os-net-config/etc/os-net-config/samples/bridge_dhcp.json

{ "network_config": [
        {
            "type": "ovs_bridge",
            "name": "br-ctlplane",
            "ovs_extra": [
              "br-set-external-id br-ctlplane bridge-id br-ctlplane"
            ],
            "use_dhcp": "true",
            "members": [
                {
                    "type": "interface",
                    "name": "em1"
                }
            ]
        }
    ]
}