In a Spanning Tree algorithm, a port selected as a designated port waits for 2 x Fwd-delay (2 x 15) seconds before transitioning to forwarding state. In RSTP, such port corresponds to a designated role and blocking state. The figure below illustrates the rapid transition of a port to forwarding state.
P0: Designated port
P1: New root port
P2: Alternate port
P3: Designated port
P4: Edge Port
If a new link is created between the Root and Switch A, then both the ports on this link are put in designated blocking state, until they receive a BPDU from their counterpart. When a designated port is in discarding or learning state (and only in this case), it sets the proposal bit on the sent BPDU. This happens for port P0 of the root bridge, as shown in step 1 of the above shown figure. Because Switch A receives superior information, it immediately knows that P1 will be its new root port. Switch A then starts a sync operation to ensure that all of its ports are in-sync with this new information. A port is in-sync if it meets either of the following criteria:
- The port is in blocking state.
- The port is an edge port.
If an alternate port P2, a designated forwarding port P3, and an edge port P4 exist on Switch A, P2 and P4 already meet one of the listed criteria. To be in-sync (step 2 of the diagram above), Switch A just needs to block port P3, assigning it the discarding state. If all ports are in-sync, Switch A can unblock its newly selected root port P1 and reply to the Root by sending an agreement message (step 3). This message is a copy of the proposal BPDU, with the agreement bit set instead of the proposal bit. This ensures that port P0 knows exactly to which proposal, the agreement it receives corresponds.
When port P0 receives that agreement, it can immediately transition to forwarding. Port P3 which was left in a designated discarding state after the sync, in the next step, is exactly in the same state as port P0 was in Step 1. It then starts proposing to its neighbor, attempting to quickly transition to forwarding. This handshake mechanism propagates quickly towards the edge of the network and rapidly restores connectivity after a change in the topology.