'Create a sub_port.
:param trunk_id: ID of the trunk.
:param port_id: ID of the subport.
:param segmentation_id: segmentation ID associated with this subport.
:param port_mac: MAC address of the subport.'

def add_sub_port(self, trunk_id, port_id, port_mac, segmentation_id):

    sub_port = SubPort(trunk_id, port_id, port_mac, segmentation_id)
    try:
        if (not sub_port.bridge.exists()):
            raise exc.TrunkBridgeNotFound(bridge=sub_port.bridge.br_name)
        sub_port.plug(self.br_int)
    except RuntimeError as e:
        raise TrunkManagerError(error=e)
