The Spanning Tree Protocol is an OSI layer-2 protocol that ensures a loop-free topology for any bridged LAN. Spanning tree allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of these backup links. Bridge loops must be avoided because they result in flooding the network.

The Spanning Tree Protocol (STP), is defined in the IEEE Standard 802.1D. As the name suggests, it creates a spanning tree within a mesh network of connected layer-2 bridges (typically Ethernet switches), and disables those links that are not part of the tree, leaving a single active path between any two network nodes.

But what happens if a switch doesn’t receive BPDU in a timely manner or when it doesn’t receive any? In this case the topology must have changed and blocked ports can be unblocked, so a “logical” loop can occur.

More info on http://www.ciscozine.com/2009/01/19/preventing-stp-forwarding-loops/