Fake bridge are are ovsbridge child of a parent bridge with a specific vlan tag.
```
auto br-parent
iface br-parent inet manual
ovs_type OVSBridge
auto br-fakev10
iface br-fakev10 inet manual
ovs_type OVSBridge
ovs_bridge br-parent
ovs_options br-parent 10
```
Openvswitch already manage mtu if ovs-mtu is defined.
(Ovs manage mtu in userland, and sync mtu for some interfaces in kernel).
If mtu is changed by address module, before the ovs userland mtu,
this give packets drop.
