IP Integration into IS-IS

IP routing is integrated into IS-IS via carrying IP reachability information in LSPs. All IP networks are considered externals, and they always end up as leaf nodes in the shortest path tree when IS-IS does a SPF run.

This means that changes in IP reachability alone result only in a partial SPF run (Partial Route Calculation, or PRC); the routers in the tree need to calculate only the parts of the tree in which the leaf node for that destination network resides.

Only L2 routers can summarize IP destinations to shorter masks.