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

Shantanu Pavgi pavgi at uab.edu
Wed Sep 12 17:04:55 UTC 2012

On Sep 11, 2012, at 4:12 AM, Michal Privoznik wrote:

> 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

Thanks Michal, that's helpful in understanding the network chain.


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