Catalyst 4506 router switch

Unanswered Question
Aug 5th, 2009

Good morning all,

I am having trouble with one of the ports on our switch. It is telling me that I have a host flapping between ports 2/8 and 2/18. I found the port that was causeing the issue but don't know what that error means or how to fix it completely. The fix that I did as a temporary one is shutdown the port. It was on port 2/8. Any suggestions on what that error is and how to do a better fix? Thansk in advance for all your help.

I have this problem too.
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Peter Paluch Wed, 08/05/2009 - 04:52


If a host is flapping rapidly between two ports then it means that the MAC address of that host appears as the source MAC on multiple frames entering both those ports. In a stable switched topology, this is an illegal scenario, as there cannot be multiple active ways to a Layer2 destination and the host cannot move that fast from one port to another.

Very often, these kinds of problems arise because of Layer2 loops. If some of your ports is physically failing it might be that the STP has been repeatedly opening and blocking another port and the frames sent by the respective host indeed started appearing on both ports - because of the instability in your STP.

Can you double check if there is no Layer2 loop in your network and if the STP is stable and running?

Best regards,


rhilliker Wed, 08/05/2009 - 05:10

Thanks very much peter for responding to this. But please be patient with me on this. I am new to this and I really don't have much experiance with this so can you please tell me how to check for the layer2 loop and what STP is? Sorry for bothering you with this. Thanks Richie

Leo Laohoo Wed, 08/05/2009 - 14:44

In addition to Peter's comment and post ...

Firstly, this commonly happens with servers with 2 or more active NICs.

You should get the MAC address and port numbers. Make sure that both ports are not switches (one method is by using the command "sh mac-address-table ". If the output is more than one, chances are the ports have a switch connected to it.

If there's only one MAC address on each port, physically trace or identify what machine this is. I wouldn't recommend trying to find the IP address of both because you are likely to find the IP address of only ONE of the MAC address (sh mac-address-table ).

Once you've found the machine, then you can disconnect one of the two connections.

Hope this helps.

iyde Thu, 08/06/2009 - 12:34

It can also happen if you have wireless Access Points connected to the two ports in question and a wireless client is roaming back and forth between the two.



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