Configuring IS-IS

The essential tasks to begin IS-IS routing are:

Enable IS-IS on the router:
Router(config)#router isis

Configure each router’s NET:
Router(config-router)#net 49.0010.1111.2222.3333.00

Enable IS-IS on the router’s interfaces:
Router(config)#interface s0/0/0
Router(config-int)#ip router isis


You may wish to do some tuning of IS-IS routing. Following are the tasks:

Set the IS level. Cisco routers are L1-2 by default. If the router is completely an internal area router, set the IS level to L1. If the router routes only to other areas and has no internal area interfaces, set the IS level to L2. If the router has both internal and external area interfaces, leave the IS level at L1-2.

Router(config-router)is-type {level-1 | level 1-2 | level-2-only}


Set the circuit type on L1-2 routers. On L1-2 routers, all interfaces send out both L1 and L2 hellos, trying to establish both types of adjacencies. This can waste bandwidth. If only an L1 router is attached to an interface, then change the circuit type for that interface to L1, so that only L1 hellos are sent. If there is only a L2 router attached to an interface, change the circuit type for that interface to L2:

Router(config-int)#isis circuit-type {level-1 | level 1-2 | level-2-only}


Summarize addresses. Although IS-IS does CLNS routing, it can summarize the IP addresses that it carries. Summarized routes can be designated as Level 1, Level 2, or Level 1-2 routes. The default is Level 2:

Router(config-router)#summary-address prefix mask [level-1 | level-2 | level-1-2]


Adjust the metric. IS-IS uses a metric of 10 for each interface. You can manually assign a metric that more accurately reflects the interface characteristics, such as bandwidth:

Router(config-int)#isis metric metric {level-1 | level-2}


Verifying and Troubleshooting IS-IS
Table 4-2 shows some IS-IS verification and troubleshooting commands, and describes the information you obtain from these commands.