A complex routing protocol that shares information with other routers in order to determine the best path. IS-IS was the first comprehensive link state protocol, and OSPF and NLSP evolved from it. Link state protocols use characteristics of the route such as speed and cost as well as current congestion to determine the best path, which is typically computed by the Dijkstra algorithm.

Link state routers are updated from all the routers in the entire network by passing information from router to nearest router. Rather than continuously broadcast its routing tables as does a distance vector protocol, a link state protocol router only notifies its neighboring routers when it detects a change. Contrast with distance vector protocol and path vector protocol. See routing protocol, OSPF, NLSP and IS-IS.