Verify Neighbors or Adjacencies at the IP Layer
Purpose
If the IP addressing is configured incorrectly then the OSPF neighbors or IS-IS adjacencies both need to be checked to determine if one or both of them are established.
Action
To verify neighbors (OSPF) or adjacencies (IS-IS), enter the following commands from the ingress, transit, and egress routers:
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
Meaning
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.
