Description of problem: in Mitaka, there is a new MTU logic in place, where network calculated MTU is applied to all ports belonging to the network, including router legs and TAP devices.

It was revealed in upstream that the logic does not work for OVS in some cases, when there are ports that belong to networks with different MTUs. In which case MTU setup may not take effect.


Version-Release number of selected component (if applicable): this does not affect any current OSP/Liberty, but it will affect it once a rebase for a new tarball occurs. This affects all OSP/Mitaka installations.


How reproducible: always.


Steps to Reproduce:
- observe that br-int mtu = 1500
- create vlan network (mtu = 1450)
- start a nova instance using vlan network
- observe br-int mtu is now 1450
- create gre network (1458) with a subnet
- plug in the gre subnet into the router
- observe that qr device in qrouter namespace for gre network is 1450 (not 1458 as expected)


Expected results: MTU on qr device reflects network MTU.

The issue is fixed in upstream with: https://review.openstack.org/#/q/Ibd9866d2d8fe12bb253c6fb9978f583293b32734,n,z