BSCI - ISIS - Adjacency requirements

For an ISIS adjacency to come up the following criteria must match:
i) MTU  - default is 1497
ii) Router Levels - L1, L1/L2, L2 only
iii) If L1 router, the IS must be in the same area
iv) System ID's must be unique
v) Authentication (if used)

Adjacency is formed after a 3-way handshake. The stages are DOWN - INIT- UP

The adjacency is up if the neighbor has put you identity in their hello packet.