Decide the port of the side near a root bridge.
            It is compared by the following priorities.
             1. root path cost
             2. designated bridge ID value
             3. designated port ID value

def compare_root_path(path_cost1, path_cost2, bridge_id1, bridge_id2,
                          port_id1, port_id2):
        """ Decide the port of the side near a root bridge.
            It is compared by the following priorities.
             1. root path cost
             2. designated bridge ID value
             3. designated port ID value """
        result = Stp._cmp_value(path_cost1, path_cost2)
        if not result:
            result = Stp._cmp_value(bridge_id1, bridge_id2)
            if not result:
                result = Stp._cmp_value(port_id1, port_id2)
        return result