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Difficulty: EasyRapid PVST+ Spanning Tree Protocol Operations and Roles

In Cisco Rapid PVST+, a switch constructs an 8-byte (64-bit) Bridge ID (BID) for each VLAN instance to participate in Root Bridge election. Arrange the three components of the Rapid PVST+ Bridge ID in order from most significant bits (leftmost) to least significant bits (rightmost).

  1. 14-bit Bridge Priority
  2. 212-bit System ID Extension (VLAN ID)
  3. 348-bit MAC Address

Answer

The correct sequence from most significant bits to least significant bits is: 4-bit Bridge Priority, 12-bit System ID Extension (VLAN ID), and 48-bit MAC Address.
The 8-byte (64-bit) Rapid PVST+ Bridge ID is ordered from most significant to least significant bits as follows: 4-bit Bridge Priority (bits 63–60), 12-bit System ID Extension representing the VLAN ID (bits 59–48), and the 48-bit switch MAC Address (bits 47–0).

Step-by-Step Solution

1
Examine the 2-byte (16-bit) Priority field structure in the Rapid PVST+ Bridge ID.
The upper 4 bits represent the configurable Bridge Priority, making them the most significant bits of the entire 64-bit Bridge ID.
This layout ensures that user-defined priority configurations take precedence over VLAN IDs and MAC addresses.
2
Identify the remaining sub-field of the 2-byte Priority field.
The lower 12 bits of the Priority field carry the System ID Extension (VLAN ID).
Carrying the VLAN ID within these 12 bits allows Rapid PVST+ to maintain unique Bridge IDs per VLAN without requiring separate MAC addresses for each instance.
3
Identify the final 6 bytes (48 bits) of the Bridge ID.
The lowest 48 bits consist of the switch's burned-in MAC address.
The MAC address serves as the final tie-breaker during Root Bridge election when Bridge Priority and VLAN ID values are equal.

Key Concept

Rapid PVST+ Bridge ID (BID) Structure and System ID Extension
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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