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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

An administrator issues the global configuration command `spanning-tree vlan 50 priority 28672` on a switch running Rapid PVST+. What total Bridge Priority value will be populated in the Bridge Identifier (BID) field of outgoing BPDUs for VLAN 50?

  1. A
    28672
  2. B
    32818
  3. 28722Answer
  4. D
    28622

Answer

28722
Rapid PVST+ utilizes the IEEE 802.1t System ID Extension by default. The 16-bit priority portion of the Bridge Identifier (BID) is calculated by taking the configured 4-bit base priority (which must be a multiple of 4096) and adding the 12-bit VLAN ID (System ID Extension). Adding VLAN ID 50 to the configured base priority of 28672 produces a total Bridge Priority of 28722.

Step-by-Step Solution

1
Identify the base priority configured for the VLAN
The configured base priority is 28672.
The command `spanning-tree vlan 50 priority 28672` sets the base bridge priority for VLAN 50.
2
Identify the System ID Extension value
The System ID Extension value is 50.
In Rapid PVST+, the System ID Extension field automatically carries the 12-bit VLAN ID.
3
Calculate the total Bridge Priority field in the Bridge Identifier (BID)
Total Bridge Priority = 28672 + 50 = 28722.
The 16-bit priority portion of the BID is composed of the 4-bit base priority multiplier plus the 12-bit System ID Extension.

Key Concept

Rapid PVST+ Bridge Identifier (BID) Structure and System ID Extension
Estimated Time:1m 0s
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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