As shown in the figure, both SWA.SWB.SWC run RSTP. What is the port role of GE0/0/2 port on SWB and GE0/0/1 port on SWC?

Question

Options

• AAlternative port, Backup port
• BBackup port, Root port
• CRoot port, Designated port
• DBackup port, Alternative port

• A
  4% (1)
• B
  7% (2)
• C
  11% (3)
• D
  78% (21)

Explanation

Option D is correct because in this topology, SWB has two ports connected to the same LAN segment - the better-cost port becomes the Designated Port while GE0/0/2 loses the competition on that shared segment, making it a Backup port. Meanwhile, SWC's GE0/0/1 receives a superior BPDU from a different switch (not through SWC's own Root Port path), so it is blocked as an Alternate port, ready to take over if the Root Port fails.

Why distractors fail: Option A reverses the roles - Alternate and Backup are swapped. Option B incorrectly labels GE0/0/1 on SWC a Root port; SWC already has a Root port on a different interface with a better path to the root bridge. Option C assigns forwarding roles (Root/Designated) to blocked ports, which contradicts the topology's path costs and BPDU competition results.

Memory tip: Use the phrase "Backup = same box, same bus; Alternate = another switch wins" - a Backup port loses to another port on the same switch on the same segment (like two ports into a hub), while an Alternate port loses to a BPDU arriving from a different switch.

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