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Difficulty: EasyRapid PVST+ Spanning Tree Protocol Operations and Roles

Place the tie-breaker criteria used by a non-root switch running Rapid PVST+ to select its Root Port in the correct order, from first evaluated (highest priority) to last evaluated (lowest priority).

  1. 1Lowest cumulative root path cost
  2. 2Lowest sender Bridge ID
  3. 3Lowest sender port priority
  4. 4Lowest sender port number

Answer

The correct evaluation sequence for electing a Root Port is: 1) Lowest cumulative root path cost, 2) Lowest sender Bridge ID, 3) Lowest sender port priority, and 4) Lowest sender port number.
When a non-root switch receives BPDUs across multiple active interfaces, it runs the Rapid PVST+ Root Port election process in a strict sequential order: first, it selects the port with the lowest cumulative root path cost. If path costs are equal, it compares the neighbor Bridge IDs and picks the lowest. If multiple connections lead to the same neighboring switch, it compares the sender's port priority, and finally breaks any remaining tie using the sender's lowest port number.

Step-by-Step Solution

1
Identify the primary metric for Root Port selection in Rapid PVST+.
The switch evaluates the total path cost (cumulative root path cost) to reach the Root Bridge.
STP prioritizes forwarding traffic across the most direct/highest bandwidth path available.
2
Determine the tie-breaker when path costs from multiple interfaces are equal.
The switch evaluates the Bridge ID of the neighboring (sender) switches.
If equal-cost paths exist through different upstream switches, preferring the neighbor with the lower BID ensures determinism.
3
Determine the tie-breaker when parallel links connect to the exact same upstream neighbor switch.
The switch compares the Port Priority value advertised by the sending switch interface.
Port priority is configured on the upstream switch to influence path selection on downstream switches.
4
Identify the final tie-breaker when all upstream port priorities are identical.
The switch selects the link connected to the lowest physical port number on the upstream sender switch.
This guarantees a deterministic, unambiguous decision even when default configurations are unchanged.

Key Concept

Rapid PVST+ Root Port Election Tie-Breaker Decision Tree
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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