Comment 50 for bug 1815989

live_migration_wait_for_vif_plug=true is the default value.
if you have set live_migration_wait_for_vif_plug=false then yes it would result in
packet loss and the mac learning frame not propagating if the vlan tag application took a protracted period of time.

when live_migration_wait_for_vif_plug=true we wait for the network-vif-plugged events to be recived from neutron in pre-livemigrate before we tell libvirt to migrate the guests.
if its set to false we do not wiat for the event and just proceed to start the migration once the rpc call too plug the vifs completes without waiting for neutron to wire up the ports with the vlans and flow rules.

so if you have live_migration_wait_for_vif_plug=false you will see similar issue to the behavior with this bug bug since true is the default you should only see that behavior if you opted in by setting it to false. if you currently have it set to false and you are using ml2/ovs then you shoudl set it to true.

it only exts because some network backend do not reliably send event. ovs is not one fo the backed that is broken in that way so it shoudl always be set to true when using ml2/ovs and ovs in general.