Troubleshooting IS-IS Neighbor Relationships
There are two instances where IS-IS will not form neighbor adjacencies correctly. The first is with misconfigured NSAPs. For example, Router C has just been attached to the network in Figure 6-8, and it isn't forming L1 adjacencies with Routers A and B as was expected.
Figure 6-8. IS-IS Neighbors in Different Areas
On Router A, you see the following:
A#sh clns neiSystem Id SNPA Interface State Holdtime Type ProtocolB 00C0.174A.08FD Et0 Up 27 L1 IS-ISC 00C0.0c76.f096 Et0 Up 26 L2 IS-IS
It's easy to see from this ...