Cisco CCNA And CCNP BSCI Exam Troubleshooting Exercise:

Loss Of OSPF Neighbor Adjacency

By Chris Bryant, CCIE #12933

OSPF isn't just a huge part of your CCNA and CCNP BSCI exams, it's a protocol you'll run into over and over in real-world networking. Let's take a look at an OSPF adjacency issue and talk about how to diagnose the issue and what we can do about it.

In this example, we've got two routers on the same network segment, 172.23.23.0 /24. They have IP connectivity as verified by a ping (not shown). Each router has an IP address on its Ethernet interface that uses the router number for the last octet.

For some reason, the routers are not forming an adjacency. Running show interface ospf on both routers verifies the mask and address settings:

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)

The answer may be in that output, and it may not be. :) In this exercise, I've got a few questions for you...

1. What debug will help you spot the exact issue?

2. What exactly is the problem?

3. What's the most efficient way to solve the issue?

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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 - the problem is the dead and hello timer values. This debug actually tells us exactly what the problem is - mismatched hello timers.

The default hello and dead OSPF timers on an Ethernet segment are 10 and 40 seconds, respectively. According to this debug, that's exactly what we're getting from R3, so... the problem must be on R2, and the command we ran earlier, show ip ospf interface e0, indicates that someone has changed the default timers on R2.

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)

To get the OSPF adjacency to come up, we need to change the OSPF timers on one of the routers so the values match on R2 and R3. We could go to R3's ethernet interface and use the command ip ospf hello 15, which would change both the hello and dead timers. (The OSPF dead time defaults to four times the hello time.)

Since we're on R2, we'll use the no ip ospf hello 15 command on the ethernet interface, which will set the hello time back to the default.

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

In a matter of seconds, R2 has an adjacency with R3, verified with 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

And as always, when you're done.... turn off your debugs! You can turn them off on an individual basis by entering the particular debug command with the word no in front ("no debug ip ospf adj"), or you can make sure you've turned off all debugs with the "u all" command, short for undebug all.

R2#undebug all
All possible debugging has been turned off

When you're practicing your Cisco skills in a lab environment, make sure to run debugs as you go along - that's the real secret to absolutely mastering switching and routing!

And while you're here, check out my other Network Troubleshooting Challenges!

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