[libvirt-users] virtual networking - virbr0-nic interface

Michal Privoznik mprivozn at redhat.com
Tue Sep 11 09:12:10 UTC 2012

On 10.09.2012 23:09, Shantanu Pavgi wrote:
> I need some help in understanding libvirt's virtual networking. I have configured bridged networking (shared physical device) on libvirt+KVM system which is working fine. Also, I am using default NAT network on with virbr0 bridge device and virbr0-nic. I would like to get better understanding on virbr0-nic works in this virtual network configuration. I understand that traffic from virbr0 bridge is forwarded through host system's physical interface eth0 using iptables rules, but I am not following virbr0-nic's working here. I see it attached to virbr0 bridge device though.
> {{{
> $ brctl show
> bridge name      bridge id	                 STP enabled	   interfaces
> br0               8000.14feb5dc4f06	     no                      eth0
>                                                                                       vnet1
> virbr0           8000.525400f5a4ed         yes                 virbr0-nic
>                                                                                        vnet2
> }}}
> Any pointers on how virbr0 and virbr0-nic work would be really helpful.

It's a workaround for kernel bug/feature. The bridge's MAC is copied
from the first NIC attached. So if one detach all interfaces from a
bridge and then attach just one, the bridge will lost previous MAC and
gain a new one - just the same as the attached interface has.

So if libvirt has to ensure a MAC for virtual bridge - it creates this
dummy device (no traffic is routed through though) and just attach it to
the virtual bridge.


> --
> Thanks,
> Shantanu
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