ISIS Adjacency

Hello,

There are only three adjacency states in IS-IS.

Down: This is the initial state. Its means that no hellos have been received from the neighbor.

Initializing: This state means that the local router has successfully received hellos from the neighboring router, however it’s not sure that the neighboring router has also successfully received local router’s hellos.

Up: Now it’s confirmed that neighboring router is receiving local router’s hellos. 

Network Diagram

The network diagram described below will be used.The addressing scheme is as follows.

Subnets are of type 192.168.X.0 where X is shown between interfaces in the diagram. The loopbacks are of type 192.168.YY.YY, where Y is 1 when Router is R1. So for R1 loopback ip will be 192.168.11.11. 

L1, L1/L2 and L2 are Level 1, Level 1-2 and Level 2 routers respectively. 

Thanks

Hi,
Please refer the reasons below: 

1. IS-IS circuit type. Level 1 and level 2 circuit cannot establish the neighbor relationship. Level 1 circuit only establish the IS-IS neighbor with the level 1 or level-1-2 circuit, while Level 2 circuit only establish the IS-IS neighbor with the level 2 or level-1-2 circuit.

2. Area ID. For level 1 area, it requires that all the routers run the same area ID to establish the neighbor.

3. The same network for the broadcast network.

4. The same authentication type and password.

5. MTU. Unlike the OSPF, IS-IS will fill the hello packet to the length of the MTU. So, when the MTU dismatch, hello packet will be discarded on one side due to the packet over size.

6. Different system ID. System ID is similar with the router ID in OSPF, IS-IS requires the system ID unique.