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Zorluk: ZorRapid PVST+ Spanning Tree Protocol Operations and Roles

A network engineer is configuring Rapid PVST+ on a Cisco Catalyst switch to force it to become the Root Bridge for VLAN 40. The engineer attempts to configure a custom bridge priority value using the Cisco IOS CLI command line syntax. Which command entry contains a valid bridge priority value that Cisco IOS will accept, and what total bridge priority value will the switch advertise in its outbound Spanning Tree Protocol BPDUs for VLAN 40?

  1. The command 'spanning-tree vlan 40 priority 4096' will be accepted, and the advertised total bridge priority will be 4136.Cevap
  2. B
    The command 'spanning-tree vlan 40 priority 4000' will be accepted, and the advertised total bridge priority will be 4040.
  3. C
    The command 'spanning-tree vlan 40 priority 4096' will be accepted, and the advertised total bridge priority will be 4096.
  4. D
    The command 'spanning-tree vlan 40 priority 4136' will be accepted, and the advertised total bridge priority will be 4136.

Cevap

The command 'spanning-tree vlan 40 priority 4096' will be accepted, and the advertised total bridge priority will be 4136.
In Cisco Rapid PVST+, the 16-bit priority portion of the 8-byte Bridge ID (BID) is divided into a 4-bit Bridge Priority and a 12-bit System ID Extension. Because only 4 bits are allocated to user configuration (24=162^4 = 16 levels), priority values in CLI commands must increase in steps of 4096 (4096×1=40964096 \times 1 = 4096). When the command 'spanning-tree vlan 40 priority 4096' is executed, Cisco IOS validates 40964096 as a valid base priority. During BPDU generation for VLAN 40, the switch adds the VLAN ID (4040) as the System ID Extension, resulting in a total priority value of 4096+40=41364096 + 40 = 4136.

Adım Adım Çözüm

1
Verify CLI input validation rules for Rapid PVST+ bridge priority
Cisco IOS requires configurable bridge priority values to be explicit multiples of 4096 (0,4096,8192,12288,…,614400, 4096, 8192, 12288, \dots, 61440).
The 16-bit Priority field in the Bridge ID (BID) uses the 4 most significant bits for configurable priority and the remaining 12 bits for the System ID Extension (VLAN ID).
2
Calculate the total advertised Bridge Priority field value
Base Priority (40964096) + System ID Extension (4040) = Total Priority (41364136).
Rapid PVST+ dynamically inserts the 12-bit VLAN ID into the System ID Extension field for per-VLAN spanning tree identification.

Anahtar Kavram

Rapid PVST+ Bridge ID Structure and System ID Extension Calculation
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Konular

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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