they start sending and receiving c bpdus for spanning tree construction each c bpdu includes root bridge id information root path cost information and bridge id information construction of a spanning tree is thus started when one of the c bpdus is received through some one of the ports step s 100 the cpu 21 determines by referring to the assignment table 22 the spanning tree management unit according to the port through which the c bpdu has been received step s 101 this c bpdu is distributed to the determined spanning tree management unit step s 102 which executes a spanning tree management process i e a spanning tree construction processing procedure step s 103 this spanning tree construction processing procedure is executed in steps s 1 to s 15 shown in the detailed flowchart of fig6 on the basis of the specification in ieee802 1d as mentioned above it will be only out lined since it is well known first each bridge starts operating as if it is itself a root bridge since no root bridge has been determined and sends out a c bpdu by setting the bridge id as a root bridge id and by setting the root path cost to zero at this time the bridge 5 see fig7 sends out a c bpdu having root bridge id bridge id 97 through the port on the port id 1 side and a c bpdu having root bridge id bridge id 70 through the port on the port id 2 side then each bridge makes a determination described below on the basis of c bpdu information received through its ports step s 1 and its own bridge id if any one of the root bridge ids of.