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Difficulty: EasySpanning Tree Protocol Operations

A network administrator is configuring four interconnected switches running standard IEEE 802.1D Spanning Tree Protocol (STP). All switches currently maintain the default bridge priority value of 32768. Which switch configuration modification will guarantee that a specific switch becomes elected as the STP Root Bridge?

  1. Lower the bridge priority numerical value on the desired switch.Answer
  2. B
    Increase the bridge priority numerical value on the desired switch.
  3. C
    Configure the native VLAN ID to match on all connected trunk ports.
  4. D
    Assign the lowest numerical IP address to the management interface of the desired switch.

Answer

Lower the bridge priority numerical value on the desired switch.
In Spanning Tree Protocol (STP), the switch with the lowest numerical Bridge ID (BID) is elected as the Root Bridge. The BID is composed of the Bridge Priority and the switch MAC address. Because all switches default to a priority of 32768, lowering the priority value on a specific switch reduces its BID, ensuring it wins the root election.

Step-by-Step Solution

1
Identify the criteria used by 802.1D STP for root bridge election.
STP elects the switch with the lowest Bridge ID (BID) as the Root Bridge.
The BID is composed of a 2-byte Bridge Priority followed by a 6-byte MAC Address.
2
Determine how to force a specific switch to have the lowest BID.
Manually lower the Bridge Priority value (e.g., set to 4096 or 0).
Since lower numerical values win the election, reducing the priority below the default 32768 ensures that this switch wins regardless of MAC addresses.

Key Concept

STP Root Bridge Election and Priority Calibration
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Topics

Spanning Tree Protocol Operations

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