[Bridge] MAC intermittently being learnt on wrong port

Stephen Hemminger shemminger at linux-foundation.org
Mon Jan 11 10:18:48 PST 2010

On Fri, 8 Jan 2010 03:25:10 +0000
Ryan King <ryank at staff.globaldial.com> wrote:

> Hi,
> I am seeing a strange issue with a fairly simple bridge I have setup (for openvpn in bridge mode).
> eth0 ---  [ openvpn machine  (tap0) ] --- eth1
> The bridge is between tap0 and eth1 on a debian machine running on ESX 4.  (tap0 being the openvpn tunnel interface).  Intermittently, I see openvpn client MAC addresses on port 2 (eth1).  When this happens, their vpn link stops working, since arp replies are being sent back via the wrong interface.  But after a random amount of time, it will change back to port 1 (tap0) and start working again.  Sometimes this is quick, sometimes it doesn't happen for hours.
> Anyone else had these issues?  I've searched the archives, and come across several people who seem to have had similar problems - but haven't found one that has a solution yet.
> I'd appreciate any suggestions on where I should start looking to find out why/how these MAC's are being learnt on the wrong interface...

By default the address of the bridge interface is the minimum of all the interfaces,
and the tap one is generated randomly; that might be your problem.
So either:
   1. force a specific MAC address on the tap interface
or 2. force the the MAC address of the bridge interface to be that of eth

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