3com 3.01.01 Manuale Utente

Intermediate System-to-Intermediate System (IS-IS) intra-domain routing 
information exchange protocol is the dynamic routing protocol used in the AS 
issued by the International Organization for Standardization (ISO). An intermediate 
system (IS) in the OSI reference model is basically equivalent to a router in the 
TCP/IP reference model. The IS-IS protocol, based on the link state algorithm, uses 
the Shortest Path First (SPF) algorithm. It is similar to the Open Shortest Path First 
(OSPF) protocol. 

Integrated

 

IS-IS is an implementation of IS-IS for IP regulated by the IETF. 

This section introduces IS-IS routing protocol terms.

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

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End System (ES). An ES equals the host system of TCP/IP. An 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 (LSP). 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). NPDUs are the network layer packets of 
ISO and are basically equivalent to the IP packet of TCP/IP. 

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Designated Intermediate System (DIS), is the elected router on the broadcast 
network, equivalent to the DR in OSPF.

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

Configuring IS-IS is described in the following sections:

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IS-IS adopts a two-level structure, Level-1 and Level-2, in a routing domain (or an 
AS) to support a large-scale routing network. A large RD is divided into one or 
more areas. The Level-1 routers manage the intra-area routing and are responsible 
for communicating with other Level-1 routers in the same area. The Level-2 
routers manage the inter-area routing.