As shown in the figure, the four switches have run the RSTP protocol and completed spanning tree calculations. So which of the following descriptions of port roles are correct?

Question

Options

• AThe G0/0/1 port of SW3 is the RP port, and the G0/0/2 port of SW4 is the AP port.
• BThe G0/0/1 port of SW4 is the RP port, and the G0/0/2 port of SW4 is the DP port.
• CThe G0/0/2 port of SW2 is the AP port, and the G0/0/2 port of SW3 is the RP port.
• DThe G0/0/2 port of SW2 is the DP port, and the G0/0/2 port of SW3 is the AP port.

• A
  8% (4)
• B
  76% (38)
• C
  4% (2)
• D
  12% (6)

Explanation

Option B is correct because in RSTP, a non-root switch can simultaneously hold both a Root Port and a Designated Port - SW4's G0/0/1 is the RP (its best upstream path toward the Root Bridge), while its G0/0/2 wins the Designated Port election on the segment connecting to SW3, meaning SW4 is the best forwarder on that link.

Why the distractors fail:

• A incorrectly labels SW4's G0/0/2 as an Alternate Port (blocked/backup), when SW4 actually wins the DP election on that segment - directly contradicting the confirmed answer.
• C misidentifies SW2's G0/0/2 as an Alternate Port; since SW2 is typically closer to the Root Bridge, it wins the DP election on its downstream segments, making G0/0/2 a Designated Port, not a blocked AP.
• D gets SW2's G0/0/2 role right (DP) but incorrectly labels SW3's G0/0/2 as AP - in the correct topology, SW3's G0/0/2 should be its Root Port, not a blocked alternate path.

Memory tip: Think of port roles in terms of direction - RP points UP (toward the root, one per non-root switch), DP points DOWN (toward segments, can be multiple per switch), and AP is the blocked backup. A switch near the "middle" of a topology, like SW4, naturally has both an RP (facing root) and a DP (facing downstream), so seeing both roles on the same switch is expected and normal.

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