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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

A network engineer attempts to configure a Cisco switch running Rapid PVST+ to serve as a primary root bridge by issuing the command `spanning-tree vlan 100 priority 8000` in global configuration mode. Which outcome will occur on the switch upon executing this command?

  1. The switch rejects the command because bridge priority values must be specified in increments of 4096.Answer
  2. B
    The switch accepts the command and adds the VLAN ID (100) to 8000, resulting in a total priority of 8100.
  3. C
    The switch accepts the command and automatically rounds the value down to 4096, the nearest valid increment.
  4. D
    The switch accepts the command but places all interconnect trunk ports into the discarding state to prevent loops.

Answer

The switch rejects the command because bridge priority values must be specified in increments of 4096.
In Rapid PVST+, the Bridge ID incorporates a 12-bit System ID Extension containing the VLAN ID. Consequently, only the 4 most significant bits of the 16-bit priority field are configurable by the administrator, mandating that bridge priority values must be configured in multiples of 4096 (such as 0, 4096, 8192, 12288, up to 61440). Entering a value like 8000 violates this requirement, causing the switch CLI to reject the command immediately with an error message.

Step-by-Step Solution

1
Analyze the Bridge ID structure under Rapid PVST+.
The 16-bit Bridge Priority field consists of a 4-bit Priority field and a 12-bit System ID Extension (VLAN ID).
Because only the 4 most significant bits are allocated for configurable priority, valid base values must be multiples of 212=40962^{12} = 4096.
2
Evaluate the entered CLI value of 8000.
8000 is not a multiple of 4096 (the nearest valid multiples are 4096 and 8192).
Cisco IOS enforces strict syntax validation that rejects invalid priority increments.
3
Determine the switch response.
The CLI displays an error message specifying valid increments (0, 4096, 8192, ..., 61440) and rejects the command.
Invalid inputs cannot be processed or saved into the running configuration.

Key Concept

Rapid PVST+ System ID Extension and Bridge Priority 4096 Increment Rule
Estimated Time:1m 0s
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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