Question

Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

During initial topology convergence for a VLAN running Rapid PVST+, switches exchange BPDUs to determine which switch assumes the Root Bridge role. Place the evaluation criteria steps in the correct order, from the first criterion evaluated to the final step establishing the Root Bridge.

  1. 1Compare the configured Base Bridge Priority values of candidate switches.
  2. 2Add the System ID Extension (VLAN ID) to the base priority to form the total priority.
  3. 3Compare the MAC addresses of candidate switches if total bridge priorities are equal.
  4. 4Designate the switch with the overall lowest Bridge ID (BID) as the Root Bridge for the VLAN.

Answer

The correct sequence begins with comparing base priority values, adding the System ID Extension to calculate total priority, using the lowest MAC address as a tie-breaker if priorities match, and concluding with designating the switch with the lowest overall Bridge ID as the Root Bridge.
In Rapid PVST+, Root Bridge election follows a strict hierarchy based on the Bridge ID (BID). Switches first compare base bridge priority, add the System ID Extension (VLAN ID) to establish total priority, resolve any ties using the lowest MAC address, and finally designate the switch with the overall lowest BID as the Root Bridge.

Step-by-Step Solution

1
Evaluate Base Bridge Priority
Identifies the base priority component configured on each participating switch.
Base priority is the primary configurable metric evaluated in the Bridge ID.
2
Incorporate System ID Extension
Combines base priority and VLAN ID into the effective 16-bit priority value.
Rapid PVST+ requires unique per-VLAN bridge priorities by appending the VLAN number.
3
Evaluate MAC addresses for tie-breaking
Determines the winning switch when total priorities are identical.
The 48-bit switch MAC address serves as a unique secondary tie-breaker.
4
Confirm Root Bridge election outcome
The switch with the lowest resulting BID becomes the Root Bridge.
All switches in the spanning-tree domain recognize the switch with the lowest BID as Root.

Key Concept

Rapid PVST+ Root Bridge Election and Bridge ID Evaluation
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