Three switches run the RSTP protocol, and the topology and configuration are as shown in the figure. [SWA]stp enable [SWA]stp mode rstp [SWA]stp priority 4096 [SWB]stp enable [SWB]stp mode rstp…

Question

Options

• AThe root bridge is SWA
• BThe root bridge cannot be determined
• CThe root bridge is SWC
• DThe root bridge is SWB

• A
  72% (23)
• B
  6% (2)
• C
  3% (1)
• D
  19% (6)

Explanation

SWA wins the root bridge election because it has the lowest Bridge Priority (4096) among all three switches. In RSTP, the switch with the numerically lowest priority value is elected as the root bridge, and SWA's value of 4096 beats SWB's 8192 and SWC's default of 32768 (the default when no priority is explicitly configured).

Option D is wrong because SWB's priority of 8192 is higher than SWA's 4096, so it loses the election. Option C is wrong because SWC uses the default priority of 32768, the highest of all three, making it the least likely candidate. Option B is wrong because the priorities are all different, so the outcome is entirely deterministic -- there is no tie requiring a MAC address tiebreaker.

Memory tip: In STP/RSTP priority elections, treat it like a race where the lowest number crosses the finish line first. Configured priorities are always multiples of 4096, and a switch with no explicit configuration sits at the back of the pack at 32768.

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