# from compute-dvr.yaml
...
              # External bridge for DVR (no IP address required)
              - type: ovs_bridge
                name: bridge_name       # defaults to br-ex
                dns_servers:            # what for?
                  get_param: DnsServers
                use_dhcp: false
                members:
                - type: interface
                  name: nic6
                  primary: true
...

[root@overcloud-compute-0 ~]# ovs-vsctl show | grep 'Bridge br-ex' -A 10
    Bridge br-ex
        Controller "tcp:127.0.0.1:6633"
            is_connected: true
        fail_mode: secure
        Port phy-br-ex
            Interface phy-br-ex
                type: patch
                options: {peer=int-br-ex}
        Port br-ex
            Interface br-ex
                type: internal