Catalyst 4506 ROuter Switch (2)

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.

I took a look at the STP to see if it was enabled and it is on all the ports. All the ports are connected directly to a server. The one thing that I notice was that port 2/7, the config that I put in it was changed to just the vlans that I have.

On each of the ethernet ports i have switchport mode access enabled and STP enabled. Is this correct? Any suggestions on what that error is and how to do a better fix?

Thanks for all and any help you can provide.


I have this problem too.
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Collin Clark Wed, 08/05/2009 - 12:07


The two most common scenarios of host flapping is there is a loop in your network (someone plugged in a hub in to two ports) or what sounds more typical for your situation, you have a server dual connected and the NIC team is set to load-balance. Do either of these fit your situation?

rhilliker Wed, 08/05/2009 - 12:38

I know that we do have one server that has teaming of the NIC on it. and it is our file / everything server. this is the one that went down yesterday and started this whole proccess, and made me start looking at all the connections..

Collin Clark Wed, 08/05/2009 - 12:41

Check how the team is setup. It's best to have them in Active/Passive. If you need the throughput, etherchannel between the NICs and the Switch.

rhilliker Wed, 08/05/2009 - 13:16

How and what is ether channel between the nic's and switch?

Collin Clark Wed, 08/05/2009 - 13:21

It bundles the two NICs together and make it look like one connection. That will alleviate your MAC address flapping and provides bandwidth and redundancy. If one of the links fails, the etherchannel will stay up, but only uses the one NIC. When the other NIC comes back online, it will then use both again.

Here's a link with more info-


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