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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

Two Cisco Catalyst switches running Rapid PVST+ are configured with an identical base bridge priority of 40964096 for VLAN 3030. Which factor ultimately determines which switch is elected as the Root Bridge for VLAN 3030?

  1. The switch with the lowest MAC address, because both switches have an identical total Bridge Priority of 41264126.Answer
  2. B
    The switch with the highest MAC address, because Rapid PVST+ prefers higher physical hardware addresses during tie-breakers.
  3. C
    The switch with the lowest port ID connected to the neighbor, because port IDs break priority ties before MAC addresses are evaluated.
  4. D
    The switch configured first, because Rapid PVST+ retains the active Root Bridge role until an administrative preempt command is issued.

Answer

The switch with the lowest MAC address will be elected as the Root Bridge because both switches generate an identical total Bridge Priority of 41264126.
The correct answer identifies that both switches have a total Bridge Priority of 41264126 (40964096 base priority + 3030 VLAN ID extension). Because their priorities are equal, the tie-breaker rule dictates that the switch with the lowest MAC address is selected as the Root Bridge.

Step-by-Step Solution

1
Calculate the total Bridge Priority for each switch
Total Priority = Base Priority (40964096) + System ID Extension (3030) = 41264126.
Rapid PVST+ automatically adds the VLAN ID to the base priority step.
2
Compare the total Bridge IDs of both switches
Both switches share the exact same Bridge Priority (41264126).
When total priorities match, the election moves to the final component of the Bridge ID.
3
Apply the Root Bridge tie-breaker rule
The switch with the numerically lowest MAC address wins the Root Bridge election.
In Spanning Tree election criteria, lower numeric values are always preferred.

Key Concept

Rapid PVST+ Root Bridge Election and System ID Extension Calculation
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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