OSPF Area ID Mismatch Between Neighbors
Presenting Symptom
OSPF neighbors are stuck in the INIT state and do not form a full adjacency.
Network Context
Two Cisco routers (R1 and R2) are directly connected via a serial link in a small branch office. Both run IOS 15.x and are configured for OSPF area 0 on the connecting interfaces. The network uses a single area OSPF design. The problem occurs after a recent configuration change on R2.
Diagnostic Steps
Check OSPF neighbor status
show ip ospf neighborNeighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 INIT/DROTHER 00:00:32 10.1.1.2 Serial0/0/0
The neighbor is stuck in INIT state, indicating that R1 has received a Hello from R2 but the parameters do not match. Normal state should be FULL or 2WAY.
Verify OSPF interface configuration on both routers
show ip ospf interface serial0/0/0Serial0/0/0 is up, line protocol is up
Internet Address 10.1.1.1/30, Area 0
Process ID 1, Router ID 192.168.1.1, Network Type POINT-TO-POINT, Cost: 64
Transmit Delay is 1 sec, State POINT-TO-POINT,
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:03
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 0, Adjacent neighbor count is 0
Adjacent with neighbor 192.168.1.2 (Hello suppressed)
Suppress hello for 0 neighbor(s)Check the Area field. If both routers show Area 0, then area IDs match. If one shows a different area (e.g., Area 1), that indicates an area ID mismatch.
Compare OSPF parameters on both routers
show ip ospf interface serial0/0/0 | include AreaInternet Address 10.1.1.1/30, Area 0
On R1, the interface is in Area 0. Run the same command on R2. If R2 shows a different area (e.g., Area 1), the area ID mismatch is confirmed.
Check OSPF process configuration for area assignment
show running-config | section router ospfrouter ospf 1 network 10.1.1.0 0.0.0.3 area 0
Verify the network statement under the OSPF process. If the network statement on R2 specifies a different area (e.g., area 1), that is the root cause.
Root Cause
The network statement on R2's OSPF process assigns the serial interface to area 1 instead of area 0, causing an area ID mismatch. OSPF requires that interfaces on the same link belong to the same area to form an adjacency.
Resolution
Verification
Run 'show ip ospf neighbor' on R1. Expected output: Neighbor ID Pri State Dead Time Address Interface 192.168.1.2 1 FULL/ - 00:00:36 10.1.1.2 Serial0/0/0 The state should change from INIT to FULL, confirming the adjacency is established.
Prevention
["Use consistent area numbering across all routers in the same OSPF domain, especially on shared links.","Implement OSPF configuration templates or automation to avoid manual errors when assigning areas.","Verify OSPF neighbor formation immediately after any configuration change using 'show ip ospf neighbor'."]
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why OSPF neighbors are stuck in INIT state. The exam tests your ability to use 'show ip ospf neighbor' and 'show ip ospf interface' to diagnose area mismatches. A key fact is that OSPF neighbors must share the same area ID on the connecting interface.
Exam Tips
Remember that OSPF neighbors stuck in INIT state often indicate a mismatch in Hello parameters (area ID, subnet mask, hello/dead intervals, or authentication).
The exam may present a scenario where one router's interface is in area 0 and the other in area 1; you must identify the area mismatch as the root cause.
Know that 'show ip ospf interface' displays the area ID for each interface; compare outputs from both routers to spot mismatches.
Commands Used in This Scenario
show ip ospf interface
Displays OSPF interface parameters and status, used to verify OSPF configuration and troubleshoot neighbor relationships.
show ip ospf neighbor
Displays OSPF neighbor information to verify adjacency formation and troubleshoot OSPF neighbor relationships.
show running-config
Displays the current active configuration in DRAM, showing all non-default settings.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
Practice CCNA Questions