scenario: i run a mix of libvirt 1.1.1 and 1.2.2 on the source hosts, 1.2.2 on the destination hosts qemu: 1.5.0+dfsg-3ubuntu5.3~cloud0 on all the hosts openstack havana and icehouse description: i have many compute nodes running libvirt+kvm, when i try to migrate guest that were created months ago it fails, if i create a vm and then migrate it works the problem is that libvirt cannot retrieve the mtu of the bridge were the interfaces are be attached, but bridge is there (up and running) $ grep qbr32740ab3-b6 /var/log/libvirt/libvirtd.log 2014-11-05 08:44:53.013+0000: 35311: debug : udevGetDeviceProperty:121 : udev reports device 'qbr32740ab3-b6' does not have property 'DRIVER' 2014-11-05 08:44:53.013+0000: 35311: debug : udevGetDeviceType:1217 : Could not determine device type for device with sysfs name 'qbr32740ab3-b6' 2014-11-05 08:44:53.013+0000: 35311: debug : udevAddOneDevice:1392 : Discarding device -1 0x7f36e80bd5c0 /sys/devices/virtual/net/qbr32740ab3-b6 2014-11-05 08:44:53.013+0000: 35311: debug : udevProcessDeviceListEntry:1414 : Failed to create node device for udev device '/sys/devices/virtual/net/qbr32740ab3-b6' 2014-11-05 08:45:08.458+0000: 35303: error : virNetClientProgramDispatchError:175 : Cannot get interface MTU on 'qbr32740ab3-b6': No such device # ip l show dev qbr32740ab3-b6 639: qbr32740ab3-b6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP link/ether 72:d5:db:f6:b5:30 brd ff:ff:ff:ff:ff:ff # brctl show qbr32740ab3-b6 bridge name bridge id STP enabled interfaces qbr32740ab3-b6 8000.72d5dbf6b530 no qvb32740ab3-b6 tap32740ab3-b6 the difference between a new defined domain and an old one is this < <alias name='pci0'/> --- > <alias name='pci.0'/> > <timer name='hpet' present='no'/> ---> old domain does not have it i even tried patching src/util/virnetdev.c to remove the check and always return 1500 but the same thing happens i'm lost i don't know where to look at it, any clue were to look is appreciated