After the root bridge is elected the bridges other than the root just listen to the BDPUs sended by the root. This is the mehanism used by 802.1D.
Thanks for reply.
Also the BPDUs are changed by switches on the way or every switch will receive original BPDU generated by root?
The BPDUs will be changed in a hop-by-hop basis. Remember that BPDUs will have different attributes from the interfaces perspective. Downstream switches most likely receive higher cost values than upstream switches.
Yes, you are right.
Configuration BPDUs will be originated only by root bridge. other switches will be just listening to those bpdus and send them downstream.
Topology change notification BPDUs can be generated by any switch and will be passed to root bridge to inform change in topology.