Comment 26 for bug 1763608

I also have the same problem, running openstack on Ubuntu 18.04.2.
The physical interface of br-ex does not come up because there is not IP address set in netplan yaml file.

below is default ubuntu openstack config, the enp3s0 interface comes up as it is connected to a linux bridge defined in netplan file. But the enp5s0 which is handled by ovs will not come up.

network:
  version: 2
  ethernets:
    enp3s0:
      match:
        macaddress: 00:1a:4b:4c:a6:60
      set-name: enp3s0
      mtu: 1500
    enp5s0:
      match:
        macaddress: 00:1a:4b:4c:a6:5e
      set-name: enp5s0
      mtu: 1500
  bridges:
    br-enp3s0:
      interfaces: [enp3s0]
      addresses:
      - 192.168.251.209/24
      gateway4: 192.168.251.1
      nameservers:
        search: [maas]
        addresses: [192.168.251.2]
      mtu: 1500