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In STP, the bridge IDs of three switches are known: SW1 has priority 32768 and MAC address 00-00-5E-00-53-01, SW2 has priority 61440 and MAC address 00-00-5E-00-53-05, and SW3 has priority 4096 and MAC address 00-00-5E-00-53-0A. Which switch is elected as the root bridge?

  1. ASW1
  2. BSW2
  3. CSW3
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Answer: C. SW3

In STP, the switch with the lowest bridge ID becomes the root bridge. The bridge ID puts the priority in the upper part and the MAC address in the lower part, so priorities are compared first and MAC addresses only when the priorities are equal. Here SW3, with the lowest priority of 4096, becomes the root bridge regardless of its MAC address. The default priority is 32768, and if nothing is configured the switch with the lowest MAC address is chosen, so it is common practice to give the switch at the center of the network a lower priority.

  • A. Its MAC address is the lowest, but its priority is higher than SW3's, so it is not chosen.
  • B. This switch has the highest priority value. Lower values take precedence.
  • C. Correct. Its priority of 4096 is the lowest, so its bridge ID is also the lowest.
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