Rapid Reconvergence

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:

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Initial Network Configuration
../Graphics/EX_055a.svg

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:

As shown in the following figure, after the configuration update, bridge F:

As shown in the following figure, after the configuration update, bridge E:

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New Root Bridge Selected
../Graphics/EX_055c.svg
As shown in the following figure, when bridge F receives the superior BPDU and configuration update from bridge E, bridge F:
  • Decides that the receiving port is the root port.

  • Determines that bridge E is the root bridge.

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Communicating New Root Bridge Status to Neighbors
../Graphics/EX_055d.svg
Bridge D believes that bridge A is the root bridge. When bridge D receives the BPDU from bridge E on its alternate port, bridge D:
  • 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:

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Sending a Propose Message to Confirm a Port Role
../Graphics/EX_055e.svg
Upon receiving the proposal, bridge E (as shown in the following figure):
  • Performs a configuration update.

  • Changes its receiving port to a root port.

The existing designated port enters the blocking state.

Bridge E then sends:

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Communicating Port Status to Neighbors
../Graphics/EX_055f.svg
To complete the topology change (as shown in the following figure):
  • 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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Completing the Topology Change
../Graphics/EX_055g.svg
The following figure displays the new topology.
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Final Network Configuration
../Graphics/EX_055h.svg