MAC Flapping

Unanswered Question
Oct 14th, 2008

Is this a problem?

%SW_MATM-CLUSTER_MEMBER_1-4-MACFLAP_NOTIF: Host 4200.24ff.81ff in vlan 1 is flapping between port Gi5/0/14 and port Gi1/0/26

We have an AS400 that has two ports. One port is connected to the L3 switch, and the other port is connected to a Dell L2 switch. I noticed these errors today, and we've narrowed it down to the AS400. Is there a way of stopping this, or is this one of those errors we just ignore?



I have this problem too.
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Prashanth Krishnappa Tue, 10/14/2008 - 11:44

As long as a Cisco switch sees the same MAC-address being learnt on the same vlan it would print the message you are seeing.

I suspect misconfig in your AS400 redundancy due to which it is sending out frames on both interfaces at the same time.

allan.thomas Tue, 10/14/2008 - 12:28

I suspect that the AS400 has active/passive failover which essentially shares the same MAC?

If this is the case then I would not ordinarily expect to see one of the interfaces UP unless there was a failure, hence the MAC would not be learnt on that port.

This should not be ignored as I suspect it will be affecting affecting connectivity. You should investigate the operation of the AS400 nics with your OPS or whoever maintains it in order to understand the significance.



John Blakley Tue, 10/14/2008 - 13:00

According to them, the AS400 has one card that has two ports. Each port has it's own mac address, and each port is connected into individual switches like so:

switch <-----> switch

| |

AS400 NIC1 AS400 NIC2

I KNOW that this can create a loop, BUT, theoretically if the card in fact has separate mac addresses for each port, then the port that is connected to the switch shouldn't be seen on two ports. It almost acts like it's load balancing.


allan.thomas Tue, 10/14/2008 - 13:07

John, you're absolutely correct, if infact they do have unique MAC addresses then I would expect the switchport to learn same.

Find out what the IP address is of each NIC and ping each address from your PC and then check arp table for the MAC address? are they different.



John Blakley Tue, 10/14/2008 - 13:33

Resolved :)

I noticed that if I ping from a host on the Dell switch, and then at the same time ping from a host on the 3750 (where the AS400 is connected), I could replicate the problem. I'm forwarding this to the AS400 team. :o)

Thanks All!!



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