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Mac flapping: how to solve?

Hello everybody.

Got this log entry, repeated many times:

%SW_MATM-4-MACFLAP_NOTIF: Host 2c76.8aae.410e in vlan 1 is flapping between port Gi1/0/11 and port Gi2/0/12

It is happening on a stacked switch...

What's this and how to solve it? For me, it looks like a device being connected or trying to get connected on two different switch ports (loop?!)...

Kind regards and thanks.

F.

Everyone's tags (2)
3 REPLIES 3
Cisco Employee

Mac flapping: how to solve?

Hi Flavio,

SO you get some packet looped from device with MAC  2c76.8aae.410e. You can see packet is coming both from Gi1/0/11 and Gi2/0/12. You need to trace that mac down to the end device and see why this packet taking both pathes to your switch.

Causes can be:

- link flapping issue triggering packet to take another part

- that in turn can cause STP loops

etc.

Nik

HTH,
Niko

Mac flapping: how to solve?

Hi Flavio,


That error implies the switch is seeing the same MAC in serveral ports.Do you have the same server connected to different ports without configuring etherchannel on it?
You may using blade server for this one the flapping is between the blade server.

You should group those ports in a port-channel but, in your case, is impossible because one of the ports is a trunk. I recommend you change one of the ports of that Blade to another switch where you can configure a etherchannel with them (stack switches, same switch).


Please find the below some of the errors messages


18w2d: %SW_MATM-4-MACFLAP_NOTIF: Host 0201.0000.0000 in vlan 7 is flapping between port Gi0/2 and port Gi0/11
18w2d: %SW_MATM-4-MACFLAP_NOTIF: Host 0201.0000.0000 in vlan 7 is flapping between port Gi0/2 and port Gi0/11
18w2d: %SW_MATM-4-MACFLAP_NOTIF: Host 0201.0000.0000 in vlan 1 is flapping between port Gi0/16 and port Gi0/17
18w2d: %SW_MATM-4-MACFLAP_NOTIF: Host 0201.0000.0000 in vlan 7 is flapping between port Gi0/11 and port Gi0/10
18w2d: %SW_MATM-4-MACFLAP_NOTIF: Host 0201.0000.0000 in vlan 7 is flapping between port Gi0/2 and port Gi0/11


says that the server is using the links in active/active configuration.
To avoid to confuse the lan switch you need to use bundles of ports.
you can create two etherchannel bundles one made of 5 ports for vlan 1 and one made of 5 ports for vlan 7.


something like:


int gi4/x
switchport
switchport mode access
switchport access vlan 7
channel-group 7 active

on all 5 ports in vlan 7

int gi4/x
switchport
switchport mode access
switchport access vlan 1
channel-group 1 active

you can check if these etherchannel groups are free with

sh etherchannel summary

on blade switch

int gi0/y
switchport
switchport mode access
switchport access vlan 7
channel-group 7 active

on all 5 ports in vlan 7

int gi0/y
switchport
switchport mode access
switchport access vlan 1
channel-group 1 active

Note: you need to do the same on the blade switch in this way you build two bundles and all the MAC address flapping confusion will end


Please rate all the helpfull posts.
Regards,
Naidu.

Cisco Employee

Mac flapping: how to solve?

I am assuming you are using Windows Server 2003, & to get rid of this pesky MAC address that keeps appearing across your MAC  tables you need to go to Network adapter settings -> highlight your adapter --> right click --> Properties--> and Untick 'Network Load Balancing'.

Thanks

Subramanian Iyer

CISCO ASA QA

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