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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

A network engineer is configuring Rapid PVST+ on a Cisco Catalyst switch. The engineer needs to set the switch priority for VLAN 45 so that the total Bridge Identifier (BID) priority field transmitted in BPDUs equals 20525. Which global configuration command must the engineer execute on the switch?

  1. spanning-tree vlan 45 priority 20480Answer
  2. B
    spanning-tree vlan 45 priority 20525
  3. C
    spanning-tree vlan 45 priority 24576
  4. D
    spanning-tree vlan 45 priority 16384

Answer

spanning-tree vlan 45 priority 20480
In Rapid PVST+, the Bridge Identifier (BID) priority field consists of a 4-bit bridge priority and a 12-bit System ID Extension (VLAN ID). Because 4 bits are allocated to the configurable priority, values must be multiples of 212=40962^{12} = 4096. To achieve a total priority of 20525 for VLAN 45, the base priority must be configured as 20525−45=2048020525 - 45 = 20480.

Step-by-Step Solution

1
Identify the System ID Extension value for the target VLAN.
The System ID Extension equals the VLAN ID, which is 45.
Rapid PVST+ encodes the VLAN number in the 12-bit System ID Extension portion of the Bridge Identifier (BID).
2
Calculate the required base bridge priority value.
20525−45=2048020525 - 45 = 20480
Total Bridge Priority = Base Priority + System ID Extension.
3
Verify that the base priority is a valid increment of 4096.
20480/4096=520480 / 4096 = 5 (exact integer multiplier)
Cisco Catalyst switches require bridge priority to be configured in increments of 4096.

Key Concept

Rapid PVST+ System ID Extension and Priority Calculation
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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