1. Problem phenomenon

On an OSPF network, configured OSPF router IDs conflict sometimes. Once a router ID conflict occurs, OSPF LSAs are frequently aged and generated, causing network unstable. This is because a router ID uniquely identifies an OSPF device. When a router ID conflict occurs, the following situations occur:

• The CPU usage is high, which is mainly caused by the ROUT task.
• Routing flapping occurs.

2. Methods of determining an intra-area router ID conflict 

The network topology is assumed as follows:

SwitchA, SwitchB, SwitchC, and SwitchD in area 0 establish OSPF neighbor relationships.
The router IDs of SwitchA and SwitchC are 1.1.1.1. Thus, a router ID conflict occurs.
Identification method:

a. Run the display ospf lsdb command on any of the devices every one second to check whether the Age field in the Router LSA frequently changes and whether the value of the Sequence field increases quickly.

<SwitchA> display ospf lsdb

OSPF Process 1 with Router ID 1.1.1.1
Link State Database
Area: 0.0.0.0
Type LinkState ID AdvRouter Age Len Sequence
Metric
Router 4.4.4.4 4.4.4.4 1410 48 80000003 1562
Router 2.2.2.2 2.2.2.2 2 48 8000001C 1562
Router 1.1.1.1 1.1.1.1 6 36 800015D0 1562
Network 22.22.22.1 2.2.2.2 7 32 80000001 0
Network 11.11.11.2 2.2.2.2 23 32 80000002 0
<SwitchA> display ospf lsdb
OSPF Process 1 with Router ID 1.1.1.1
Link State Database
Area: 0.0.0.0
Type LinkState ID AdvRouter Age Len Sequence
Metric
Router 4.4.4.4 4.4.4.4 1411 48 80000003 1562
Router 2.2.2.2 2.2.2.2 3 48 8000001C 1562
Router 1.1.1.1 1.1.1.1 0 36 800015D3 1562

Network 22.22.22.1 2.2.2.2 8 32 80000001 0
Network 11.11.11.2 2.2.2.2 24 32 80000002 0

In this example, on the device with the router ID being 1.1.1.1, the Age field in the Router LSA frequently changes and the value of the Sequence field increases quickly.

b. Run the display ospf routing command on SwitchB every one second, and you can find that route flapping occurs. If intra-area route flapping occurs but neighbor flapping does not occur, it can be determined that a router ID conflict occurs. 

<SwitchB> display ospf routing
OSPF Process 1 with Router ID 2.2.2.2
Routing Tables
Routing for Network
Destination Cost Type NextHop AdvRouter Area
33.33.33.0/24 3124 Transit 22.22.22.2 4.4.4.4 0.0.0.0
22.22.22.0/24 1562 Transit 22.22.22.1 2.2.2.2 0.0.0.0
11.11.11.0/24 1562 Transit 11.11.11.2 2.2.2.2 0.0.0.0
Total Nets: 3
Intra Area: 3 Inter Area: 0 ASE: 0 NSSA: 0
<SwitchB> display ospf routing
OSPF Process 1 with Router ID 2.2.2.2
Routing Tables
Routing for Network
Destination Cost Type NextHop AdvRouter Area
22.22.22.0/24 1562 Transit 22.22.22.1 2.2.2.2 0.0.0.0
11.11.11.0/24 1562 Transit 11.11.11.2 2.2.2.2 0.0.0.0
Total Nets: 2
Intra Area: 2 Inter Area: 0 ASE: 0 NSSA: 0

3. Methods of determining an inter-area router ID conflict The network topology is assumed as follows:

The router ID of SwitchA conflicts with that of SwitchC, but SwitchA and SwitchC are not in the same area.
Identification method:
Run the display ospf lsdb command on any of the devices every one second. If a large number of AS external LSAs from the same device are frequently refreshed, it can be determined that an inter-area router ID conflict occurs.
<HUAWEI> display ospf lsdb

OSPF Process 1 with Router ID 1.1.1.1
Link State Database
Area: 0.0.0.0
Type LinkState ID AdvRouter Age Len Sequence Metric
Router 2.2.2.2 2.2.2.2 172 48 80000002 1562
Router 1.1.1.1 1.1.1.1 174 48 80000003 1562
Sum-Net 22.22.22.0 2.2.2.2 166 28 80000001 1562
Sum-Asbr 1.1.1.1 2.2.2.2 38 28 80000001 1562
<HUAWEI> display ospf lsdb
OSPF Process 1 with Router ID 1.1.1.1
Link State Database
Area: 0.0.0.0
Type LinkState ID AdvRouter Age Len Sequence Metric
Router 2.2.2.2 2.2.2.2 173 48 80000002 1562
Router 1.1.1.1 1.1.1.1 175 48 80000003 1562
Sum-Net 22.22.22.0 2.2.2.2 167 28 80000001 1562
Sum-Asbr 1.1.1.1 2.2.2.2 39 28 80000001 1562
AS External Database
Type LinkState ID AdvRouter Age Len Sequence Metric
External 100.1.2.0 1.1.1.1 3600 36 80000004 1
External 100.1.3.0 1.1.1.1 3600 36 80000004 1
External 100.1.1.0 1.1.1.1 3600 36 80000004 1

Generally, a router ID conflict occurs on a network sometimes. Therefore, you need to master the methods of determining a router ID conflict to quickly locate the fault. After eliminating the router ID conflict, run the reset ospf process-id command to make the configuration take effect.

good job

Thank you!