What type of BPDU is used to communicate changes in Layer 2 topology?

The Topology Change Notification BPDU is specifically designed to inform switches of changes in the Layer 2 topology. When a switch detects that a port has transitioned into a forwarding state, or when a device is added or removed from the network, it generates a Topology Change Notification BPDU to alert other switches about the change. This timely notification allows the switches to update their spanning tree calculations and ensure that the network continues to function efficiently and without loops.

Configuration BPDUs are used primarily to convey the configuration of the spanning tree protocol itself, such as the bridge ID and root port, but not necessarily focused on immediate changes in the topology. Forwarding BPDUs do not exist as a distinct category; typically, BPDUs serve the configuration and notification roles. Blocking BPDUs also do not exist as a standalone category but rather refer to the state of a port that does not participate in frame forwarding to prevent loops, rather than communicating topology changes.

Understanding the specific role of each type of BPDU helps in grasping how Spanning Tree Protocol (STP) works to maintain a loop-free network environment.