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Difficulty: HardRapid PVST+ Spanning Tree Protocol Operations and Roles

A network engineer connects a new point-to-point link between Switch-1 and Switch-2 in an enterprise network running Rapid PVST+. Place the steps of the RSTP Proposal/Agreement fast convergence mechanism in the correct chronological order from link initialization to the Designated Port entering the Forwarding state.

  1. 1Switch-1 sends a BPDU with the Proposal bit set on its Designated Port (which is currently in the Discarding state) toward Switch-2.
  2. 2Switch-2 receives the superior BPDU, selects the receiving port as its new Root Port, and enters the synchronization phase.
  3. 3Switch-2 blocks all of its non-edge Designated Ports by placing them into the Discarding state.
  4. 4Switch-2 transmits an Agreement BPDU back to Switch-1 out of its newly selected Root Port.
  5. 5Switch-1 receives the Agreement BPDU and immediately transitions its Designated Port to the Forwarding state.

Answer

The correct chronological sequence for the Rapid PVST+ Proposal/Agreement handshake is: 1) Switch-1 sends a BPDU with the Proposal bit set on its Designated Port (in Discarding state) toward Switch-2. 2) Switch-2 receives the superior BPDU, selects the receiving port as its new Root Port, and enters the synchronization phase. 3) Switch-2 blocks all of its non-edge Designated Ports by placing them into the Discarding state. 4) Switch-2 transmits an Agreement BPDU back to Switch-1 out of its newly selected Root Port. 5) Switch-1 receives the Agreement BPDU and immediately transitions its Designated Port to the Forwarding state.
In Rapid PVST+ (IEEE 802.1w), fast convergence on point-to-point links relies on an explicit proposal/agreement handshake. When a link initializes, the upstream switch sends a BPDU with the Proposal bit set while its Designated Port is in the Discarding state. The downstream switch evaluates the BPDU, adopts the interface as its new Root Port, and enters the sync phase by blocking (placing in Discarding) all non-edge Designated Ports to eliminate loop potential. Once synchronized, the downstream switch sends an Agreement BPDU back out its Root Port. Upon receiving the Agreement BPDU, the upstream switch immediately transitions its Designated Port into the Forwarding state without waiting for traditional 802.1D timers.

Step-by-Step Solution

1
Initiate the handshake proposal
Switch-1 sends a BPDU with the Proposal bit set while its port is in the Discarding state.
Rapid PVST+ relies on active proposal/agreement handshake frames across point-to-point links rather than passive timer delays.
2
Process the proposal on the receiving switch
Switch-2 identifies the interface as its new Root Port upon evaluating the superior BPDU.
Accepting a superior BPDU triggers Switch-2 to sync its topology before confirming agreement.
3
Synchronize downstream ports to prevent loops
Switch-2 forces all non-edge Designated Ports into the Discarding state (sync process).
Isolating local non-edge ports guarantees that no active switching loop can form while transitioning the new Root Port.
4
Send agreement confirmation
Switch-2 transmits an Agreement BPDU back to Switch-1.
The Agreement BPDU explicitly informs Switch-1 that Switch-2 has safely synchronized its downstream ports.
5
Complete rapid transition to Forwarding
Switch-1 transitions its Designated Port directly into the Forwarding state.
Because Switch-2 confirmed loop safety via the Agreement BPDU, Switch-1 can bypass legacy 802.1D listening and learning timers.

Key Concept

Rapid PVST+ Proposal/Agreement Handshake Mechanism
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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