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Enable IS-IS on Routers in Your Network



To enable IS-IS on routers in your network, follow these steps:

  1. In configuration mode, go to the following hierarchy level:

  2. Disable Level 1 if appropriate for your network:

  3. Configure the interface:

  4. Disable the management interface if you have included the interface all statement, as shown in Sample Output 2:

  5. Include the loopback interface (lo0) if you have listed all interfaces separately, as shown in Sample Output 1:

  6. Set the loopback interface (lo0) to passive:

  7. Verify and commit the configuration:

Sample Output 1

Sample Output 2


Sample Output 1 shows that IS-IS Level 1 is disabled, making this a Level 2 router. When all routers in the network are running at one IS-IS level (Level 2), default CSPF LSPs can occur.

Because R1 in Sample Output 1 has all IS-IS enabled interfaces listed, including the loopback interface (lo0), you do not need to include the disable statement for the management interface (fxp0). All interfaces have unit number 0, the default if a unit number is not specified. When you configure an interface at the [edit protocols isis] hierarchy level, and you do not include the logical unit, the default 0 is appended to the interface name, for example, so-0/0/1.0.

Sample Output 2 does not list the interfaces configured with IS-IS; instead, all interfaces are configured, including the loopback interface (lo0) and the management interface (fxp0). Therefore, you do not need to include a separate statement for the loopback (lo0) interface. However, in this instance, it is best practice to disable the management interface (fxp0) so that IS-IS packets are not sent over it. If you do not disable the management interface (fxp0) when you include the interface-all statement, the IS-IS protocol can form adjacencies over the management backbone, but traffic does not flow because transit traffic does not go out of the management interface.

Sample Output 2 also shows that all interfaces on R6 are configured with a metric of 15. A metric is not required to configure IS-IS on your interfaces. The default metric value is 10 (with the exception of the loopback [lo0] interface, which has a default metric of 0). A metric is included to demonstrate that you can configure a metric for IS-IS if the default (10) is not appropriate for your network.

Both sample outputs show the passive statement included in the configuration of the loopback (lo0) interface. Including the passive statement is considered best practice and ensures the following:

  • Protocols are not run over the loopback (lo0) interface

  • When the router ID (RID) is configured manually, ensures that the loopback (lo0) interface is advertised to other networks.


    It is considered best practice to configure the RID manually to avoid duplicate RID problems.