Cisco Systems 3560 사용자 설명서

The key difference between the ISO IGRP and IS-IS NSAP addressing schemes is in the definition of 
area addresses. Both use the system ID for Level 1 routing (routing within an area). However, they differ 
in the way addresses are specified for area routing. An ISO IGRP NSAP address includes three separate 
fields for routing: the domain, area, and system ID. An IS-IS address includes two fields: a single 
continuous area field (comprising the domain and area fields) and the system ID.

For more detailed information about ISO CLNS, see the Cisco IOS Apollo Domain, Banyan VINES, 
DECnet, ISO CLNS and XNS Configuration Guide, Release 12.2. For complete syntax and usage 
information for the commands used in this chapter, see the Cisco IOS Apollo Domain, Banyan VINES, 
DECnet, ISO CLNS and XNS Command Reference, Release 12.2, use the IOS command reference master 
index, or search online.

IS-IS is an ISO dynamic routing protocol (described in ISO 105890). Unlike other routing protocols, 
enabling IS-IS requires that you create an IS-IS routing process and assign it to a specific interface, 
rather than to a network. You can specify more than one IS-IS routing process per Layer 3 switch or 
router by using the multiarea IS-IS configuration syntax. You then configure the parameters for each 
instance of the IS-IS routing process.

Small IS-IS networks are built as a single area that includes all the routers in the network. As the network 
grows larger, it is usually reorganized into a backbone area made up of the connected set of all Level 2 
routers from all areas, which is in turn connected to local areas. Within a local area, routers know how 
to reach all system IDs. Between areas, routers know how to reach the backbone, and the backbone 
routers know how to reach other areas.

Routers establish Level 1 adjacencies to perform routing within a local area (station routing). Routers 
establish Level 2 adjacencies to perform routing between Level 1 areas (area routing).

A single Cisco router can participate in routing in up to 29 areas and can perform Level 2 routing in the 
backbone. In general, each routing process corresponds to an area. By default, the first instance of the 
routing process configured performs both Level 1and Level 2 routing. You can configure additional 
router instances, which are automatically treated as Level 1 areas. You must configure the parameters 
for each instance of the IS-IS routing process individually.

For IS-IS multiarea routing, you can configure only one process to perform Level 2 routing, although 
you can define up to 29 Level 1 areas for each Cisco unit. If Level 2 routing is configured on any process, 
all additional processes are automatically configured as Level 1. You can configure this process to 
perform Level 1 routing at the same time. If Level 2 routing is not desired for a router instance, remove 
the Level 2 capability using the is-type global configuration command. Use the is-type command also 
to configure a different router instance as a Level 2 router.

For more detailed information about IS-IS, see the “IP Routing Protocols” chapter of the Cisco IOS IP 
Configuration Guide, Release 12.2. For complete syntax and usage information for the commands used 
in this section, see the Cisco IOS IP Command Reference, Release 12.2. 

This section briefly describes how to configure IS-IS routing. It includes this information: