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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

A network administrator issues the command `spanning-tree vlan 20 priority 16384` on a Cisco Catalyst switch running Rapid PVST+. What priority value will be carried in the Bridge Identifier (BID) field of outgoing BPDUs for VLAN 20?

  1. 16404Answer
  2. B
    16384
  3. C
    16364
  4. D
    32788

Answer

The effective priority value carried in the Bridge Identifier for VLAN 20 is 16404.
Rapid PVST+ uses the 802.1t System ID Extension mechanism, which appends the 12-bit VLAN ID directly to the 4-bit base bridge priority. Adding the configured base priority of 16384 to the VLAN ID of 20 yields a total priority value of 16404 in transmitted BPDUs.

Step-by-Step Solution

1
Identify the base Bridge Priority configured on the switch.
The base priority configured via CLI is 1638416384.
The command `spanning-tree vlan 20 priority 16384` sets the base priority component of the Bridge ID.
2
Determine the System ID Extension value for the VLAN.
The System ID Extension value equals the VLAN number, which is 2020.
Rapid PVST+ uses 802.1t System ID Extension to carry unique STP instances per VLAN without consuming extra MAC addresses.
3
Calculate the total Bridge Priority by adding the base priority to the System ID Extension.
16384+20=1640416384 + 20 = 16404.
The 16-bit priority structure combines the 4-bit priority multiplier with the 12-bit VLAN identifier.

Key Concept

System ID Extension and Bridge Priority Calculation in Rapid PVST+
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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