Propagating Topology Change Information
When a change occurs in the topology of the network, such events are communicated through the network.
In an RSTP environment, only non-edge ports entering the forwarding state cause a topology change. A loss of network connectivity is not considered a topology change; however, a gain in network connectivity must be communicated. When an RSTP bridge detects a topology change, that bridge starts the topology change timer, sets the topology change flag on its BPDUs, floods all of the forwarding ports in the network (including the root ports), and flushes the learned MAC address entries.