Designated Port Rapid Behavior
When a port becomes a new designated port, or the STP priority changes on an existing designated port, the port becomes an unsynced designated port.
For an unsynced designated port to rapidly move into the forwarding state, the port must propose a confirmation of its role on the attached LAN segment (unless the port is an edge port). Upon receiving an “agree” message, the port immediately enters the forwarding state.
If the receiving bridge does not agree and it has a superior STP priority, the receiving bridge replies with its own BPDU. Otherwise, the receiving bridge keeps silent, and the proposing port enters the forwarding state and starts the forward delay timer.
The link between the new designated port and the LAN segment must be a point-to-point link. If there is a multi-access link, the “propose” message is sent to multiple recipients. If only one of the recipients agrees with the proposal, the port can erroneously enter the forwarding state after receiving a single “agree” message.