[Bridge] bridge port number changing

Brett Worth brett.worth at gmail.com
Fri Mar 20 05:10:02 UTC 2015

Hi all.  This is my first post to this list and I hope you'll be able to help.

I have been using the Linux bridge module for a long time under kvm on centos.  This has
not given me any problems until now.

I'm adding some host servers and am using HP BL660 blades.

The kvm nic script in /etc/kvm creates a tap device then assigns a MAC address after
modifying the first byte to FE.  This is so the port 1 host mac doesn't change.

On a working host server when running brctl showmacs br0 I see the tap mac address and the
mac address assigned to the vm on the same port. The tap mac shows as local and the vm mac
is not.

The problem:

Intermittently (once a minute or so) the port number shown for the VM will switch to port
1.  During this time the VM cannot be contacted. The tap device mac remains at it's
original value.  If I wait it will switch back to the correct port number i.e. the one
associated with the tap device at which point network connectivity is re-established.

I have looked for the VM mac address on the LAN but cannot see it so it seems to be
something internal to the bridge.

Can anyone offer a suggestion as to what might be happening?

I'm running centos 6.6 and the bridge is 2.3.


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