167IS-IS configurationIS-IS overviewIS-IS is a dynamic routing protocol designed by the ISO to operate on the CLNP.The term router in this document refers to both routers and Layer switches.The IS-IS routing protocol was modified and extended in RFC 1195 by the IETF for application inboth TCP/IP and OSI reference models, and the new one is named Integrated IS-IS or Dual IS-IS.IS-IS is an IGP used within an AS. It adopts the SPF algorithm for route calculation.Basic conceptsIS-IS terminology• IS: An IS, similar to a router in TCP/IP, is the basic unit in IS-IS to generate and propagaterouting information. In the following text, an IS refers to a router.• ES: An ES refers to a host system in TCP/IP. ISO defines the ES-IS protocol forcommunication between an ES and an IS; therefore, an ES does not participate in the IS-ISprocessing.• RD: An RD is a group of ISs exchanges routing information with each other using the samerouting protocol in a routing domain.• Area. An area is a unit in a routing domain. The IS-IS protocol allows a routing domain tobe divided into multiple areas.• LSDB: All link states in the network form the LSDB. There is at least one LSDB in each IS. TheIS uses the SPF algorithm and LSDB to generate its own routes.• LSPDU or LSP: Each IS can generate an LSP, which contains all the link state information ofthe IS.• NPDU: An NPDU is a network layer protocol packet in OSI, which is equivalent to an IPpacket in TCP/IP.• Designated IS. On a broadcast network, the designated router is also known as thedesignated IS.• NSAP: An NSAP is an OSI network layer address. It identifies an abstract network serviceaccess point and describes the network address in the OSI reference model.
