What is IS –IS protocol ?

Intermediate System to Intermediate System (IS-IS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices.

IS-IS (pronounced "i-s i-s") is an interior gateway protocol, designed for use within an administrative domain or network. This is in contrast to Exterior Gateway Protocols, primarily Border Gateway Protocol (BGP), which is used for routing between autonomous systems (RFC 1930).

IS-IS is a link-state routing protocol, operating by reliably flooding link state information throughout a network of routers. Each IS-IS router independently builds a database of the network's topology, aggregating the flooded network information. Like the OSPF protocol, IS-IS uses Dijkstra's algorithm for computing the best path through the network. Packets (datagrams) are then forwarded, based on the computed ideal path, through the network to the destination