As shown in the figure, STA and STB run the RSTP protocol, and the Cost of Path remains the default. Which of the following descriptions of device and port status is correct?

Question

Options

• AGE0/0/0 on STA is the Backup port
• BGE0/0/1 on STB is the Alternate port
• CSTB will become the root bridge
• DGE0/0/2 on the STB is the Alternate port

• A
  7% (2)
• B
  85% (23)
• C
  4% (1)
• D
  4% (1)

Explanation

In RSTP, an Alternate port provides a redundant path to the root bridge but loses out to the Root Port - it blocks traffic unless the Root Port fails. Option B is correct because STB's GE0/0/1 is the non-preferred path to the root bridge (STA), making it an Alternate port by definition.

Why the distractors are wrong:

• A is wrong - STA is the root bridge (lower Bridge ID), so all of its ports are Designated ports, not Backup ports. A Backup port only appears when a single switch has two ports on the same LAN segment.
• C is wrong - The root bridge is elected by the lowest Bridge ID (priority + MAC). STA wins, so STB cannot be root.
• D is wrong - GE0/0/2 on STB is the Root port (the best-cost path toward the root bridge), not an Alternate port. Only one port role can be "best path to root."

Memory tip: Think of roles by function - Root port = Route to root (best path, one per non-root bridge); Alternate port = Alternative to root (second-best, stays blocked); Backup port = Backing up same segment (same switch, same LAN). Confusing Alternate with Backup is the most common trap - the key distinction is Alternate = different path to root, Backup = redundant link to same segment.

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