1      How to Determine that Configured OSPF Router IDs Conflict?

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:

Figure 1 Networking diagram

SwitchA, SwitchB, SwitchC, and SwitchD in area 0 establish OSPF neighbor relationships. The router IDs of SwitchA and SwitchC are 10.1.1.1. 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 10.1.1.1

                  Link State Database

                          Area: 0.0.0.0

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric

 Router    10.4.4.4         10.4.4.4           1410  48    80000003    1562

 Router    10.2.2.2         10.2.2.2              2  48    8000001C    1562

 Router    10.1.1.1         10.1.1.1              6  36    800015D0    1562

 Network   10.22.22.1      10.2.2.2               7  32    80000001       0

 Network   10.11.11.2      10.2.2.2              23  32    80000002       0

<SwitchA> display ospf lsdb

          OSPF Process 1 with Router ID 10.1.1.1

                  Link State Database

                          Area: 0.0.0.0

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric

 Router    10.4.4.4         10.4.4.4           1411  48    80000003    1562

 Router    10.2.2.2         10.2.2.2              3  48    8000001C    1562

 Router    10.1.1.1         10.1.1.1              0  36    800015D3    1562

 Network   10.22.22.1      10.2.2.2               8  32    80000001       0

 Network   10.11.11.2      10.2.2.2              24  32    80000002       0

In this example, on the device with the router ID being 10.1.1.1, 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 10.2.2.2

                   Routing Tables

 Routing for Network

 Destination        Cost  Type       NextHop         AdvRouter       Area

 10.33.33.0/24      3124  Transit    10.22.22.2      10.4.4.4         0.0.0.0

 10.22.22.0/24      1562  Transit    10.22.22.1      10.2.2.2         0.0.0.0

 10.11.11.0/24      1562  Transit    10.11.11.2      10.2.2.2         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 10.2.2.2

                   Routing Tables

 Routing for Network

 Destination        Cost  Type       NextHop         AdvRouter       Area

 10.22.22.0/24      1562  Transit    10.22.22.1      10.2.2.2         0.0.0.0

 10.11.11.0/24      1562  Transit    10.11.11.2      10.2.2.2         0.0.0.0

 Total Nets: 2

 Intra Area: 2  Inter Area: 0  ASE: 0  NSSA: 0

      Methods of determining an inter-area router ID conflict

The network topology is assumed as follows:

Figure 2 Networking diagram

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 10.1.1.1

                  Link State Database

                          Area: 0.0.0.0

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric

 Router    10.2.2.2         10.2.2.2            172  48    80000002    1562

 Router    10.1.1.1         10.1.1.1            174  48    80000003    1562

 Sum-Net   10.22.22.0      10.2.2.2             166  28    80000001    1562

 Sum-Asbr  10.1.1.1         10.2.2.2             38  28    80000001    1562

<HUAWEI> display ospf lsdb

          OSPF Process 1 with Router ID 10.1.1.1

                  Link State Database

                          Area: 0.0.0.0

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric

 Router    10.2.2.2         10.2.2.2            173  48    80000002    1562

 Router    10.1.1.1         10.1.1.1            175  48    80000003    1562

 Sum-Net   10.22.22.0       10.2.2.2            167  28    80000001    1562

 Sum-Asbr  10.1.1.1         10.2.2.2             39  28    80000001    1562

                  AS External Database

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric

 External  10.100.2.0       10.1.1.1           3600  36    80000004       1

 External  10.100.3.0       10.1.1.1           3600  36    80000004       1

 External  10.100.1.0       10.1.1.1           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.