One of the most commonly used routing protocols, the Intermediate System-to-Intermediate System protocol (IS-IS) is based on a routing method known as DECnet Phase V routing, in which routers known as intermediate systems exchange data about routing using a single metric to determine the network topology. IS-IS was developed by the International Organization for Standardization (ISO) as part of their Open Systems Interconnection (OSI) model. The first versions of IS-IS were used to manage routing within ISO Connectionless Network Protocol (CLNP) networks. IS-IS was ratified as a standard in 1990 (OSI IS-IS Intra-domain Routing Protocol, IETFRFC 1142 , ISO/IEC 10589 ).
In the OSI context, an intermediate system refers to a router, as opposed to an end system (ES), which refers to a node. ES-IS protocols allow routers and nodes to identify each other; IS-IS performs the same service between nodes for routing purposes. In common with other routing protocols such as OSPF (Open Shortest Path First), IS-IS is a link state protocol: it stores information about the state of links and uses that data to select paths. IS-IS is used to intermittently send out link state information across the network, so that each router can maintain a current picture of network topology. Optional metrics can be used to identify network delay, expense, and error involved with the use of a particular link.