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Difficulty: EasyRapid PVST+ Spanning Tree Protocol Operations and Roles

A network administrator needs to manually configure the base bridge priority for VLAN 30 on a Cisco Catalyst switch running Rapid PVST+. Which configured base priority value is valid and will be accepted by Cisco IOS?

  1. 4096Answer
  2. B
    4126
  3. C
    4000
  4. D
    3000

Answer

4096 is a valid base priority because Rapid PVST+ requires bridge priority values to be configured in increments of 4096.
In Rapid PVST+, the 16-bit Priority field is divided into a 4-bit base priority field and a 12-bit System ID Extension field (which carries the VLAN ID). Because only 4 bits are used for the base priority, configurable values must be increments of 40964096 (such as 0,4096,8192,122880, 4096, 8192, 12288, etc.). Selecting 4096 provides a valid base priority value.

Step-by-Step Solution

1
Identify the STP bridge priority configuration requirement for Rapid PVST+.
Bridge priority values must be configured in multiples of 4096 due to the 12-bit System ID Extension field in the 802.1t bridge ID structure.
Cisco IOS reserves 12 bits for the VLAN ID (System ID Extension) within the 16-bit Priority field, leaving 4 bits for the base priority.
2
Evaluate the given options to find a valid multiple of 4096.
4096 is equal to 4096×14096 \times 1, making it a valid configurable base priority value.
Values such as 3000, 4000, and 4126 are not valid base priority multiples.

Key Concept

Rapid PVST+ Bridge Priority and System ID Extension
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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