In normal STP (802.1D), whenever a topology change is detected by any bridge in the network then immediately the root bridge is informed.
Then the root bridge tells all the bridges in the network about this topology change. As soon as the bridge learns about the Topology change from the root bridge, it clears all the information it has about the old topology.
Topology change information is passed by the root bridge by making the TC flag bitset in the BPDU'S.
But in RSTP (802.1D) the topology change mechanism is different from 802.1D. In RSTP, if any port move to blocking state, then it is not considered as topology change. Only the non-edge ports moving to the forwarding state generate the topology change.
In RSTP, if any bridge detects a topology change, then a TCwhile timer is started on all non-edge designated and root ports and all the MAC address are cleared on these ports.
The value of TCwhile timer is double the hello-time.
The bridge then starts sending out BPDU'S to all the other bridges in the network by making the TC flag bit set. These BPDU'S are sent for the duration of TCwhile timer.
