code: def link_down(self, ofp_port):
          
          port = self.ports[ofp_port.port_no]
          init_stp_flg = bool(port.role is ROOT_PORT)
  
          port.down(PORT_STATE_DISABLE, msg_init=True)
          self.ports_state[ofp_port.port_no] = ofp_port.state
          if init_stp_flg:
              self.recalculate_spanning_tree()
docs: DESIGNATED_PORT/NON_DESIGNATED_PORT: change status to DISABLE.
              ROOT_PORT: change status to DISABLE and recalculate STP.