This chapter is from the book
Rules for IS-IS Adjacencies
- L1 routers form L1 adjacencies with L1 and L1-L2 routers in their area.
- L2 routers form L2 adjacencies with L2 and L1-L2 routers in their area or another area.
- L1/L2 routers form L1 and L2 adjacencies with each other in their area or another area.
- An L1 router does not form an adjacency with an L2 router, regardless of area.
- The system ID must be unique to each router.
- Hello intervals and hold times do not have to match.