OSPF NEIGHBORSHIP FLAPPING

TROUBLESHOOTING STEPS

GIVEN SCENARIO

STEP 1: FAULTY SWITCH PORT

STEP 2: FAULTY  CABLE

STEP 3: FAULTY ROUTER PORT

STEP 4: SPEED / DUPLEX SETTINGS

STEP 5: OSPF HELLO / DEAD TIMER

STEP 6: CHECK CPU UTILIZATION

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GIVEN SCENARIO

We have a Customer edge device or WAN device or Router at the Data center. The, comes the connected switch.

When we check the OSPF neighborship at  Router, neighborship with the connected switch is flapping continously.

This can be checked using command: show ip ospf nei | i Up

we need to troubleshoot the ospf neighborship flapping issue

 

PHYSICAL CONNECTIVITY

CUSTOMER EDGE DEVICE / WAN DEVICE / ROUTER (Gi0/1)  =============  SWITCH  

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STEP 1: FAULTY SWITCH PORT

There might be the case that switch port is faulty. So, simply swap the switch port with any vacant port.

How to check vacant port ?

Port can be admin down, check using command: sh int des

If there is no admin down port, use command : sh int status, output will show “notconnected” ports

Suppose you ae able to find the notconnected port, as an example: Gi0/0

Check running configuration of Gi0/0. If the configuration is present, then check below steps to check further

Use command: sh ip arp Gi0/0, if interface is getting arp response or not, Port should not get arp response to use it further.

or,

Use command: sh int summary, to cross-check if there is any traffic on interface

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STEP 2: FAULTY  CABLE

Cable connected between Router & Switch can be faulty.

Replace the cable between Router & Switch with a new one

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STEP 3: FAULTY ROUTER PORT

Here, we suppose that the Router port is faulty.

OSPF is running on Router port. So, look for some other vacant port on Router.

Example: Current port is Gi0/1 & vacant port is Gi0/2.

First, remove the configuration from Gi0/1 & then paste the configuration on Gi0/2.

Take care of one more thing, new port Gi0/2 should be ” no shut ”

Symptoms

OSPF neighborship will go down during this process.

There will be no output in command: sh ip ospf nei

After changes,

You will see the output in below commands:

sh ip ospf int br

sh ip ospf nei

 

It means, things are fine as per changes performed

Now, check if OSPF is still flapping or not using command: sh ip ospf nei de | i Up

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STEP 4: SPEED / DUPLEX SETTINGS

Confirm the speed / duplex settings on the Router & Switch port. Both should be matched

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STEP 5: OSPF HELLO / DEAD TIMER

Check the OSPF Hello / Dead timers using command: sh ip ospf int, if the value is very low, not as per Cisco recommendations

As an example, if OSPF Hello timer is configured as 1/10th of a second, which is not recommended.

Then, there might be the case that due to high CPU utilization or busy CPU, Hello packets might get missed.

So, solution is to configure the Hello packet timers as 10 sec, as per Cisco recommendation

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STEP 6: CHECK CPU UTILIZATION

Check the CPU utilization on Router & Switch both, if CPU utilization is high, Hello packets might get missed.

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