Evolution of Spanning Tree Protocols

Multiple redundant paths between switches can cause loops in the network topology. If a loop exists, the potential for message duplication exists. When loops occur, some switches see stations appear on both sides of the switch. This condition confuses the forwarding algorithm and enables duplicate frames to be forwarded. To prevent loops while providing path redundancy, Spanning Tree Protocol (STP) defines a tree that spans all switches in an extended network. STP will allow only one active path and block any redundant paths, as shown in Figure 3-1. In case of failure of the active path, one of the redundant paths may become the active path.

Figure 3-1. Spanning Tree Protocol