This section describes the RSTP rapid behavior following a topology change.
In this example, the bridge priorities are assigned based on the order of their alphabetical letters; bridge A has a higher priority than bridge F.
Suppose you have a network, as shown in the following figure, with six bridges (bridge A through bridge F) where the following is true:
Bridge A is the root bridge.
Bridge D contains an alternate port in the blocking state.
All other ports in the network are in the forwarding state.
The network reconverges in the following way:
If the link between bridge A and bridge F goes down, bridge F detects the root port is down. At this point, bridge F:
Immediately disables that port from the STP.
Performs a configuration update.
As shown in the following figure, after the configuration update, bridge F:
Considers itself the new root bridge.
Sends a BPDU message on its designated port to bridge E.
Determines that it received an inferior BPDU.
Immediately begins the max age timer on its root port.
Performs a configuration update.
As shown in the following figure, after the configuration update, bridge E:
Regards itself as the new root bridge.
Sends BPDU messages on both of its designated ports to bridges F and D, respectively.
Decides that the receiving port is the root port.
Determines that bridge E is the root bridge.
Immediately begins the max age timer on its alternate port.
Performs a configuration update.
As shown in the following figure, after the configuration update, bridge D:
Moves the alternate port to a designated port.
Sends a “propose” message to bridge E to solicit confirmation of its designated role and to rapidly move the port into the designated state.
Performs a configuration update.
Changes its receiving port to a root port.
The existing designated port enters the blocking state.
Bridge E then sends:
A “propose” message to bridge F.
An “agree” message from its root port to bridge D.
Bridge D moves the port that received the “agree” message into the forwarding state.
Bridge F confirms that its receiving port (the port that received the “propose” message) is the root port, and immediately replies with an “agree” message to bridge E to unblock the proposing port.
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