During an STP topology change, what does the root bridge do upon receiving a TCN BPDU?

When a root bridge receives a Topology Change Notification (TCN) Bridge Protocol Data Unit (BPDU) during a Spanning Tree Protocol (STP) topology change, it responds by creating a new configuration BPDU. This is crucial in maintaining the integrity and stability of the spanning tree.

The creation of a new configuration BPDU serves to inform all switches in the network about the topology change. It helps in recalculating the path costs and determining the best forwarding paths. By sending out this new configuration BPDU, the root bridge essentially signals that there has been a change in the network topology, prompting other switches to update their own STP states.

This is part of STP's design to ensure that all devices in the network have a synchronized understanding of the current topology and to efficiently manage traffic flows while preventing loops. The process of acknowledging, flooding, or blocking traffic is not the primary response of the root bridge when handling a TCN BPDU; rather, adapting to the change by sending a configuration BPDU is the key action that drives the network's topology convergence.