code: def transmit_tc_bpdu(self):
          
          if not self.send_tc_flg:
              timer = datetime.timedelta(seconds=self.port_times.max_age
                                         + self.port_times.forward_delay)
              self.send_tc_timer = datetime.datetime.today() + timer
              self.send_tc_flg = True
docs: Set send_tc_flg to send Topology Change BPDU.