As shown in the figure, SWA, SWB, and SWC all run RSTP. What are the port roles of the GE0/0/2 port on SWB and the GE0/0/1 port on SWC?

Question

Options

• ABackup port, Root port
• BAlternative port, Backup port
• CRoot port, Designated port
• DBackup port, Alternative port

• A
  11% (5)
• B
  2% (1)
• C
  4% (2)
• D
  83% (38)

Explanation

Option D is correct because the topology places SWB with two ports on the same shared segment - one SWB port wins the designated port election, making GE0/0/2 on SWB a Backup port (a blocked port that backs up another port on the same switch for the same segment). SWC GE0/0/1 is an Alternate port because SWC already reaches the root bridge via a better path through another port, so GE0/0/1 becomes blocked as a secondary route to root through a different switch.

Why the distractors fail:

• A (Backup, Root): SWC GE0/0/1 cannot be a Root port - it's blocked because a better root path already exists on SWC via another interface.
• B (Alternate, Backup): The roles are swapped; same-switch/same-segment redundancy produces a Backup port (SWB), not Alternate; cross-switch blocked paths produce Alternate (SWC), not Backup.
• C (Root, Designated): Both ports are actually in a discarding/blocked state - neither forwards traffic as a Root or Designated port.

Memory tip: Use the phrase "Backup = backs up yourself; Alternate = backs up someone else's route." If your own switch already has a Designated port on the same segment, your blocked port is a Backup. If another switch's port gives root a better path than yours does, your blocked port is an Alternate.

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