Electing the Root Bridge in PVST+ by Priority and MAC

200-301 By islever Editorial Last updated

Interpret basic operations of Rapid PVST+ Spanning Tree Protocol

Short answer

D SW4 is elected root: it ties SW3 for the lowest priority (8192) and then wins on the lower MAC address (0C:0A:18... beats 0C:0A:A8...).

Question

Refer to the exhibit. Which switch in this configuration will be elected as the root bridge? SW1 0С:0A:05:22:05:97 - SW2 0С:4A:82:07:57:58 - SW3 0C:0A:A8:1A:3C:9D - SW4 0С:0A:18:A1:B3:19 - Exhibit: Electing the Root Bridge in PVST+ by Priority and MAC
  1. SW1
  2. SW2
  3. SW3
  4. SW4 Correct Answer

Key Takeaways

Priority is compared before MAC. Even though SW1's MAC (0C:0A:05...) is numerically lowest, its default priority 32768 loses to SW3/SW4's 8192, so MAC only breaks the tie between those two.

STP/PVST+ elects the root bridge by the lowest bridge priority, then by the lowest MAC address as a tiebreaker. SW3 and SW4 share priority 8192 (lower than the 32768 default on SW1/SW2), and SW4's MAC is lower than SW3's, so SW4 becomes root.

Common mistake: Picking SW1 because its MAC looks lowest, or SW3 without comparing the MAC tiebreaker. Always apply priority first, then MAC within the same priority.

Expert Analysis

Why the Answer Is Correct

D (SW4) is correct. Root election compares bridge priority first: SW3 and SW4 both run priority 8192, lower than SW1/SW2's default 32768. With priority tied, the lower MAC wins — SW4 (0C:0A:18:A1:B3:19) is lower than SW3 (0C:0A:A8:1A:3C:9D), so SW4 is root.

Why the Other Options Are Wrong

A (SW1) and B (SW2) have the higher default priority 32768, so they cannot win. C (SW3) loses the MAC tiebreaker to SW4 despite sharing the lowest priority.

Community Comment Notes

Detailed comments (71 votes for D) confirm: SW3 and SW4 have lowest priority 8192, and SW4's MAC is lower, so SW4 is root. A few comments mistakenly pick SW3 by stopping after priority.

Official Reference

https://learningnetwork.cisco.com/s/ccna-exam-topics

This link opens the official vendor documentation in a new tab.

Community Votes

D
71%
C
29%

71% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 7 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 7

  1. adrianspa 👍 6
    wronh answer. It should be Sw4, it has the lowest MAC
  2. Shaifalimittal02 👍 2 Answered D
    In STP, the switch with the lowest priority + MAC address becomes the root bridge. SW3 and SW4 have the lowest priority (8192). Between them, SW4 has the lower MAC address. The correct answer is D. SW4.
  3. SnowWhiteYeti 👍 2 Answered D
    Answer is SW4, lowest Priority, then lowest MAC.
  4. 26acfae 👍 2 Answered C
    correct answer is sw4, because the lowest MAC
  5. exiledwl 👍 1 Answered D
    SW4 has the lowest priority tied with SW3, but Sw4 has lower mac address than sw3 so we go with D
Show 2 more comments
  1. f2bdbb0 👍 3
    correct answer is sw 4
  2. 3040636 👍 2
    SW4 is the right answer

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