IS-IS Adjacency Formation and Neighbor State Machine
What the exam tests
The exam checks the IIH fields that must match, the three adjacency states, and the common reasons an adjacency stays in Initializing.
Core rules
- IIH carries circuit type, system ID, holding time, area addresses, protocols supported, interface addresses and authentication.
- L1 adjacency requires a matching area address. L2 adjacency does not require matching area addresses.
- Levels must match: an L1-only interface cannot form an L2 adjacency.
- MTU, authentication and interface type must be compatible; a broadcast-versus-point-to-point mismatch blocks adjacency.
- IS-IS states are Down, Initializing (Init) and Up. There is no two-way or exchange state as in OSPF.
- Point-to-point links can use the optional three-way handshake TLV.
Worked example
R1 is L1 in area 49.0001 and R2 is L1 in area 49.0002 on the same LAN. Each sees the other's IIH, so both show Init, but neither reaches Up because the area addresses do not intersect. Adding 49.0002 as a second area on R1, or changing both interfaces to L2, resolves it. Raising the level to L2 works because L2 ignores the area-address mismatch.
Traps
Believing L2 requires an area match; confusing Init with a routing failure; overlooking MTU or hello padding; mismatching authentication; setting one side as point-to-point and the other as broadcast.