[Bridge] MAC intermittently being learnt on wrong port

Robert LeBlanc robert at leblancnet.us
Mon Jan 11 10:42:36 PST 2010

On Mon, Jan 11, 2010 at 11:18 AM, Stephen Hemminger <
shemminger at linux-foundation.org> wrote:

> 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
I'm seeing the same problem on a static bridge (no tap) with the MAC address
set to locally administrated MACs. I sent the e-mail to the list on Friday.
This is a good thought, but I don't believe it is the issue.

Robert LeBlanc
Life Sciences & Undergraduate Education Computer Support
Brigham Young University
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