About the MAC address flapping

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May 12th, 2009
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We have a stack switch (two phsical ones) connected to two NICs with one server. We setup NICteaming on server side. We also configure Etherchannel (PAGP)on cisco side. Since we configured two NICs as active/avtive, I think the network connection is ok even though we disable one NIC. But the result is negative.So I tried to disable etherchannel on switch side, the connection could move back active one when I tired to disable one.But I got the following logs: "Host 0050.xxxx.yyyy in vlan zzz is flapping between port Gi2/0/10 and port Gi1/0/30", I am not sure if it is normal behaviour. I was pretty sure it is not related to SPT issue. Please give me two cents. Thanks a lot!

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Laurent Aubert Tue, 05/12/2009 - 17:44
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The message means the switch received the same mac-address on both ports which is usually highlighting STP loop.

In your case, I think it's because your etherchannel between your server and the cisco was not negotiated.

PAGP is cisco proprietary, you need to use LACP or static configuration depending on what is supported on the server side.



HWangLoyalty_2 Wed, 05/13/2009 - 07:08
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Thanks for your suggestion.

All severs are running on the Vmware box. But it looks like the Vmware does not support LACP so far. You mean I could setup 802.3ad static configuration on Cisco switch.

How to configure it? it is the same as PAGP command "channek-group xx mode on". Please advice.

iyde Sun, 05/17/2009 - 07:00
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channel-group xx mode on means that negotiation of either PAgP or LACP is disabled and that the port will attempt to be active in an etherchannel.

Your NIC's will need a similar config as well.



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