Relationship to the IP route table

The IS-IS routes are calculated and first placed in the IS-IS route table. The routes are then transferred to the IP route table.

The best IS-IS path for a given destination is sent to the IP route table for comparison to the best paths from other protocols to the same destination. The CPU selects the path with the lowest administrative distance and places that path in the IP route table:

• If the path provided by IS-IS has the lowest administrative distance, then the CPU places that IS-IS path in the IP route table.
• If a path to the same destination supplied by another protocol has a lower administrative distance, the CPU installs the other protocol‘s path in the IP route table instead.

The administrative distance is a protocol-independent value from 1 - 255. Each path sent to the CPU, regardless of the source of the path (IS-IS, OSPF, static IP route, and so on) has an administrative distance.

Each route source has a default administrative distance. The default administrative distance for IS-IS is 115.

You can change the administrative distance for IS-IS and other routes sources.