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On 11.06.2015 18:52, Andreas Scheuring wrote:
> Maybe this helps (taken from [1])
>
> "Actually there is one way that the MAC address of the tap device
> affects
> proper operation of guest networking - if you happen to set the tap
> device's MAC identical to the MAC used by the guest, you will get errors
> from the kernel similar to this:
>
>
>    kernel: vnet9: received packet with own address as source address"
>
>
>
> [1] http://www.redhat.com/archives/libvir-list/2012-July/msg00984.html
I was wondering the same question myself one day and found this 
explanation from the same mail list:

vnet0 is the backend of the guest NIC, and its MAC addr
is more or less irrelevant to functioning of the guest
itself, since traffic does not originate on this NIC.
The only important thing is that this TAP device must
have a high value MAC address, to avoid the bridge
device using the TAP device's MAC as its own. Hence
when creating the TAP Device  libvirt takes the guest
MAC addr and simply sets the top byte to 0xFE

http://www.redhat.com/archives/libvir-list/2012-June/msg01330.html