[Bridge] bridge port number changing

Stephen Hemminger stephen at networkplumber.org
Fri Mar 20 19:51:30 UTC 2015

On Fri, 20 Mar 2015 16:10:02 +1100
Brett Worth <brett.worth at gmail.com> wrote:

> 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.
> Regards
> Brett

No idea, you might try running 'ip monitor' and put output to a log file.
Something i changing things underneath you. Maybe Network Manager?

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