Step 3: Verify Neighbors or Adjacencies at the IP Layer

Action

To verify neighbors (OSPF) or adjacencies (IS-IS), enter the following commands from the ingress, transit, and egress routers:

user@host> show ospf neighbor extensive

user@host> show isis adjacency extensive

Sample Output 1

user@R1> show ospf neighbor extensive

Address         Interface             State      ID              Pri  Dead

10.1.12.2        so-0/0/0.0             Full      10.0.0.2         128   34  

  area 0.0.0.0, opt 0x42, DR 0.0.0.0, BDR 0.0.0.0

  Up 1d 04:45:20, adjacent 1d 04:45:20

10.1.15.2        so-0/0/1.0             Full      10.0.0.5         128   35  

  area 0.0.0.0, opt 0x42, DR 0.0.0.0, BDR 0.0.0.0

  Up 1d 04:45:20, adjacent 1d 04:45:10 <<< no adjacency with R3 so-0/0/2

user@R3> show ospf neighbor extensive 

Address         Interface             State      ID              Pri  Dead

10.1.23.1        so-0/0/1.0             Full      10.0.0.2         128   35  

  area 0.0.0.0, opt 0x42, DR 0.0.0.0, BDR 0.0.0.0

  Up 1w2d 04:54:30, adjacent 1w2d 04:54:21

10.1.36.2        so-0/0/3.0             Full      10.0.0.6         128   39  

  area 0.0.0.0, opt 0x42, DR 0.0.0.0, BDR 0.0.0.0

  Up 1w2d 04:54:30, adjacent 1w2d 04:54:30 <<< no adjacency with R1 so-0/0/2

user@R6> show ospf neighbor extensive 

Address         Interface             State      ID              Pri  Dead

10.1.56.1        so-0/0/0.0             Full      10.0.0.5         128   39  

  area 0.0.0.0, opt 0x42, DR 0.0.0.0, BDR 0.0.0.0

  Up 1d 02:59:35, adjacent 1d 02:59:35

10.1.26.1        so-0/0/2.0             Full      10.0.0.2         128   36  

  area 0.0.0.0, opt 0x42, DR 0.0.0.0, BDR 0.0.0.0

  Up 1w2d 04:57:30, adjacent 1w2d 04:57:30

10.1.36.1        so-0/0/3.0             Full      10.0.0.3         128   36  

  area 0.0.0.0, opt 0x42, DR 0.0.0.0, BDR 0.0.0.0

  Up 1w2d 04:56:11, adjacent 1w2d 04:56:11

Sample Output 2

user@R1> show isis adjacency extensive 

R2

  Interface: so-0/0/0.0, Level: 2, State: Up, Expires in 23 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 05:57:16 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.12.2

  Transition log:

  When                  State        Reason

  Fri Oct 15 14:58:35   Up           Seenself

R5

  Interface: so-0/0/1.0, Level: 2, State: Up, Expires in 26 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 05:56:52 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.15.2

  Transition log:

  When                  State        Reason

  Fri Oct 15 14:59:00   Up           Seenself

R3

  Interface: so-0/0/2.0, Level: 2, State: Up, Expires in 26 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 05:56:51 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.13.2

  Transition log:

  When                  State        Reason

  Fri Oct 15 14:59:01   Up           Seenself

user@R3> show isis adjacency extensive 

R4

  Interface: so-0/0/0.0, Level: 2, State: Up, Expires in 25 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 1w1d 00:22:51 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.34.2

  Transition log:

  When                  State        Reason

  Thu Oct 28 15:13:12   Up           Seenself

R2

  Interface: so-0/0/1.0, Level: 2, State: Up, Expires in 25 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 2w2d 18:02:48 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.23.1

  Transition log:

  When                  State        Reason

  Tue Oct 19 21:33:15   Up           Seenself

R1

  Interface: so-0/0/2.0, Level: 2, State: Up, Expires in 22 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 2w2d 17:24:06 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.13.1

  Transition log:

  When                  State        Reason

  Tue Oct 19 22:11:57   Up           Seenself

R6

  Interface: so-0/0/3.0, Level: 2, State: Up, Expires in 21 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 2w1d 00:07:00 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.36.2

  Transition log:

  When                  State        Reason

  Thu Oct 21 15:29:03   Up           Seenself

user@R6> show isis adjacency extensive 

R5

  Interface: so-0/0/0.0, Level: 2, State: Up, Expires in 23 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 1w2d 01:10:03 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.56.1

  Transition log:

  When                  State        Reason

  Wed Oct 27 14:35:32   Up           Seenself

R4

  Interface: so-0/0/1.0, Level: 2, State: Up, Expires in 25 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 1w1d 00:26:50 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.46.1

  Transition log:

  When                  State        Reason

  Thu Oct 28 15:18:45   Up           Seenself

R2

  Interface: so-0/0/2.0, Level: 2, State: Up, Expires in 24 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 2w1d 00:11:40 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.26.1

  Transition log:

  When                  State        Reason

  Thu Oct 21 15:33:55   Up           Seenself

R3

  Interface: so-0/0/3.0, Level: 2, State: Up, Expires in 19 secs

  Priority: 0, Up/Down transitions: 1, Last transition: 2w1d 00:11:40 ago

  Circuit type: 2, Speaks: IP, IPv6

  Topologies: Unicast

  Restart capable: Yes

  IP addresses: 10.1.36.1

  Transition log:

  When                  State        Reason

  Thu Oct 21 15:33:55   Up           Seenself

What It Means

Sample Output 1 from the ingress, transit, and egress routers shows that R1 and R3 are not established OSPF neighbors. Considering that the two interfaces so-0/0/2.0 (R1 and R3) are configured with identical IP addresses, you would expect this. The OSPF protocol routes IP packets based solely on the destination IP address contained in the IP packet header. Therefore, identical IP addresses in the autonomous system (AS) result in neighbors not establishing.

Sample Output 2 from the ingress, transit, and egress routers shows that R1 and R3 have established an IS-IS adjacency despite the identical IP addresses configured on interfaces so-0/0/2.0 on R1 and R3. The IS-IS protocol behaves differently from the OSPF protocol because it does not rely on IP to establish an adjacency. However, if the LSP is not up, it is still useful to check the IP subnet addressing in case there is a mistake in that layer. Correcting the addressing error might bring the LSP back up.