35 IS-IS CONFIGURATIONIS-IS Overview The intermediate system-to-intermediate system (IS-IS) is a dynamic routingprotocol standardized by the International Organization for Standardization (ISO)to operate on connectionless network protocol (CLNP).The IS-IS routing protocol has been adopted in RFC 1195 by the InternationalEngineer Task Force (IETF) to be applied in both TCP/IP and OSI reference models,and this form is called Integrated IS-IS or Dual IS-IS.The IS-IS routing protocol, based on the link state algorithm, is an interior gatewayprotocol (IGP) used within an Autonomous System. It is similar to open shortestpath first (OSPF) using shortest path first (SPF) algorithm to calculate best paths inthe network.Basic Concept IS-IS terminology■ Intermediate system (IS). An IS, similar to a router in TCP/IP, is the basic unit inIS-IS protocol to generate and propagate routing information. In the followingtext, an IS equals to a router.■ End system (ES). An ES refers to a host system in TCP/IP. ISO uses ES-IS protocolto specify the communication between an ES and an IS, therefore an ES doesnot participate in the IS-IS process and can be ignored in the IS-IS protocol.■ Routing domain (RD). A group of ISs exchange routing information with thesame routing protocol in a routing domain.■ Area. An area is a division unit in a routing domain. The IS-IS protocol allows arouting domain to be divided into multiple areas.■ Link state database (LSDB). All link states in the network consist of the LSDB.There is at least one LSDB in each IS. The IS uses SPF algorithm and LSDB togenerate its own routes.■ Link state protocol data unit or link state packet (LSP). In the IS-IS routingprotocol, each IS can generate a LSP which contains all the link stateinformation of the IS. Each IS collects all the LSPs in the local area to generateits own LSDB.■ Network protocol data unit (NPDU). An NPDU is a network layer protocolpacket in ISO, which is equivalent to an IP packet in TCP/IP.■ Designated IS. On a broadcast network, the designated router is also known asthe designated IS or a pseudonode.■ Network service access point (NSAP). The NSAP is the ISO network layeraddress. It identifies an abstract network service access point and describes thenetwork address in the ISO reference model.
