1      How to Determine that the IP Addresses of Interfaces in an OSPF Area Conflict?

1.  Problem phenomenon

On a network, correct routes fail to be generated sometimes, because interfaces on different devices are configured with the same IP address. On an OSPF network, when the IP addresses of interfaces conflict, OSPF LSAs are frequently aged and generated, causing the network unstable. When the IP addresses of interfaces conflict, the following situations may occur:

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

2.  Methods of determining an intra-area IP address conflict

The network topology is assumed as follows:

Figure 1 Networking diagram

a.  Conflict between the IP addresses of a DR and a non-DR

The interface with the IP address being 10.112.1.2 on SwitchA functions as a DR, and the interface with the IP address being 10.112.1.2 on SwitchC is not a DR. The IP addresses of the two interfaces conflict.

Identification method:

Run the display ospf lsdb command on SwitchC every one second. You can find that the Age field of the Network LSA is always 3600 or the Network LSA does not exist once in a while and the value of the Sequence field increases quickly on the network where the IP addresses of interfaces conflict.

<SwitchC> display ospf lsdb

          OSPF Process 1 with Router ID 10.3.3.3

                  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              2  48    8000000D       1

 Router    10.3.3.3         10.3.3.3              6  72    80000016       1

 Router    10.2.2.2         10.2.2.2            228  60    8000000D       1

 Router    10.1.1.1         10.1.1.1            258  60    80000009       1

 Network   10.112.1.4       10.4.4.4            121  32    80000001       0

 Network   10.112.1.2       10.1.1.1           3600  32    80000015       0

 Network   10.222.1.3       10.3.3.3            227  32    80000003       0

 Network   10.111.1.1       10.1.1.1            259  32    80000002       0

                  AS External Database

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric

 External  10.33.33.33     10.4.4.4            206  36    800001D7       1

 External  10.125.1.2      10.4.4.4            206  36    80000032       1

Run the display ospf lsdb command on the other devices. You can find that the Age field of the Network LSA switches between 3600 and a smaller value and the value of the Sequence field increases quickly on the network where the IP addresses of interfaces conflict. 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 an IP address conflict or a router ID conflict occurs. For details about how to eliminate a router ID conflict, see "How to Determine that the Configured OSPF Router IDs Conflict?"

<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

 10.213.1.0/24       2     Stub       10.222.1.3       10.3.3.3         0.0.0.0

 10.112.1.0/24       2     Transit    10.222.1.3       10.4.4.4         0.0.0.0

 10.212.1.0/24       2     Transit    10.222.1.3       10.4.4.4         0.0.0.0

 10.111.1.0/24       1     Transit    10.111.1.2       10.2.2.2         0.0.0.0

 10.134.1.1/32       2     Stub       10.111.1.1       10.1.1.1         0.0.0.0

 10.222.1.0/24       1     Transit    10.222.1.2       10.3.3.3         0.0.0.0

 Routing for ASEs

 Destination        Cost      Type       Tag         NextHop         AdvRouter

 10.125.1.2/32      1         Type2      1           10.222.1.3       10.4.4.4

 10.33.33.33/32     1         Type2      1           10.222.1.3       10.4.4.4

 Total Nets: 8

 Intra Area: 6  Inter Area: 0  ASE: 2  NSSA: 0

b.  Conflict between the IP addresses of two DRs

The interface with the IP address being 10.112.1.2 on SwitchA functions as a DR, and the interface with the IP address being 10.112.1.2 on SwitchC also functions as a DR. The IP addresses of the two interfaces conflict.

Identification method:

Run the display ospf lsdb command on any device every one second. You can find that there are two Network LSAs with the LinkState ID being 10.112.1.2, and the values of the Age fields of the two LSAs are always small and the values of the Sequence fields increase quickly.

<SwitchC> display ospf lsdb

          OSPF Process 1 with Router ID 10.3.3.3

                  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             17  48    8000011D       1

 Router    10.3.3.3         10.3.3.3             21  72    8000015A       1

 Router    10.2.2.2         10.2.2.2            151  60    80000089       1

 Router    10.1.1.1         10.1.1.1           1180  60    8000002A       1

 Network   10.112.1.2       10.3.3.3              3  32    8000016A       0

 Network   10.112.1.2       10.1.1.1              5  32    80000179       0

 Network   10.222.1.3       10.3.3.3            145  32    8000002D       0

 Network   10.212.1.4       10.4.4.4             10  32    80000005       0

 Network   10.111.1.2       10.2.2.2            459  32    80000003       0

                  AS External Database

 Type      LinkState ID    AdvRouter          Age  Len   Sequence   Metric

 External  10.33.33.33     10.4.4.4             30  36    800001DC       1

 External  10.125.1.2      10.4.4.4             30  36    80000037       1

      Methods of determining an intra-area IP address conflict

 .  For the conflict between the IP address of a DR and the IP address of a non-DR, you can find the conflicting IP addresses based on the LinkState ID in the flapping Network LSA, and then find one of the device based on AdvRouter and locate the interface. The other device can be found only based on network IP address planning. It is difficult to find the other conflicting device based on information carried by OSPF LSAs.

In this example, you can determine that the conflicting IP address is 10.112.1.2, and the router ID of one of the devices is 10.1.1.1. The other device with the router ID being 10.3.3.3 cannot be found based on the information carried by OSPF LSAs.

a.  When the addresses of two DRs conflict, you can determine the IP addresses of which interfaces on which devices conflict based on LinkState Id and AdvRouter fields of the two Network LSAs with the same LinkState IDs.

In this example, it is easy to find the devices (with router IDs being 10.3.3.3 and 10.1.1.1 respectively) whose interface addresses conflict and the corresponding interfaces based on LinkState IDs.

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 problem and solve the problem.