OSPFv2 Neighbor Adjacency Troubleshooting

Learning Objectives#

• Identify the mandatory operational parameters that must match for two OSPFv2 routers to achieve FULL adjacency.
• Diagnose Hello and Dead interval timer mismatches using show ip ospf interface and verification commands.
• Troubleshoot OSPF Area ID mismatches between transit router interfaces across a point-to-point link.
• Identify missing network statements that prevent LAN prefixes from being injected into the OSPF Link-State Database (LSDB).
• Configure appropriate passive interfaces to protect access subnets while maintaining WAN neighbor adjacencies.
• Verify complete SPF route convergence and end-to-end data plane reachability across a multi-router topology.

Prerequisites#

• Thorough understanding of RFC 2328 OSPFv2 adjacency mechanics, neighbor states (DOWN, INIT, 2-WAY, EXSTART, EXCHANGE, LOADING, FULL), and Link-State Advertisements.
• Proficiency with Cisco IOS CLI diagnostic commands (show ip ospf neighbor, show ip ospf interface, show ip route ospf).
• Understanding of IPv4 point-to-point transit circuits and CIDR subnetting (/30 and /24).

Topology#

The topology models an enterprise hub-and-spoke WAN architecture connecting a corporate headquarters router (HQ-R1) to two remote facilities: Branch-R2 and Remote-R3. Both remote sites support local access LANs connected through Catalyst 2960 switches (S1 and S2) servicing end stations (PC-Branch and Server-Remote).

                    +--------------------+
                    |       HQ-R1        |
                    | Loopback:          |
                    | 192.168.1.1/24     |
                    +---------+----------+
            Gi0/0/0           |          Gi0/0/1
          10.1.12.1/30        |          10.1.13.1/30
                              |
               Link 1         |         Link 2
           (Timer Fault)      |       (Area Fault)
                              |
          10.1.12.2/30        |          10.1.13.2/30
            Gi0/0/0           |          Gi0/0/0
     +--------------------+   |   +--------------------+
     |     Branch-R2      |   |   |     Remote-R3      |
     +---------+----------+   |   +---------+----------+
        Gi0/0/1| 192.168.2.1  |      Gi0/0/1| 192.168.3.1
               |              |             |
          Fa0/1|              |        Fa0/1|
            +--+---+          |          +--+---+
            |  S1  |          |          |  S2  |
            +--+---+          |          +--+---+
          Fa0/2|              |        Fa0/2|
               |              |             |
           Fa0|               |          Fa0|
       [ PC-Branch ]          |     [ Server-Remote ]
       192.168.2.10           |       192.168.3.10

Addressing Table#

Lab Tasks#

1. Verify Baseline Layer 3 Point-to-Point Connectivity:
   • Confirm ICMP echo reachability across transit circuits 10.1.12.0/30 and 10.1.13.0/30.
   • Confirm local gateway reachability from PC-Branch (192.168.2.1) and Server-Remote (192.168.3.1).
2. Diagnose OSPF Neighbor Failure on Link 1 (HQ-R1 to Branch-R2):
   • Run show ip ospf neighbor on HQ-R1 and observe that no neighbor is formed on GigabitEthernet0/0/0.
   • Compare show ip ospf interface GigabitEthernet0/0/0 across HQ-R1 and Branch-R2.
   • Identify that Branch-R2 has modified timers (Hello 5, Dead 20) differing from HQ-R1 (Hello 10, Dead 40).
3. Remediate Fault 1 on Branch-R2:
   • Restore default timers (ip ospf hello-interval 10, ip ospf dead-interval 40) on Branch-R2 GigabitEthernet0/0/0.
   • Verify that the neighbor adjacency transitions immediately to FULL.
4. Diagnose OSPF Neighbor Failure on Link 2 (HQ-R1 to Remote-R3):
   • Observe that HQ-R1 has no neighbor on GigabitEthernet0/0/1.
   • Inspect show ip ospf interface GigabitEthernet0/0/0 on Remote-R3 and discover it is running in Area 1 while HQ-R1 is in Area 0.
5. Remediate Fault 2 & Fault 3 on Remote-R3:
   • Remove network 10.1.13.0 0.0.0.3 area 1 and add network 10.1.13.0 0.0.0.3 area 0 under router ospf 1.
   • Add the missing LAN statement: network 192.168.3.0 0.0.0.255 area 0.
   • Configure passive-interface GigabitEthernet0/0/1 to protect the remote LAN.
6. Verify Complete Topology Convergence:
   • Confirm HQ-R1 neighbor table shows both 2.2.2.2 and 3.3.3.3 in the FULL state.
   • Confirm all routers have learned the remote subnets in show ip route ospf.
   • Verify end-to-end communication from PC-Branch (192.168.2.10) to Server-Remote (192.168.3.10) and HQ-R1 (192.168.1.1).

Configuration Walkthrough#

Step 1: Repair Timer Mismatch on Branch-R2#

Synchronize OSPF Hello and Dead timers on Branch-R2 GigabitEthernet0/0/0:

Branch-R2(config)# interface GigabitEthernet0/0/0
Branch-R2(config-if)# ip ospf hello-interval 10
Branch-R2(config-if)# ip ospf dead-interval 40
Branch-R2(config-if)# exit
Branch-R2(config)# router ospf 1
Branch-R2(config-router)# passive-interface GigabitEthernet0/0/1
Branch-R2(config-router)# exit

Step 2: Repair Area Mismatch and Missing Network on Remote-R3#

Move transit circuit to Area 0 and advertise local LAN:

Remote-R3(config)# router ospf 1
Remote-R3(config-router)# no network 10.1.13.0 0.0.0.3 area 1
Remote-R3(config-router)# network 10.1.13.0 0.0.0.3 area 0
Remote-R3(config-router)# network 192.168.3.0 0.0.0.255 area 0
Remote-R3(config-router)# passive-interface GigabitEthernet0/0/1
Remote-R3(config-router)# exit

Step 3: Harden HQ-R1 Management Loopback#

HQ-R1(config)# router ospf 1
HQ-R1(config-router)# passive-interface Loopback0
HQ-R1(config-router)# exit

Verification#

1. Neighbor Adjacencies on HQ-R1#

Verify that HQ-R1 establishes full adjacencies with both spoke routers:

HQ-R1# show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
3.3.3.3           1   FULL/DR         00:00:38    10.1.13.2       GigabitEthernet0/0/1
2.2.2.2           1   FULL/DR         00:00:39    10.1.12.2       GigabitEthernet0/0/0

2. Synchronized Timer Verification#

Verify that interface timer intervals match across links:

Branch-R2# show ip ospf interface GigabitEthernet0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 10.1.12.2/30, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Remote-R3# show ip ospf interface GigabitEthernet0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
  Internet address is 10.1.13.2/30, Area 0
  Process ID 1, Router ID 3.3.3.3, Network Type BROADCAST, Cost: 1
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

3. Complete Routing Table Convergence#

Verify that HQ-R1 learns both remote branch and remote server subnets:

HQ-R1# show ip route ospf
O    192.168.2.0 [110/2] via 10.1.12.2, 00:05:56, GigabitEthernet0/0/0
O    192.168.3.0 [110/2] via 10.1.13.2, 00:05:33, GigabitEthernet0/0/1

Verify that Branch-R2 has routes to HQ loopback and the remote server LAN:

Branch-R2# show ip route ospf
     10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
O       10.1.13.0 [110/2] via 10.1.12.1, 00:05:30, GigabitEthernet0/0/0
     192.168.1.0/32 is subnetted, 1 subnets
O       192.168.1.1 [110/2] via 10.1.12.1, 00:05:59, GigabitEthernet0/0/0
O    192.168.3.0 [110/3] via 10.1.12.1, 00:05:30, GigabitEthernet0/0/0

4. End-to-End Inter-Spoke Host Reachability#

From PC-Branch, verify connectivity across the hub to Server-Remote:

C:\> ping 192.168.3.10

Pinging 192.168.3.10 with 32 bytes of data:

Reply from 192.168.3.10: bytes=32 time<1ms TTL=125
Reply from 192.168.3.10: bytes=32 time<1ms TTL=125
Reply from 192.168.3.10: bytes=32 time<1ms TTL=125
Reply from 192.168.3.10: bytes=32 time<1ms TTL=125

Ping statistics for 192.168.3.10:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)

C:\> tracert 192.168.3.10

Tracing route to 192.168.3.10 over a maximum of 30 hops: 

  1   0 ms      0 ms      0 ms      192.168.2.1
  2   0 ms      0 ms      0 ms      10.1.12.1
  3   0 ms      0 ms      0 ms      10.1.13.2
  4   0 ms      0 ms      0 ms      192.168.3.10

Trace complete.

Troubleshooting#

• Symptom: OSPF neighbor stuck in INIT state.
  • Cause: The local router received a Hello packet from the remote router, but the remote router did not see the local router's Router ID in its own received Hello. This commonly indicates an asymmetric layer 1/2 issue, an access list filtering inbound multicast, or a unicast issue.
• Symptom: OSPF neighbor stuck in 2-WAY state.
  • Cause: On broadcast multi-access networks, two DROTHER routers normally stay in 2-WAY state with each other. If this occurs on a point-to-point link, verify network type and priority settings.
• Symptom: OSPF neighbor stuck in EXSTART or EXCHANGE state.
  • Cause: Usually caused by an MTU mismatch between connected interfaces. In Cisco IOS, OSPF includes the interface MTU in Database Description (DBD) packets; mismatched MTUs prevent completion of the exchange.
• Symptom: Neighbors form FULL adjacency but remote routes are missing.
  • Cause: The interface is participating in OSPF, but the remote router's LAN interfaces have not been advertised under the OSPF process using a network statement or ip ospf <proc> area <area> interface command.

Key Takeaways#

• Mandatory Adjacency Criteria: For two OSPF routers to form a neighbor relationship, the following must match:
  1. Area ID and Area Type (e.g., standard, stub, NSSA).
  2. Hello and Dead intervals.
  3. Primary subnet IP and subnet mask on broadcast interfaces.
  4. Authentication type and key (if configured).
  5. Unique Router IDs (routers cannot have duplicate RIDs).
• Hello/Dead Ratio: Cisco IOS defaults to Dead interval = 4 $\times$ Hello interval (e.g., 10s Hello / 40s Dead). Manually changing one interval requires adjusting the corresponding peer interface.
• Systematic Isolation: When troubleshooting OSPF:
  1. Verify Layer 1/2 line protocols and Layer 3 ping.
  2. Inspect neighbor table (show ip ospf neighbor).
  3. Compare interface settings (show ip ospf interface).
  4. Verify routing table (show ip route ospf).

References#

• Cisco Systems: Troubleshooting Common OSPF Neighbor Issues (cisco.com)
• Cisco IOS IP Routing: OSPF Configuration Guide, Cisco IOS Release 15M&T (cisco.com)
• Cisco CCNA 200-301 Official Cert Guide: OSPF Neighbor Troubleshooting

Files and Validation Status#