Wierd MAC Flapping Errors

Unanswered Question
Mar 9th, 2009

We had an issue with a MAC flapping in our network today. We've dealt with this before and for the most part have eliminated it with BPDU guard.

Today's MAC flapping seemed different since instead of a bunch of different MACs flapping between their port and the port towards the loop. Something like below with Gi1/32 being the port towards the loop:

%C4K_EBM-4-HOSTFLAPPING: Host AAA in vlan 100 is flapping between port Gi1/1 and port Gi1/32

%C4K_EBM-4-HOSTFLAPPING: Host BBB in vlan 100 is flapping between port Gi1/2 and port Gi1/32

%C4K_EBM-4-HOSTFLAPPING: Host CCC in vlan 100 is flapping between port Gi1/3 and port Gi1/32

We saw the same MAC flapping between many “Access Ports” and the port towards the loop. Something like this:

%C4K_EBM-4-HOSTFLAPPING: Host XXX in vlan 100 is flapping between port Gi1/1 and port Gi1/32

%C4K_EBM-4-HOSTFLAPPING: Host XXX in vlan 100 is flapping between port Gi1/2 and port Gi1/32

%C4K_EBM-4-HOSTFLAPPING: Host XXX in vlan 100 is flapping between port Gi1/3 and port Gi1/32

The former case make sense to me since frames with the MAC addresses of the hosts on ports Gi1/1-3 get forwarded down Gi1/32 got looped and come back to the switch. But the case we saw today make no sense to me since ports Gi1/1-3 are not loops and do not have hosts with MAC address XXX on them.

It's still not clear how this loop happened, but it looks like something to do with a host with two NICs with unknown OS or config.

Can anyone shed any light on how we could have seen the behavior we saw today?

Thanks,

Joe

I have this problem too.
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Sebastian Helmer Mon, 03/09/2009 - 23:00

Hello,

is this maybe a problem in a link aggregation configuration?

Or, we had this problem on Ports where a Access Point was connected to..

best regards, Sebastian

c2jkeegan Tue, 03/10/2009 - 08:13

Yes, I've seen that site, but I believe that only explains the first scenario in my post (i.e. Different MAC addresses showing up in the messages).

If this does explain the second scenario, I must be missing something, and I was hoping someone could explain.

Thanks.

jorge.calvo Tue, 03/10/2009 - 16:38

Hi,

As s.helmer said. This situation occurs if you have access points where the clients are roamming. Is this the case?

c2jkeegan Tue, 03/10/2009 - 16:45

This is not the case, No APs involved.

As I mentioned in my post the loop was caused by a system with two NICs and the other ports were just normal access ports with either a host, a host and phone or just a phone attached to them.

Sebastian Helmer Wed, 03/11/2009 - 12:58

Maybe it would help to give us a short overview which device is connected to which port..

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