IS-IS natively supports only broadcast and point to point network types, and there are no special provisions needed to correctly operate over partially meshed data link layer topologies. However running Is-Is over those types of networks can result in partial visibility or incomplete routing tables, its best to configure point to point sub interfaces and run IS-IS over the point to point links.
IS-IS neighbors have three adjacency states:
- Down – initial state no IIHs have been received
- Initializing – IIHs have been received but its not certain that the originating router is receiving the IIHs from the receiving router.
- Up – IIHs have been received from the neighbor and both neighbors can see each other.