Cisco Systems 78-11741-02 manual IPC-278, Routerconfig# router isis area tag

Configuring Integrated IS-IS

IS-IS Configuration Task List

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 (intra-area routing). Routers establish Level 2 adjacencies to perform routing between Level 1 areas (interarea routing).

Some networks use legacy equipment that supports only Level 1 routing. These devices are typically organized into many small areas that cannot be aggregated due to performance limitations. Cisco routers are used to interconnect each area to the Level 2 backbone.

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 1 and 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 router. 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-typerouter configuration command. Use the is-typerouter configuration command also to configure a different router instance as a Level 2 router.

Network entity titles (NETs) define the area addresses for the IS-IS area and the system ID of the router. Refer to the “Configuring ISO CLNS” chapter in the Cisco IOS Apollo Domain, Banyan VINES, ISO CLNS, and XNS Configuration Guide for a more detailed discussion of NETs.

To enable IS-IS and specify the area for each instance of the IS-IS routing process, use the following commands in global configuration mode:

	Command	Purpose
Step 1
	Router(config)# router isis [area tag]		Enables IS-IS routing for the specified routing process, and places
			the router in router configuration mode.
			Use the area tag arguments to identify the area to which this IS-IS
			router instance is assigned. A value for tag is required if you are
			configuring multiple IS-IS areas.
			The first IS-IS instance configured is Level 1-2 by default. Later
			instances are automatically Level 1. You can change the level of
			routing to be performed by a particular routing process using the
			is-typerouter configuration command.
Step 2
	Router(config)# net network-entity-title		Configures NETs for the routing process. Specify a NET for each
			routing process if you are configuring multiarea IS-IS. You can
			specify a name for a NET and for an address.
			Note	Multiarea IS-IS is supported only for ISO CLNS.

See the “IS-IS Configuration Examples” section at the end of this chapter for examples of configuring IS-IS as an IP routing protocol.

Cisco IOS IP Configuration Guide

IPC-278