PURPOSE: To minimize influence given to the transmission function of LAN by changing the state of a blocking port connected to same LAN as a root port to a forwarding state when time is out at the time of receiving BPDU to make the port of the designated bridge of LAN comes to a blocking state.

CONSTITUTION: The time-out of C=BPDU is detected at high speed by using the fact that two ports 3-7 and 3-8 have compatibility on a topology when the two ports 3-7 and 3-8 of the designated bridge 3-4 are connected to same LAN. Namely, the reception stop of C=BPDU of the port 3-7 occurs when a fault occurs in the part of a transmission line 3-10. But, the reception of the blocking port 3-8 continues. In such a case, the state of the port 3-8 is set from the blocking state to the forwarding state and the root port of the bridge is changed from 3-7 to 3-8.