code: def add_nio(self, nio, port_number):
          
  
          if port_number in self._nios:
              raise DynamipsError("Port {} isn't free".format(port_number))
  
          log.info('Frame Relay switch "{name}" [{id}]: NIO {nio} bound to port {port}'.format(name=self._name,
                                                                                               id=self._id,
                                                                                               nio=nio,
                                                                                               port=port_number))
  
          self._nios[port_number] = nio
          yield from self.set_mappings(self._mappings)
docs: Adds a NIO as new port on Frame Relay switch.
  
          :param nio: NIO instance to add
          :param port_number: port to allocate for the NIO