A switch runs MSTP. The configuration is shown in the figure. What is the role of this switch in MST1?

Question

Options

• ARoot switch
• BNon-root switch
• CDesignated root switch
• DDesignated switch

• A
  73% (19)
• B
  4% (1)
• C
  15% (4)
• D
  8% (2)

Explanation

In MSTP, the Root switch is the switch with the lowest Bridge ID (priority + MAC address) within a given MST instance. The configuration shown assigns this switch the lowest bridge priority in MST1, so it wins the root election - all its ports become designated ports and it serves as the topology reference point for MST1.

Why the distractors are wrong:

• B (Non-root switch): Non-root switches are every switch except the root; since this switch has the lowest Bridge ID in MST1, it cannot be a non-root.
• C (Designated root switch): This is not a valid MSTP role - it conflates "Root switch" and "Designated switch," two distinct concepts. Expect the exam to use this type of hybrid distractor to test terminology precision.
• D (Designated switch): A designated switch wins the designated port election on a specific segment, but that role applies to individual ports/segments on non-root switches too - it does not describe the instance-wide root.

Memory tip: In STP/MSTP, lowest always wins - lowest priority (then lowest MAC as tiebreaker) = Root. If the config shows a switch with priority 0 or the minimum value for an instance, it's the Root switch. No ports on the Root are ever root ports; they're all designated.

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