Step 2: Verify IS-IS Adjacencies and Interfaces

Purpose

When you check the IS-IS layer, you verify that IS-IS adjacencies are up, and that the IS-IS interfaces are included at the protocol level.

Action

To verify the functioning of adjacent interfaces, enter the following commands from the relevant routers:

user@host> show isis adjacency

user@host> show isis interface

Sample Output 1

user@R1> show isis adjacency 

Interface             System         L State        Hold (secs) SNPA

so-0/0/0.0            R2             2 Up                   20

so-0/0/1.0            R5             2 Up                   23

so-0/0/2.0            R3             2 Up                   26

user@R3> show isis adjacency 

Interface             System         L State        Hold (secs) SNPA

so-0/0/0.0            R4             2 Up                   23

so-0/0/1.0            R2             2 Up                   21

so-0/0/2.0            R1             2 Up                   19

so-0/0/3.0            R6             2 Down                  0

user@R6> show isis adjacency 

user@R6>                     <<< No IS-IS adjacencies are established

Sample Output 2

user@R1> show isis interface 

IS-IS interface database:

Interface             L CirID Level 1 DR        Level 2 DR        L1/L2 Metric

lo0.0                 0   0x1 Passive           Passive                 0/0

so-0/0/0.0            2   0x1 Disabled          Point to Point         10/10

so-0/0/1.0            2   0x1 Disabled          Point to Point         10/10

so-0/0/2.0            2   0x1 Disabled          Point to Point         10/10

user@R3> show isis interface 

IS-IS interface database:

Interface             L CirID Level 1 DR        Level 2 DR        L1/L2 Metric

lo0.0                 0   0x1 Passive           Passive                 0/0

so-0/0/0.0            2   0x1 Disabled          Point to Point         10/10

so-0/0/1.0            2   0x1 Disabled          Point to Point         10/10

so-0/0/2.0            2   0x1 Disabled          Point to Point         10/10

so-0/0/3.0            2   0x1 Disabled          Point to Point         10/10

user@R6> show isis interface 

IS-IS interface database:

Interface             L CirID Level 1 DR        Level 2 DR        L1/L2 Metric

lo0.0                 0   0x1 Passive           Passive                 0/0

so-0/0/0.0            1   0x1 Point to Point    Disabled               10/10

so-0/0/1.0            1   0x1 Down              Disabled               10/10

so-0/0/2.0            1   0x1 Point to Point    Disabled               10/10

so-0/0/3.0            1   0x1 Point to Point    Disabled               10/10

What It Means

Sample Output 1 shows that ingress router R1 has established adjacencies with the relevant routers. Transit router R3 does not have an adjacency with egress router R6, and egress router R6 has no adjacencies established in the network shown in Figure 15, indicating that the problem might be at the IS-IS protocol level.

Sample Output 2 shows that R1 and R2 are Level 2 routers, in contrast to R6 which is a Level 1 router. When a router is configured explicitly as a Level 1 or Level 2 router, it does not communicate with routers configured at a different level. Level 1 routers communicate with other Level 1 routers within their area, while Level 2 routers communicate with other Level 2 routers, and towards other autonomous systems. Because all the routers in this network are configured for Level 2, they cannot form an adjacency with R6, which is incorrectly configured as a Level 1 router.