In a network running RSTP where some devices use STP, how are topology changes notified?

In a network that employs Rapid Spanning Tree Protocol (RSTP) but contains devices operating under the older Spanning Tree Protocol (STP), topology changes are notified through the use of STP Bridge Protocol Data Units (BPDUs) that include a topology change (TC) notification.

The rationale behind this is that when RSTP is mixed with STP, the communication for topology changes must be compatible with the older STP devices, which do not recognize or interpret RSTP-specific BPDUs correctly. It is essential for the BPDUs to adhere to the protocol standards that the STP devices expect. In this case, the use of STP BPDUs ensures that all devices in the network can understand the topology change notifications.

When there is a topology change, the switch that detects the change sends a BPDU with the TC flag set, notifying other switches about this change. This helps in minimizing address learning times and allows them to update their MAC address tables accordingly.

Therefore, the correct choice regarding how topology changes are communicated in a mixed RSTP and STP environment is indeed through STP BPDUs with the TC notification.