An oscillation almost ruined my work. It turned out to be caused by an IP address conflict!

Switches are in the arena, and it is not okay to not abide by the rules of the arena and do whatever you want. Even if you make a mistake accidentally, it may cause the switch's reputation in the arena to decline.

This issue introduces the following: Routing oscillations caused by IP address configuration conflicts can cause user services to be intermittent.

Background knowledge:

The IP address is unique within a LAN, which means that the same IP address cannot be configured for different devices within the LAN.

Networking:

Configure OSPF protocol for RTA, RTB, RTC, and RTD. The Router ID and IP address are shown in the figure.

Fault phenomenon:

• The CPU usage is very high. When you run the display cpu-usage command to view the CPU usage, the CPU usage of the ROUT task is significantly higher than that of other tasks.
• Route flapping occurs

Cause Analysis:

In an OSPF network, interface IP address configuration conflicts may cause OSPF LSAs to frequently age and be generated, leading to network instability, causing route flapping, and consuming CPU processing resources.

Processing steps:

(1) Run the display ospf lsdb command on each device once every second to view the OSPF link state database (LSDB) information of each switch.

(2) Determine the fault point based on the output information of each device.

a. Situation 1

On one switch, the aging time (Age) of the network segment LSA is 3600 or the LSA is occasionally absent, and the Sequence field increases rapidly. On other switches, the Age of the same network segment LSA keeps switching between 3600 and other smaller values, and the Sequence field increases rapidly.

If the above conditions are met at the same time, it means that LSA aging is abnormal.

For example:

 < RTC > display ospf lsdb

 OSPF Process 1 with Router ID 3.3 .3 .3
 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 2 48 8000000 D 1
 Router 3.3 .3 .3 3.3 .3 .3 6 72 80000016 1
 Router 2.2 .2 .2 2.2 .2 .2 228 60 8000000 D 1
 Router 1.1 .1 .1 1.1 .1 .1 258 60 80000009 1
 Network 112.1 .1 .4 4.4 .4 .4 121 32 80000001 0
 Network 112.1 .1 .2 1.1 .1 .1 3600 32 80000015 0
 Network 222.1 .1 .3 3.3 .3 .3 227 32 80000003 0
 Network 111.1 .1 .1 1.1 .1 .1 259 32 80000002 0

 AS External Database
 Type LinkState ID AdvRouter Age Len Sequence Metric
 External 33.33 .33 .33 4.4 .4 .4 206 36 800001 D7 1
 External 125.12 .1 .2 4.4 .4 .4 206 36 80000032 1

At this time, enter the display ospf routing command on each switch every second. If you see that a route is flapping and no neighbor is flapping, it can be determined that there is an IP address conflict or a router ID conflict. Combined with the display ospf lsdb command output, it can be determined that there is an IP address conflict between the DR and non-DR.

AdvRouter finds one of the devices and then locates the interface. The conflicting device can only be found through network IP address planning. It is difficult to find the conflicting device through the information carried by OSPF itself.

In this example, we can first determine that the conflicting IP address is 112.1.1.2. The Router ID of one of the conflicting devices is 1.1.1.1. The other conflicting device (3.3.3.3) cannot be found through the information carried by OSPF itself.

b. Case 2

If two Network LSAs with LinkState ID 112.1.1.2 appear on any switch, and the Age fields of the two LSAs are always small, and the Sequence fields increase rapidly, it indicates that an IP address conflict occurs on the DR and BDR.

 < RTC > display ospf lsdb

 OSPF Process 1 with Router ID 3.3 .3 .3
 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 17 48 8000011 D 1
 Router 3.3 .3 .3 3.3 .3 .3 21 72 8000015 A 1
 Router 2.2 .2 .2 2.2 .2 .2 151 60 80000089 1
 Router 1.1 .1 .1 1.1 .1 .1 1180 60 8000002 A 1
 Network 112.1 .1 .2 3.3 .3 .3 3 32 8000016 A 0
 Network 112.1 .1 .2 1.1 .1 .1 5 32 80000179 0
 Network 222.1 .1 .3 3.3 .3 .3 145 32 8000002 D 0
 Network 212.1 .1 .4 4.4 .4 .4 10 32 80000005 0
 Network 111.1 .1 .2 2.2 .2 .2 459 32 80000003 0

 AS External Database
 Type LinkState ID AdvRouter Age Len Sequence Metric
 External 33.33 .33 .33 4.4 .4 .4 30 36 800001 DC 1
 External 125.12 .1 .2 4.4 .4 .4 30 36 80000037 1

(3) Change the IP address of the conflicting party according to the plan.

Lessons learned:

In an OSPF network, sudden configuration of interface IP addresses may cause OSPF LSAs to age and be generated frequently, leading to network instability and causing route flapping.

Therefore, the IP addresses of interfaces in the network need to be configured according to the plan, and the network planning parameters should not be changed at will. If an IP address conflict has occurred, it is necessary to quickly and accurately locate the device with the IP address conflict and solve such problems in the shortest possible time.