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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

A Cisco Catalyst switch running Rapid PVST+ receives BPDUs on multiple interfaces from different neighboring switches connected to the same Root Switch. Place the decision criteria in the exact sequence the switch uses to select its Root Port, starting from the first criterion evaluated to the final tie-breaker.

  1. 1Lowest cumulative Root Path Cost received in the BPDUs
  2. 2Lowest Sender (Designated) Bridge ID
  3. 3Lowest Sender (Designated) Port ID
  4. 4Lowest local (Receiver) Port ID

Answer

The correct evaluation sequence for Root Port selection in Rapid PVST+ is: 1. Lowest cumulative Root Path Cost received in the BPDUs, 2. Lowest Sender (Designated) Bridge ID, 3. Lowest Sender (Designated) Port ID, 4. Lowest local (Receiver) Port ID.
Rapid PVST+ follows a strict 4-step decision sequence to select a single Root Port on non-root switches: 1) Lowest cumulative Root Path Cost, 2) Lowest Sender Bridge ID, 3) Lowest Sender Port ID, and 4) Lowest local (Receiver) Port ID.

Step-by-Step Solution

1
Evaluate incoming Root Path Cost
The switch identifies the port(s) offering the shortest cumulative cost to the Root Bridge.
Path cost reflects link speed and topology length, making it the primary factor for optimal routing toward the root.
2
Break ties using upstream switch identity
If path costs are equal, the port receiving BPDUs from the upstream switch with the lowest Bridge ID (Priority + MAC) is preferred.
Bridge ID serves as the first tie-breaker when multiple paths offer equal cost through different neighboring switches.
3
Break ties using upstream port identity
If candidate ports connect to the exact same upstream switch, the port connected to the lower upstream Port ID (Priority + Port Number) is selected.
Designated Port ID identifies which specific interface on the neighboring switch is sending the BPDU.
4
Break ties using local port identity
If all preceding criteria are identical (e.g., connected via a hub), the switch selects its own local interface with the lowest Port ID.
Local Port ID guarantees a deterministic decision when upstream information offers no distinction.

Key Concept

Rapid PVST+ Root Port Selection Tie-Breaker Logic
Estimated Time:1m 30s
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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