3com 8807 User Guide

Intermediate System-to-Intermediate System (IS-IS) intra-domain routing 
information exchange protocol is designed by the international organization for 
standardization (ISO) for connection-less network protocol (CLNP). This protocol is 
a dynamic routing protocol. To let this protocol support IP routing, IETF expands 
and modifies IS-IS in RFC1195, applying the protocol to TCP/IP and OSI. The 
modified IS-IS is called Integrated IS-IS or Dual IS-IS.

IS-IS is a link state protocol, which uses shortest path first (SPF) algorithm. IS-IS and 
the OSPF protocol are similar in many aspects. As an interior gateway protocol 
(IGP), IS-IS is applied inside an AS.

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Intermediate System (IS). IS equals a router of TCP/IP. It is the basic unit in IS-IS 
protocol used for propagating routing information and generating routes. In 
the following text, the IS shares the same meaning with the router.

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End System (ES). It equals the host system of TCP/IP. ES does not process the 
IS-IS routing protocol, and therefore it can be ignored in the IS-IS protocol.

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Routing Domain (RD). A group of ISs exchange routing information with the 
same routing protocol in a routing domain.

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Area. Area is the division unit in the routing domain.

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Link State DataBase (LSDB). All the link states in the network form the LSDB. In 
an IS, at least one LSDB is available. The IS uses the SPF algorithm and the LSDB 
to generate its own routes.

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Link State Protocol Data Unit (LSPDU). In the IS-IS, each IS will generate an LSP 
which contains all the link state information of the IS. Each IS collects all the 
LSPs in the local area to generate its own LSDB.

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Network Protocol Data Unit (NPDU). It is the network layer packets of OSI and 
equals the IP packet of TCP/IP.

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Designated IS (DIS). It is the elected router on the broadcast network.

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Network Service Access Point (NSAP) is the network layer address of OSI. It 
identifies an abstract network service access point and describes the very 
network address structure for the OSI model.

IS-IS routing protocol can run on point to point Links, such as PPP, HDLC and 
others. IS-IS routing protocol can also run on broadcast links, such as Ethernet, 
Token-Ring and others.