As shown in the figure, SW1 and SW2 have just run RSTP and are now performing spanning tree calculation. So, which of the following descriptions about the port synchronization process in the P/A…

Question

Options

• AAfter the ports of SW2 are synchronized, all downstream ports enter the Discarding state
• BSW2 receives the RSTBPDU with the Proposal bit set from the root bridge, and starts to put all its ports into the synchronous state
• CWhen each port in SW2 receives the Sync from the upstream RSTPBPDU, it sets the status of the port
• DWhen the downstream port of SW2 is synchronized, the status of the AP port remains unchanged

• A
  77% (24)
• D
  23% (7)

Explanation

In RSTP's Proposal/Agreement (P/A) mechanism, B is correct because synchronization is triggered the moment SW2 receives a Proposal-flagged BPDU from an upstream superior bridge - SW2 immediately begins placing all non-edge designated ports into a synchronized (Discarding) state. A is correct because this forced Discarding on all downstream ports is the very definition of synchronization - it guarantees no loop can form before the Agreement is sent upstream. C is correct because each individual port on SW2 acts on the sync condition independently, setting its own state when it processes the upstream BPDU's sync signal.

Option D is wrong because the AP (root port / Agreement port) absolutely does change state - once synchronization of all downstream ports completes, SW2 sends an Agreement back upstream and the root port immediately transitions to Forwarding. Claiming it "remains unchanged" contradicts the entire purpose of P/A, which is to rapidly move the root port to Forwarding.

Memory tip: Think of P/A as a "freeze-then-unlock" handshake - upstream says "Propose", SW2 freezes all downstream ports (Discarding), then says "Agreed", and the root port unlocks to Forwarding. Any answer saying the root port stays frozen after agreement is a trap.

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