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#### Proposal Agreement Sequence

In the spanning tree algorithm, a port selected as a designated port waits for 2 x forward-delay (2 x 15) seconds before transitioning to the forwarding state. In RSTP, this port corresponds to a designated role and blocking state.

- P0 - Designated port

- P1 - New root port

- P2 - Alternate port

- P3 - Designated port

- P4 - Edge port

If a new link is created between root and switch 1, both ports go into a designated blocking state until they receive a BPDU from their counterpart. The proposal bit on the BPDUs is set and sent out only when a designated port is in discarding or learning state. This process occurs for port P0 of the root bridge. Because switch 1 receives superior information, switch 1 immediately knows that port P1 is the new root port. Switch 1 then ensures that all of the ports are in sync with this new information.

A port is in sync only if the port meets one of the following criteria:

- The port is in the 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 1, P2 and P4 already meet one of the listed criteria. To be in sync, switch 1 must block port P3, assigning it the discarding state. If all ports are in sync, switch 1 can unblock its newly selected root port P1 and reply to the root by sending an agreement message. This message is a copy of the proposal BPDU with the agreement bit set instead of the proposal bit. This copy ensures that port P0 knows exactly which proposal that the agreement it receives corresponds to.

When port P0 receives the agreement, port P0 can immediately transition to forwarding. Port P3, which was left in a designated discarding state after the sync, is in exactly the same state as port P0 before receiving the agreement. Port P0 then starts proposing to its neighbor, attempting to quickly transition to forwarding. This handshake mechanism propagates quickly towards the edge of the network, and quickly restores connectivity after a change in the topology.

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