Thank you for the answer. I just tried it, but it doesn't change anything. I can only see the vnet0 interface "entering forwarding state" faster in /var/log/messages on host :

May 25 21:27:50 lilith kernel: device vnet0 entered promiscuous mode
May 25 21:27:50 lilith kernel: br0: topology change detected, propagating
May 25 21:27:50 lilith kernel: br0: port 2(vnet0) entering forwarding state

I'll post my question on libvirt mailing-list.

2009/5/25 Jasper Capel <capel@stone-it.com>
On 05/24/2009 10:01 PM, Fabien Dupont wrote:
> As far as I can see, it seems vnet0 doesn't forward its trafic to br0
> like there's no link.
> I have also seen that the vnet0 interface seen from host (opposed to
> guest) doesn't have the MAC address asked for vnet0, but this may be normal.
>
> I may post the same question on libvirt mailing list...
>

I think it might be a bridge forwarding issue.

Try:
# brctl setfd br0 0

This eliminates the forwarding delay, so your virtual machine may have
networking at the time it sends it's dhcp request.

-Jasper
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