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Difficulty: EasyRapid PVST+ Spanning Tree Protocol Operations and Roles

A Cisco Catalyst switch is operating with default Rapid PVST+ settings. What total bridge priority value will the switch advertise in its BPDUs for VLAN 100?

  1. A
    32768
  2. 32868Answer
  3. C
    36864
  4. D
    36964

Answer

32868
In Rapid PVST+, the 16-bit Priority field of the Bridge ID is divided into a 4-bit Bridge Priority and a 12-bit System ID Extension. The System ID Extension automatically equals the VLAN ID. With the default base bridge priority set to 32768, the advertised total priority for VLAN 100 is calculated as 32768+100=3286832768 + 100 = 32868.

Step-by-Step Solution

1
Identify the default base bridge priority
Default base priority is 32768
Cisco switches running PVST+/Rapid PVST+ use a default base priority of 32768.
2
Determine the System ID Extension value
System ID Extension is 100
In PVST+ and Rapid PVST+, the System ID Extension carries the VLAN ID for which the BPDU is generated.
3
Calculate the total bridge priority
32768 + 100 = 32868
Total Bridge Priority = Base Priority + System ID Extension (VLAN ID).

Key Concept

Rapid PVST+ System ID Extension and Total Bridge Priority Calculation
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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