What happens in STP (Spanning Tree Protocol) when a network link fails?

STP recalculates the topology and unblocks previously blocked ports to restore connectivity.

This is one of STP's most important features — automatic failover. When a link goes down:

1. Switches detect the failure (no more BPDUs (Bridge Protocol Data Units) on that link)
2. STP recalculates the spanning tree
3. Previously blocked ports are transitioned to forwarding state
4. Traffic flows through the alternate path

In the example, when Trunk1 between S1 and S2 fails, STP unblocks the Trunk2 path so S2 can reach S1 through S3 instead.

Tip: STP doesn't just prevent loops — it also provides resilience. The blocked paths are your backup routes waiting to activate.

Go deeper:

• Spanning Tree Protocol — reconvergence (Wikipedia) — on link loss STP recomputes a new least-cost tree and unblocks an alternate port.
• How Spanning-Tree works (NetworkAcademy.IO) — the failover mechanic: lost BPDUs, re-elected paths, a standby port takes over.