OSPF Troubleshooting Examples

OSPF issues typically involve neighbor adjacency failures, routing problems, or configuration mismatches. Here are common scenarios with troubleshooting commands and solutions.

Problem 1: Neighbors Not Forming Adjacency

Symptom:

R1# show ip ospf neighbor
(No output or neighbor stuck in INIT/2WAY state)

Troubleshooting Steps:

R1# show ip ospf interface GigabitEthernet0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.1.1.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST
  Timer intervals configured, Hello 10, Dead 40, Wait 40
  
R1# debug ip ospf adj
OSPF-1 ADJ Gi0/0: Rcv pkt from 10.1.1.2 : Mismatched hello parameters from 10.1.1.2
OSPF-1 ADJ Gi0/0: Dead timer mismatch

Common Causes and Solutions:

Issue: Mismatched Hello/Dead Timers

R1# show ip ospf interface brief
Interface    PID   Area   IP Address      Cost  State Nbrs
Gi0/0        1     0      10.1.1.1/24     1     DR    0

R2# show ip ospf interface brief
Interface    PID   Area   IP Address      Cost  State Nbrs
Gi0/0        1     0      10.1.1.2/24     1     BDR   0

Fix on R2:

R2(config)# interface GigabitEthernet0/0
R2(config-if)# ip ospf hello-interval 10
R2(config-if)# ip ospf dead-interval 40

Issue: Area Mismatch

R1 in Area 0, R2 in Area 1:

R1# show ip ospf interface Gi0/0 | include Area
  Internet Address 10.1.1.1/24, Area 0

R2# show ip ospf interface Gi0/0 | include Area
  Internet Address 10.1.1.2/24, Area 1

Fix on R2:

R2(config)# router ospf 1
R2(config-router)# no network 10.1.1.0 0.0.0.255 area 1
R2(config-router)# network 10.1.1.0 0.0.0.255 area 0

Issue: Authentication Mismatch

R1# debug ip ospf adj
OSPF-1 ADJ Gi0/0: Send with youngest Key 1
OSPF-1 ADJ Gi0/0: Rcv pkt from 10.1.1.2 : Mismatch Authentication type

Fix on R2:

R2(config)# interface GigabitEthernet0/0
R2(config-if)# ip ospf message-digest-key 1 md5 SECRET_PASS
R2(config-if)# ip ospf authentication message-digest

Problem 2: Routes Not Appearing in Routing Table

Symptom:

R1# show ip route ospf
(Missing expected routes)

Troubleshooting:

R1# show ip ospf database

            OSPF Router with ID (1.1.1.1)
            
            Router Link States (Area 0)
Link ID         ADV Router      Age   Seq#       Checksum
1.1.1.1         1.1.1.1         345   0x80000005 0x00A8B3
2.2.2.2         2.2.2.2         123   0x80000003 0x009A45

R1# show ip ospf database router 2.2.2.2
(Check if expected networks are advertised)

Issue: Network Statement Missing

R2# show ip protocols
Routing Protocol is "ospf 1"
  Router ID 2.2.2.2
  Routing for Networks:
    10.1.1.0 0.0.0.255 area 0
  (Missing 10.2.2.0 network)

Fix on R2:

R2(config)# router ospf 1
R2(config-router)# network 10.2.2.0 0.0.0.255 area 0

Issue: Passive Interface Blocking Adjacency

R1# show ip ospf interface
GigabitEthernet0/0 is up, line protocol is up
  OSPF not enabled on this interface

Check and Fix:

R1# show ip protocols
Routing Protocol is "ospf 1"
  Passive Interface(s):
    GigabitEthernet0/0

R1(config)# router ospf 1
R1(config-router)# no passive-interface GigabitEthernet0/0

Problem 3: Stuck in EXSTART State

Symptom:

R1# show ip ospf neighbor
Neighbor ID     Pri   State      Dead Time   Interface
2.2.2.2          1    EXSTART    00:00:35    Gi0/0

Cause: MTU mismatch

Troubleshooting:

R1# show interfaces GigabitEthernet0/0 | include MTU
  MTU 1500 bytes

R2# show interfaces GigabitEthernet0/0 | include MTU
  MTU 1400 bytes

Fix on R2:

R2(config)# interface GigabitEthernet0/0
R2(config-if)# mtu 1500

Alternative (ignore MTU mismatch):

R2(config)# interface GigabitEthernet0/0
R2(config-if)# ip ospf mtu-ignore

Problem 4: Duplicate Router IDs

R1# show ip ospf
 Routing Process "ospf 1" with ID 1.1.1.1
 
%OSPF-4-DUP_RTRID: Duplicate router ID 1.1.1.1 detected

Fix:

R2(config)# router ospf 1
R2(config-router)# router-id 2.2.2.2
R2(config-router)# end
R2# clear ip ospf process
Reset ALL OSPF processes? [no]: yes

Essential Troubleshooting Commands

R1# show ip ospf neighbor
R1# show ip ospf interface
R1# show ip ospf database
R1# show ip route ospf
R1# show ip protocols
R1# debug ip ospf adj
R1# debug ip ospf hello
R1# debug ip ospf packet

Quick Checklist

1. ✓ Interface up/up status
2. ✓ IP addresses in same subnet
3. ✓ Area ID matches
4. ✓ Hello/Dead timers match
5. ✓ Authentication configured identically
6. ✓ Network types compatible
7. ✓ MTU values match
8. ✓ Router IDs unique
9. ✓ ACLs not blocking OSPF (protocol 89)
10. ✓ Not configured as passive interface

Systematic troubleshooting using these commands resolves most OSPF issues efficiently.