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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

A network administrator is manually configuring Rapid PVST+ on a Cisco Catalyst switch to act as a secondary root bridge for VLAN 75. The administrator attempts to enter a custom priority value using the `spanning-tree vlan 75 priority <value>` command in global configuration mode. Which priority value will be accepted by Cisco IOS for this command?

  1. 28672Answer
  2. B
    28747
  3. C
    30000
  4. D
    32843

Answer

The priority value 28672 is accepted because Cisco IOS requires the priority argument in the 'spanning-tree vlan X priority' command to be specified in increments of 4096.
In Rapid PVST+, Cisco switches enforce the 802.1t System ID Extension. The 16-bit Priority field is split into a 4-bit Bridge Priority field and a 12-bit VLAN ID field. Consequently, the user-configurable base bridge priority in the CLI must be an explicit multiple of 4096 (4096×7=286724096 \times 7 = 28672). When 28672 is entered, the switch adds VLAN 75 internally to yield a total priority of 28747.

Step-by-Step Solution

1
Identify the standard requirement for Rapid PVST+ Bridge Priority values in Cisco IOS.
Rapid PVST+ utilizes 802.1t System ID Extension, dividing the 16-bit Bridge Priority field into a 4-bit Priority field and a 12-bit System ID Extension (VLAN ID).
Because the upper 4 bits control the configurable priority, configured values must be multiples of 212=40962^{12} = 4096.
2
Evaluate the CLI input requirements for the command 'spanning-tree vlan 75 priority <value>'.
The CLI argument accepts only base priority increments: 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, etc.
The switch software automatically appends the VLAN ID (75) to the base priority to form the actual 16-bit priority carried in BPDUs (28672+75=2874728672 + 75 = 28747).
3
Select the option that represents a valid 4096 multiple.
28672 is equal to 4096×74096 \times 7, making it a valid configuration parameter.
All other choices represent non-4096 multiples or values resulting from manually pre-adding the VLAN ID.

Key Concept

Rapid PVST+ System ID Extension and Bridge Priority CLI configuration constraints.
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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