When do bridges generate Configuration BPDUs?
Bridges generate Configuration BPDUs in the following instances:
Every Hello Time seconds on all ports of the Root Bridge (unless there is a Physical-Layer loop.
When a non-Root Bridge receives a Configuration BPDU on its Root Port, it sends an updated version of this BPDU out every Designated Port.
When a Designated Port hears a less attractive BPDU from a neighboring bridge.