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.
This post was last edited by 神出鬼没 at 2017-08-28 03:23.