3Com SuperStack II Manual page 195

What is STP? 195
Figure 46 Traffic flowing through Bridges C and A
Figure 47 Traffic flowing through Bridge B
STP determines which is the most efficient path between each bridged
segment and a specifically assigned reference point on the network. Once
the most efficient path has been determined, all other paths are disabled.
Thus, in the example above, STP initially decided that the path through
Bridge C was the most efficient, and so blocked the path through Bridge
B. After the failure of Bridge C, STP re-evaluated the situation and
opened the path through Bridge B.