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OSPFCCNA 200-301

OSPF Router ID Conflict Causing Instability

Presenting Symptom

OSPF neighbors are flapping between routers, causing intermittent routing table changes and network instability.

Network Context

A small enterprise campus network with two Cisco 4321 routers (R1 and R2) running IOS XE 16.9, connected via a Gigabit Ethernet link. OSPF is configured in area 0 with default settings. The network has about 50 hosts and uses a single OSPF area.

Diagnostic Steps

1

Check OSPF neighbor state

show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
10.0.0.1         1    FULL/DR         00:00:35    10.0.0.2        GigabitEthernet0/0
10.0.0.1         1    FULL/DR         00:00:35    10.0.0.2        GigabitEthernet0/0

If the same neighbor ID appears multiple times or the state flips between FULL and INIT/EXSTART, it indicates a router ID conflict. A stable neighbor should show a single entry with FULL state.

2

Verify router IDs on each router

show ip ospf | include Router ID
R1: Routing Process "ospf 1" with ID 10.0.0.1
R2: Routing Process "ospf 1" with ID 10.0.0.1

If both routers show the same router ID, that confirms the conflict. Each router must have a unique router ID.

3

Check OSPF database for duplicate LSAs

show ip ospf database router
   LS age: 10
   Options: (No TOS-capability, DC)
   LS Type: Router Links
   Link State ID: 10.0.0.1
   Advertising Router: 10.0.0.1
   ...
   LS age: 15
   Options: (No TOS-capability, DC)
   LS Type: Router Links
   Link State ID: 10.0.0.1
   Advertising Router: 10.0.0.1

Multiple LSAs with the same Link State ID and Advertising Router indicate duplicate router IDs. This causes OSPF to process conflicting information, leading to flapping.

4

Examine OSPF events with debug

debug ip ospf events
OSPF: Rcv pkt from 10.0.0.2, GigabitEthernet0/0, area 0.0.0.0, packet type: 1
OSPF: Send pkt to 10.0.0.2, GigabitEthernet0/0, area 0.0.0.0
OSPF: Neighbor change event: neighbor 10.0.0.1 state changed from FULL to EXSTART
OSPF: Neighbor change event: neighbor 10.0.0.1 state changed from EXSTART to INIT

Frequent neighbor state changes (e.g., FULL to EXSTART to INIT) indicate instability. The debug output may show the same router ID in packets from different sources, confirming the conflict.

Root Cause

Both routers have the same OSPF router ID (10.0.0.1) because the router ID was manually configured identically on both devices. OSPF requires each router to have a unique router ID; a conflict causes neighbor flapping and routing instability.

Resolution

On one of the routers, change the router ID to a unique value. For example, on R2: 1. Enter global configuration mode: configure terminal 2. Change the router ID: router ospf 1 router-id 10.0.0.2 3. Exit and clear OSPF process to apply: end clear ip ospf process (Answer yes to reset) Alternatively, if using loopback interfaces, ensure the highest loopback IP is unique. The router ID can also be set by configuring a loopback interface with a unique IP address.

Verification

After the fix, run: show ip ospf neighbor Expected output: R1# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.0.0.2 1 FULL/DR 00:00:35 10.0.0.2 GigabitEthernet0/0 R2# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.0.0.1 1 FULL/DR 00:00:35 10.0.0.1 GigabitEthernet0/0 Both routers show unique neighbor IDs and stable FULL state.

Prevention

["Always configure unique router IDs manually or rely on loopback interfaces with unique IP addresses.","Use a consistent naming convention for router IDs (e.g., use the router's loopback IP).","Before adding a new router to an OSPF domain, verify that its router ID does not conflict with existing routers."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify the cause of OSPF neighbor flapping. The exam tests your ability to use show commands (show ip ospf neighbor, show ip ospf) to diagnose router ID conflicts. Key fact: OSPF router ID must be unique; it is chosen from the highest loopback IP, then highest physical IP, or manually configured.

Exam Tips

1.

Memorize the OSPF router ID selection order: manual > loopback > highest active interface IP.

2.

In troubleshooting questions, if neighbors are flapping, always check for duplicate router IDs first.

3.

Know that changing the router ID requires clearing the OSPF process (clear ip ospf process) to take effect.

Commands Used in This Scenario

clear ip ospf process

Resets the OSPF process on the router, forcing it to re-establish all neighbor adjacencies and re-learn routes.

debug ip ospf events

Enables real-time debugging of OSPF event messages to troubleshoot neighbor adjacency issues, route propagation problems, or OSPF state changes.

show ip ospf

Displays general information about OSPF routing process, including router ID, areas, and LSDB statistics, used to verify OSPF configuration and operational status.

show ip ospf neighbor

Displays OSPF neighbor information to verify adjacency formation and troubleshoot OSPF neighbor relationships.

Test Your CCNA Knowledge

Practice with scenario-based questions to prepare for the CCNA 200-301 exam.

Practice CCNA Questions