CYCLE OF BRIDGES:
A bridges network can join several segments. One bridge is useded to connect every segment to the rest of the bridge network. This is given in the figure below:
DISTRIBUTED SPANNING TREE:
If a bridge network creates a cycle, then not all bridges on the network have be allowed to forward broadcast packets.
The bridges configure themselves automatically to select which bridge will forward broadcast frames and which bridge will not forward packets. The bridges interact with each other on the network and use Distributed Spanning Tree (DST) algorithm to select which bridge will not forward frames if a cycle happens.