In an RSTP environment, a point-to-point link LAN segment has two bridges.
A switch that considers itself the unique, designated bridge for the attached LAN segment sends a “propose” message to the other bridge to request a confirmation of its role. The other bridge on that LAN segment replies with an “agree” message if it agrees with the proposal. The receiving bridge immediately moves its designated port into the forwarding state.
Before a bridge replies with an “agree” message, it reverts all of its designated ports into the blocking state. This introduces a temporary partition into the network. The bridge then sends another “propose” message on all of its designated ports for further confirmation. Because all of the connections are blocked, the bridge immediately sends an “agree” message to unblock the proposing port without having to wait for further confirmations to come back or without the worry of temporary loops.
Beginning with the root bridge, each bridge in the network engages in the exchange of “propose” and “agree” messages until they reach the edge ports. Edge ports connect to non-STP devices and do not participate in RSTP. Their role does not need to be confirmed. If you have an 802.1D-2004 compliant edge port, the bridge detection mechanism causes the edge port to transition to a non-edge port upon receiving a BPDU. If the former edge port does not receive a subsequent BPDU during a pre-determined interval, the port attempts to become an edge port.
RSTP attempts to transition root ports and designated ports to the forwarding state and alternate ports and backup ports to the blocking state as rapidly as possible.
A port transitions to the forwarding state if any of the port:
Has been in either a root or designated port role long enough that the spanning tree information supporting this role assignment has reached all of the bridges in the network;
NoteRSTP is backward-compatible with STP, so if a port does not move to the forwarding state with any of the RSTP rapid transition rules, a forward delay timer starts and STP behavior takes over.
Is now a root port and no other ports have a recent role assignment that contradicts with its root port role;
Is a designated port and attaches to another bridge by a point-to-point link and receives an “agree” message from the other bridge port; or
Is an edge port. An edge port is a port connected to a non-STP device and is in the forwarding state.
The following sections provide more information about RSTP behavior.