[libvirt-users] Networking problem - multiple guests using MacVTap in bridge mode

[Laine Stump]

On 02/01/2013 08:26 AM, Filip Korzeniowski wrote:
> Dear list,
>
> I am getting strange problems using a setup outlined in the subject. I
> want to run three guests on a single kvm host. The guests are connected
> through macvtap directly to the network device 'eth0' of the host using
> this configuration:
>
>   <interface type='direct'>
>     <mac address='52:54:00:86:3f:24'/>
>     <source dev='eth0' mode='bridge'/>
>     <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
>   </interface>
>
> Each guest is assigned a static ip address, no DHCP is involved.
>
> When I start the guests the macvtap links are generated automatically,
> ifconfig outputs this:
>
>   macvtap2  Link encap:Ethernet  HWaddr 52:54:00:86:3f:24
>             inet6 addr: fe80::5054:ff:fe86:3f24/64 Scope:Link
>             UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
>             RX packets:98 errors:0 dropped:0 overruns:0 frame:0
>             TX packets:31 errors:0 dropped:0 overruns:0 carrier:0
>             collisions:0 txqueuelen:500
>             RX bytes:10946 (10.6 KiB)  TX bytes:2184 (2.1 KiB)
>
> However, depending on the startup order, only the first two guests are
> able to reach the network! I can't access the third one. After the
> initial startup, the last host won't connect to the network even if I restart
> all the guests in a differnt order. I can only reset this by restarting
> the host.

Assuming that each guest's interface definition has a unique MAC
address, this really sounds like a bug in macvtap on your kernel. There
is probably some limit on the number of macvtap interfaces attached to a
single physical interface, but it's certainly much higher than 3, and if
you hit it you'd be more likely to get an error when attempting to
create the macvtap device.