Troubleshooting ospf flapping

OSPF route flapping occurs, routes areconstantly added and deleted, LSAs are constantly refreshed, and the CPU usageis high.

  Possible Causes

(1) Neighbor flapping causes route flapping.

(2) Interface IP addressesconflict, network LSAs frequently age and refresh, and route calculation arefrequent.

(3) Network flapping, namely, flapping of interface or introduced external routes, causes OSPF route flapping.

Troubleshooting ProcedureStep1: Check whether the neighbor flaps.

1.Check whether the neighbor flaps through Syslog.

The following Syslog is displayed when the OSPF neighbor flaps:

%OSPF-5-ADJCHG: Process [dec], Nbr [chars1] from [chars2] to[chars3],[chars4]. For example:  

%OSPF-5-ADJCHG: Process 1, Nbr 1.1.1.1-GigabitEthernet 0/1 from Full toDown, InactivityTimer.

2.If the neighbor flaps, the route flapping is caused by the neighbor flapping.

3.If the neighbor does not flap, the possible causes for route flapping are as follows:

1) Network flapping occurs.  

2) There are IP addresses in conflict in the area.

Step2: Check whether network flapping occurs.

If network flapping occurs, you can find the following symptoms:

1. LSAs frequently refresh.  

2. LSAs frequently age and refresh.

The procedure for checking whether network flapping occurs is as follows:

① Identify the LSA for the flapping route.

1) Identify the type of the flapping route: intra-area route,inter-area route or external route.

2) If the route is an intra-area route, focus on only Router-LSA and Network-LSA in subsequent operations.

If the route is an inter-area route, focus on Router-LSA, Network-LSA and Summary-LSA in subsequent operations.

If the route is an external route, focus on Router-LSA, Network-LSA,ASBR-Summary-LSA, External-LSA and NSSA-LSA in subsequent operations.

② Collect LSA summary information.

show ip ospf database    //Collectsummary information consecutively for three times at an interval of 1 minute.

③ Compare the LSA information collected at differenttimes.

1) If the serial number of an LSA changes significantly (with a differencegreater than 2), it indicates that the LSA frequently refreshes.  

2) If the Age value of an LSAswitches between 3600s and the normal state (between 0 and 1800), it indicates that the LSA constantly ages and refreshes.

④ Check whether network flapping occurs.

If it is found that Network-LSA frequently ages and refreshes in step ③, it indicates that there may be IP addresses inconflict in the network.

If it is found that an LSA frequently refreshes in step ③, it indicates that network flapping occurs. In this case, go to step ⑤.

If it is found that an LSA (not Network-LSA) frequently ages and refreshes in step ③, it indicates that network flapping occurs. In this case, go to step ⑤.

⑤ Collect detailed information about the changedLSAs.

debug ip ospf lsa (If there are massive LSAs in the environment,screen refreshing may be caused. Perform the operation at low-traffic periods.)

show ip ospf database [ router | network | summary |asbr-summary | external | nssa-external ] [ adv-router A.B.C.D ] [ A.B.C.D ]

Collect information for 10 times at an interval of 30 seconds.

You can specify the type and announcer based on the LSA that frequently changes found in step ③, specify a unique LSA for information collectionand check the details only about the specified LSA.

Debugging operations cause risks (The worst case is to restart the switch for recovery). Perform debugging only after informing customers of the risks and get them accepted. It is recommended debugging at low-traffic periods (Be more cautious when dealing with core switches). If packet capturingis also required for troubleshooting, remember to collect information by debugging and packet capturing at the same time.

Step3: Check whether there are IP addresses in conflict in the area.

You can check whether there are IP addresses in conflict by using the following steps:

① Collect information on the faulty device.

show ip ospf database network

Display information about Network-LSA. If there are IP addresses inconflict in the area, Network-LSA constantly switches between aging and refreshing. According to the outputs of the show command, Network-LSA constantly switches between 3600s and the normal state (between 0 and 1800s).

② Display static configurations on other devices in the area.

Run the show running-config command on devices in the area to check whether their IP addresses are inconflict.

Note: If the IP address of an interface is in conflict against the interface IP address of another device in the area, OSPF route flapping may becaused even though the interface is in the down state.

Step4:Fault Information Collection

Collect log information (pay attention to the time switch and the time accuracy) for fault analysis.

[Device debugging information, configuration, hardware and softwareversions, device logs and operation logs]

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show version

show version slots

show run

show log

show ip ospf

show ip ospf neighbor

show ip ospf interface

show ip ospf border-routers

show ip ospf spf

show ip ospf route count

show ip ospf route

show ip ospf database database-summary

show ip ospf database

show ip ospf database router

show ip ospf database network

show ip ospf database summary

show ip ospf database asbr-summary

show ip ospf database external

show ip ospf database nssa-external

Debug ip ospf lsa

Debugging operations cause risks (The worst case is to restartthe switch for recovery). Perform debugging only after informing customers ofthe risks and get them accepted. It is recommended debugging at low-traffic periods (Be more cautious when dealing with core switches). If packet capturingis also required for troubleshooting, remember to collect information by debugging and packet capturing at the same time.