In MSTP, MSTI and MSTI are both calculated based on priority vectors. Which of the following descriptions of priority vectors is wrong?

Question

Options

• AThe root switching device ID is used to select the root switching device in CIST.
• BThe internal path cost refers to the path cost from this bridge to the main root.
• CThe domain root ID is used to select the main root in MSTI 0.
• DThe priority in the main switch device ID is the priority of MSTI 0

• A
  3% (1)
• B
  3% (1)
• C
  79% (23)
• D
  14% (4)

Explanation

Option C is wrong because in MSTI 0 (the CIST), the overall main root is elected using the CIST Root Bridge ID, not the domain root ID -- the domain root ID is used to elect the regional root bridge within an MST region, which is a separate role entirely. Option A is correct because the CIST root bridge ID is precisely the field used to determine which switch becomes the CIST root. Option B is correct because internal path cost accumulates within the MST region and reflects the cost from a bridge to the relevant root in that context. Option D is correct because the master bridge ID's priority in an MSTI is derived from the MSTI 0 (CIST) bridge priority, since master bridge election is driven by CIST calculations.

Memory tip: MSTP has two distinct root roles -- the overall main root (CIST root, elected by CIST Root Bridge ID) and the regional/domain root (elected by domain root ID). Whenever an answer swaps which ID governs which root election, it is wrong.

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