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Zorluk: OrtaRapid PVST+ Spanning Tree Protocol Operations and Roles

A network administrator is configuring a Cisco Catalyst switch running Rapid PVST+ to serve as a secondary root bridge for VLAN 20. The administrator intends to set a custom bridge priority using the command `spanning-tree vlan 20 priority <value>`. Which value will Cisco IOS accept for this command?

  1. 24576Cevap
  2. B
    24596
  3. C
    20000
  4. D
    8212

Cevap

The correct option is 24576 because base bridge priority in Rapid PVST+ must be configured as a strict multiple of 4096.
In Rapid PVST+, Cisco IOS enforces that bridge priority settings configured via CLI must be exact multiples of 4096 (range 0 to 61440). The value 24576 equals 6 × 4096, making it a valid base priority parameter.

Adım Adım Çözüm

1
Understand Rapid PVST+ Priority Architecture
Rapid PVST+ uses the 802.1t System ID Extension, which splits the 16-bit Bridge Priority field into a 4-bit Priority field and a 12-bit System ID Extension (VLAN ID).
Because only the top 4 bits are allocated for configurable priority, priority values can only change in steps of 212=40962^{12} = 4096.
2
Evaluate CLI configuration requirements
When executing `spanning-tree vlan <vlan-id> priority <value>`, the `<value>` parameter must be a multiple of 4096 (e.g., 0, 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, etc.).
Cisco IOS automatically adds the VLAN ID to the base priority when constructing the BPDU, so the user must specify the unshifted base priority.
3
Test candidate values
Calculating 24576/4096=624576 / 4096 = 6, which is an exact integer. Other choices (24596, 20000, 8212) leave non-zero remainders when divided by 4096.
Only 24576 satisfies the increment requirement and will be accepted by Cisco IOS.

Anahtar Kavram

Rapid PVST+ Bridge Priority Configuration and 4096 Increment Constraint
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Konular

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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