'Return bridge which contain port.
:param port_name: Name of port.
:return: Bridge name or None if there is no bridge which contain port.'

def port_to_br(self, port_name):

    bridge = None
    try:
        bridge = self.ovs_vsctl(['port-to-br', port_name]).stdout
    except error.CmdError as e:
        if (e.result_obj.exit_status == 1):
            pass
    return bridge
