In your preparation for your CCNA and CCNP exams, you should run as many debugs as you can in a lab environment. (Never practice debugs at work.) Debugs also help you spot issues with Cisco router configurations that you might not otherwise identify just by looking at the output of show commands.

In this exercise, R2 and R3 have OSPF enabled on their ethernet interfaces. IP connectivity has been verified via ping. OSPF has been correctly configured, but we don’t have an adjacency. A good place to start troubleshooting OSPF adjacencies is show ip ospf interface.

R2#show ip ospf interface ethernet0
Ethernet0 is up, line protocol is up
Internet Address 172.23.23.2 /24, Area 0
Process ID 1, Router ID 172.23.23.2, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 172.23.23.2, Interface address 172.23.23.2
No backup designated router on this network
Timer intervals configured, Hello 15, Dead 60, Wait 60, Retransmit 5
Hello due in 00:00:04
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)

R3#show ip ospf interface ethernet0
Ethernet0 is up, line protocol is up
Internet Address 172.23.23.3 /24, Area 0
Process ID 1, Router ID 172.23.23.3, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 172.23.23.3, Interface address 172.23.23.3
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)

You might have spotted the issue just by looking at the configurations, but it's a good idea to know the debug that helps you detect the reason(s) for OSPF adjacencies that do not form as expected - debug ip ospf adj.

R2#debug ip ospf adj
OSPF adjacency events debugging is on
R2#
00:54:19: OSPF: Rcv hello from 172.12.23.3 area 0 from Ethernet0 172.12.23.3
00:54:19: OSPF: Mismatched hello parameters from 172.12.23.3
00:54:19: Dead R 40 C 60, Hello R 10 C 15 Mask R 255.255.255.128 C 255.255.255.128

The problem is not a lack of Hello packets from R3, it’s the mismatched hello value contained in the hello packet. When potential OSPF neighbors disagree on the hello and/or dead timers, they’ll remain potential neighbors until this situation is corrected.

The default OSPF hello and dead timers are 10 and 40 seconds, and those are the values contained in the hello received from R3. Therefore, someone must have changed the timers on R2. Let’s look at the output of show ip ospf interface again to see if that’s the case.

R2#show ip ospf interface ethernet0
Ethernet0 is up, line protocol is up
Internet Address 172.23.23.2 /24, Area 0
Process ID 1, Router ID 172.23.23.2, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 172.23.23.2, Interface address 172.23.23.2
No backup designated router on this network
Timer intervals configured, Hello 15, Dead 60, Wait 60, Retransmit 5
Hello due in 00:00:04
Neighbor Count is 0, Adjacent neighbor count is 0
Suppress hello for 0 neighbor(s)

Yep! Both timers have been changed on R2, so we’ve got to either change the hello and dead tiemrs on R3 to match those on R2, or reset the timers on R2 back to the default. We're on R2 already, so we'll use no ip ospf hello 15 to change the interface settings back to the default.

R2(config)#int e0
R2(config-if)#no ip ospf hello 15

The adjacency comes up seconds later, as verified by show ip ospf neighbor.

R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.23.23.3 1 FULL/DR 00:00:36 172.23.23.3 Ethernet0

Don’t count on being able to spot problems just by looking at router configs – make sure to run debugs during your CCNA and CCNP preparation to see how things work, and that will help you to spot the problem when things aren’t working!