Yet another difference b/w IS-IS and OSPF

Neighbour is someone who lives nearby, normally in a house or apartment that is next door or, in the case of houses, across the street. Some neighbors are adjacent while some are not. People form friendships with their neighbors, and help them by sharing their information & tasks.

Similarly, in Network & routing world there is a difference in neighborship & adjacency.

OSPF neighbourship is straightforward while IS-IS needs a bit deeper thought to get it fully. Lets take a closer look at both.

For OSPF, in order to form OSPF adjacencies, the following must match on the neighboring routers:

• Hello and Dead intervals
• Area ID
• Authentication type and password
• Stub Area flag
• Subnet ID and Subnet mask

Most importantly. both the routers need to be in same subnet and directly connected or directly attached L3 hops as OSPF send hello messages with TTL value=1, so it is not possible for OSPF to cross another gateway device.The routers must pass through all the OSPF states to become fully adjacent.  The 2-way state means that the two routers have established bidirectional communication but they have not yet exchanged database information.

In case of two IS’s (Routers in ISO world) to become IS-IS neighbors:

• Interface MTUs
• Areas must match (if level 1)
• System IDs must be unique
• Authentication must succeed
• Levels must match

IS-IS uses Hello PDUs to establish adjacencies with other routers

• L1-only routers and L1/L2 routers à L1 adjacency
• L2-only routers and L1/L2 routers à L2 adjacency
• L1-only and L2-only routers à No adjacency

In general, rules to follow for IS-IS:

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.

L1L2 routers form L1 and L2 adjacencies with each other in their area or another area.

L1 router does not form an adjacency with an L2 router