In 802.1D, when a port has been selected by spanning tree to become a
designated port, it must wait twice the Forward Delay before transitioning the
port to a forwarding state. RSTP significantly speeds up the recalculation
process after a topology change occurs in the network as it converges on a
link-by-link basis and does not rely on timers expiring before ports can
transition. Rapid transition to forwarding state can only be achieved on edge
ports and on point-to-point links. In RSTP, this condition corresponds to a
port with a designated role that is in a blocking state. The figure illustrates
how rapid transition is achieved step-by-step.

- A new link is created between the root and Switch A and both ports are in
designated blocking state until they receive a BPDU from their counterpart.
When a designated port is in a discarding or learning state (and only in this
case), it sets the proposal bit on the BPDUs it sends out. This is what happens
for port p0 of the root bridge.
- Switch A sees the proposal BPDU with a superior Root ID. It blocks all
non-edge designated ports other than the one over which the proposal and
agreement process are occurring. This operation is called "sync" and
prevents switches below A from causing a loop during the proposal agreement
process. Edge ports need not be blocked and remain unchanged during sync.
- Bridge A explicitly sends an Agreement which allows the root bridge to put
the Root Port 0 in forwarding state. Port 1 becomes the root port for A.
Downstream RSTP Proposal Process
Once Switch A and the root
bridge are synchronized, the proposal or agreement process continues on Switch
A out of all of its downstream, designated, non-edge ports as shown in Figure
.
- Switch B on P5 will see that Switch A is discarding and will also
transition to the designated discarding state. Switch A then sends its proposal
BPDU down to B with the Root ID of the root bridge.
- Switch B sees a proposal with the superior BPDU from A and blocks all
non-edge designated ports other than the one over which the proposal and
agreement process is occurring.
- Switch B sends a BPDU with the Agreement bit set and Switch A P3
transitions to forwarding state. The synchronization process continues with
switches downstream from B.