Core Issue
An Intermediate System-to-Intermediate System (IS-IS) level router is aware only of the topology of its own area. To route packets destined for networks outside its own area, the Level 1 (L1) router relies on the default route injected by a Level 1/Level 2-capable (L1/L2) router within the same area. The L1/L2-capable router has reachability information for all the other areas in the L2 link-state database, and it is able to route the packet appropriately. In case more than one L1/L2-capable routers exist, the L1 router hands over the packet to the nearest L1/L2-capable router. This could result in sub-optimal routing, since the nearest L1/L2-capable router might not lie on the shortest path to the destination.
Resolution
To reduce sub-optimal routing, L2 information can be selectively introduced into L1 areas. The L1 router is able to choose the best path based on the metric, resulting in optimal routing. Selective introduction of L2 information into an L1 area is called route leaking. To configure route leaking, issue the redistribute isis command in router configuration mode. You can optionally configure a distribute list or a route map to strictly control the L2 routes introduced into the L1 area.
For more information on IS-IS route leaking, refer to IS-IS Route Leaking Overview.
