As shown in the figure, the four switches have been running and configured with the MSTP protocol. SW2 is the root bridge of Instance 20 and the backup root bridge of Instance 10. Which of the…

Question

Options

• AResult 1
• BResult 2
• CResult 3
• DResult 4

• A
  11% (2)
• B
  6% (1)
• C
  6% (1)
• D
  78% (14)

Explanation

Option D is correct because it accurately reflects SW2's port roles and states across both MSTP instances simultaneously. As the root bridge of Instance 20, SW2 has all ports in the Designated (DESI) role and FORWARDING state for that instance. As the backup root bridge of Instance 10, SW2 has one Root (ROOT) port in FORWARDING state pointing toward the actual root bridge, while its remaining ports are Alternate (ALTE) in DISCARDING state.

Options A, B, and C are wrong because they misrepresent one or both instances: common errors include showing all ports as Designated in Instance 10 (which would only be correct if SW2 were the root bridge there), showing Root ports in Instance 20 (impossible for the root bridge itself), or assigning incorrect forwarding and discarding states to port roles.

Memory tip: Root bridge means every port is a Designated FORWARDING port - there is no upstream to point to. Backup root bridge means exactly one Root FORWARDING port exists (toward the true root), and all others are Alternate DISCARDING ports. Keeping these two rules separate for each instance is the key to reading "display stp brief" output correctly.

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