Terminology

Intermediate system—Similar to a router in TCP/IP, IS is the basic unit used in an IS-IS routing domain to generate and propagate routing information. Throughout this chapter, an IS refers to a router.

End system—Similar to a host in TCP/IP, an ES does not run IS-IS. ISO defines the ES-IS protocol for communication between an ES and an IS.

Routing domain—An RD comprises a group of ISs that exchange routing information with each other by using the same routing protocol.

Area—An IS-IS routing domain can be split into multiple areas.

Link State Database—All link states in the network form the LSDB. Each IS has at least one LSDB. An IS uses the SPF algorithm and LSDB to generate IS-IS routes.

Link State Protocol Data Unit or Link State Packet —An IS advertises link state information in an LSP.

Network Protocol Data Unit—An NPDU is a network layer protocol packet in OSI, similar to an IP packet in TCP/IP.

Designated IS—A DIS is elected on a broadcast network.

Network service access point—An NSAP is an OSI network layer address. The NSAP identifies an abstract network service access point and describes the network address format in the OSI reference model.