A port will be in the state of DISABLE or BLOCK,
             and be stopped.

def down(self, state, msg_init=False):
        """ A port will be in the state of DISABLE or BLOCK,
             and be stopped.  """
        assert (state is PORT_STATE_DISABLE
                or state is PORT_STATE_BLOCK)
        if not self.config_enable:
            return

        if msg_init:
            self.designated_priority = None
            self.designated_times = None

        self._change_role(DESIGNATED_PORT)
        self._change_status(state)