3Com SuperStack 3C17300 Implementation Manual page 39

What is STP? 41
Enable one of the less efficient paths if the most efficient path fails.
RSTP provides the same functionality as STP. For details on how the two
systems differ, see "How RSTP Differs to STP" on page 45.
As an example, Figure 7 shows a network containing three LAN segments
separated by three bridges. With this configuration, each segment can
communicate with the others using two paths. Without STP enabled, this
configuration creates loops that cause the network to overload.
Figure 7 A network configuration that creates loops
Figure 8 shows the result of enabling STP on the bridges in the
configuration. STP detects the duplicate paths and prevents, or blocks,
one of them from forwarding traffic, so this configuration will work
satisfactorily. STP has determined that traffic from LAN segment 2 to LAN
segment 1 can only flow through Bridges C and A, because, for example,
this path has a greater bandwidth and is therefore more efficient.