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. Regards, Michal