STP - what is a topology change

Spanning Tree Protocol (STP) is designed to respond to topology changes that may change the active spanning tree. STP goes through a topology change process once a topology change is detected. But what constitutes a topology change?

With classic STP, a topology change occurs whenever there is a failure on a link between switches. The failure of a link to an end device does not constitute a topology change.

Another way of saying it is that a link failure along the active path would trigger a topology change.

With RSTP however, a topology change occurs when a non-edge interface moves to the forwarding state.

Links

https://forum.networklessons.com/t/rapid-spanning-tree-rstp/1138/127?u=lagapides

https://networklessons.com/spanning-tree/spanning-tree-topology-change-notification-tcn

https://networklessons.com/spanning-tree/rapid-spanning-tree-rstp