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Cisco 3750 Switch Port Flapping on Port group

Posted on 2011-02-22
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Last Modified: 2012-05-11
i am recieving the following in my logs...

000176: Feb 17 05:03:06: %SW_MATM-4-MACFLAP_NOTIF: Host 0015.001d.c50c in vlan 998 is flapping between port Po2 and port Po3

can anyone offer some insight on why this is happening and a possible resolution? Thanks.
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Question by:glpro76
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by:glpro76
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a few more logs...

000175: Feb 16 19:48:32: %SYS-5-CONFIG_I: Configured from console by admin on vty0 (172.20.23.110)
000176: Feb 17 05:03:06: %SW_MATM-4-MACFLAP_NOTIF: Host 0015.001d.c50c in vlan 998 is flapping between port Po2 and port Po3
000177: Feb 17 06:18:44: %SW_MATM-4-MACFLAP_NOTIF: Host 001e.e5e7.38fd in vlan 998 is flapping between port Po3 and port Po2
000178: Feb 17 09:21:11: %SW_MATM-4-MACFLAP_NOTIF: Host 0015.70f0.6b2a in vlan 998 is flapping between port Po2 and port Po3
000179: Feb 18 02:20:55: %SW_MATM-4-MACFLAP_NOTIF: Host 0024.2bda.48f8 in vlan 999 is flapping between port Po3 and port Po4
000180: Feb 18 03:04:18: %SW_MATM-4-MACFLAP_NOTIF: Host 0024.2bda.48f8 in vlan 999 is flapping between port Po3 and port Po4
000181: Feb 18 03:32:15: %SW_MATM-4-MACFLAP_NOTIF: Host 0024.2bda.48f8 in vlan 999 is flapping between port Po3 and port Po4
000182: Feb 18 15:22:04: %SW_MATM-4-MACFLAP_NOTIF: Host 0024.2bda.48f8 in vlan 999 is flapping between port Po3 and port Po4
000183: Feb 18 15:48:48: %SW_MATM-4-MACFLAP_NOTIF: Host 0024.2bda.48f8 in vlan 999 is flapping between port Po3 and port Po4
000184: Feb 18 15:49:45: %SW_MATM-4-MACFLAP_NOTIF: Host 0024.2bda.48f8 in vlan 999 is flapping between port Po3 and port Po4
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by:torvir
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There are datagrams coming in from the same MAC but they are coming from different portchannels.
Can't tell more from your information. A drawing of the topology would be helpful.
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by:SeeMeShakinMyHead
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you probably have some cables reversed for the port-channels.  Verify the physical cabling.  
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by:Kendzast
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Are you using Xenservers of other virtualization platform?
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by:Fidelius
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You probably have 3750 in stack and servers with teaming NIC's connected to each switch working in active-active configuration. Try to change teaming (or bonding) to active-backup configuration.

Regards!  
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by:glpro76
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The switch is in a stack and the port-groups connect 3 Wireless Lan Controllers.
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by:SeeMeShakinMyHead
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Have you verified the cables. That would cause this.
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by:Fidelius
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So you have one port-channel per WLAN controller? Are all ports from one port-channel on same switch?

Also note:
•LAG requires the Etherchannel to be configured for the "on" mode on both the controller and the Catalyst switch.
•Once the Etherchannel is configured as "on" at both ends of the link, it does not matter if the Catalyst switch is configured for either Link Aggregation Control Protocol (LACP) or Cisco proprietary Port Aggregation Protocol (PAgP) because no channel negotiation is done between the controller and the switch. Additionally, LACP and PAgP are not supported on the controller.
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by:glpro76
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yes i do have one port-channel per WLC controller
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by:SeeMeShakinMyHead
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Also, make sure that LAG is set to enabled on the WLC.

on cisco switch
channel-group (port-channel #) mode on
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by:glpro76
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they are set to on on wlc and on port-groups
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by:SeeMeShakinMyHead
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have you also physically verified the cables to port-channels.  
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Fidelius earned 500 total points
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By looking, once more, closely to log entries you post, I noticed that flapping is not occurring all the time. That fact leads me to conclusion it happens only when wireless clients switch to access point that is connected to different WLAN controller than the previous one was. That is only reasonable explanation I can think of.

To elaborate more precisely, I will assume following:
Po2 - WLC1 -> AP1, AP2
Po3 - WLC2 -> AP3

Client is connected to AP1. So 3750 learns client's MAC through Po2 from WLC1. As client moves around building, AP1 handover client to AP2, 3750 still sees it's MAC through Po2 as it is connected to same WLC1. When client switches to AP3, 3750 learns it's MAC from WLC2 over Po3, but it already has in MAC address table same MAC address learned over Po2, and you see MAC address flapping.
So as long as client is jumping from AP2 to AP3 and back, you will see MAC address flapping between port groups.

Regards!
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by:glpro76
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Thanks! Well done.
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