithm according to a similar concept as Open Shortest Path First( OSPF), except that it is designed for OSI addresses and is altogether more global than the OSPF protocol.
The IS-IS protocol supports dynamic routing. It is based on a domain concept and thus enables global, flexible and hierarchical modeling. The algorithm is suitable for both small and large networks (up to 10,000 routers and up to 100,00 end nodes). IS-IS supports four different metrics for line capacity, processing time, cost, and linkerror rate.
An Intermediate system can support any combination of these metrics; for each metric, it calculates its own SPF tree, Shortest Path First (SPF), and creates its own routing table. The use of different metrics makes sense because of the different possible networks( Ethernet, Token Ring, Satellite), as utilization may be the more important criterion in one case and cost in another.
In the IS-IS protocol, there are four different message types for determining the network topology. These include the Link State Packet( LSP), the Partial Sequence Number PDU( PSNP) and the Complete Sequence Number PDU( CSNP). The network topological information from these message types is stored and updated in the Link State Database( LSDB).