code: def down(self, state, msg_init=False):
          
          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)
docs: A port will be in the state of DISABLE or BLOCK,
               and be stopped.