Redundant Ethernet links improve fault tolerance, but a forwarding loop can make flooded frames circulate indefinitely because ordinary Ethernet frames have no hop limit.

Spanning-tree forwarding prevents this by selecting a loop-free subset of the physical links. Conceptually, switches cooperate to choose one logical tree that still connects the participating network while blocking redundant forwarding paths.

If a link on the active tree fails, a previously blocked redundant link can become active after the topology is recomputed.

The important idea is the trade-off: the physical network may contain cycles for resilience, while the active layer-2 forwarding topology is kept acyclic to prevent broadcast storms and repeated frame delivery.

Modern protocols refine convergence and topology selection, but the spanning-tree principle is the reusable concept.