Problem we are trying to solve: repeat traffic over the same link, particularly broadcast storms. If a layer-2 network has a loop, the same traffic may be forwarded over the same physical network segment repeatedly until its time-to-live expires. Network loops can bring a network down by buying it in traffic.

A collection of switches can be organized into a spanning tree. A spanning tree is a concept from graph theory that describes paths to interconnect all nodes without creating cycles (or loops). So, for instance, nodes A, B,C and D can be interconnected in multiple ways, including:

The topology is a tree. Spanning tree protocols detect cycles (loops) by listening for Bridge Protocol Data Units (BPDUs), which are messages that start at a node designated as the root, and propagate through the tree. When a repeat BPDU is detected, a node makes a calculation about which is the best propagation link among the duplicates. The other, duplicate links are placed into a blocking state. By this mechanism, a spanning tree can be formed among arbitrarily interconnected nodes.

Spanning tree protocols have been used to create resiliency in networks; if one of multiple links goes down, spanning tree protocols may resurrect another. However, multi-path resilience built from link aggregation groups--multiple links cooperating as one--provide combined bandwidth and faster convergence, and are a better design.

Loop detection is a single-switch capability for detecting the interconnection of two or more ports through a another device that doesn't participate in spanning tree. Imagine, for example, two Ethernet ports in a classroom being looped through an unmanaged desktop switch; loop detection will shut down one of the ports.

-Kevin Dowd