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.
--
Shantanu
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