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Difficulty: EasySpanning Tree Protocol Operations

A network administrator is deploying a network of switches using the IEEE 802.1D Spanning Tree Protocol (STP). Currently, all switches share the default bridge priority value of 3276832768. Which configuration change on a specific switch will force it to be elected as the STP root bridge?

  1. Decreasing the bridge priority value of the switch to a number lower than 32768Answer
  2. B
    Increasing the bridge priority value of the switch to the maximum value of 65535
  3. C
    Assigning the highest numerical MAC address to the primary management interface of the switch
  4. D
    Changing the native VLAN on the switch trunk interfaces to an unused VLAN ID

Answer

Decreasing the bridge priority value of the switch to a number lower than 32768 will ensure it has the lowest Bridge ID and becomes the STP root bridge.
In Spanning Tree Protocol operations, the root bridge election is determined by the lowest Bridge ID (BID). The BID is calculated using the bridge priority plus the switch MAC address. Because all switches initially share the default priority of 3276832768, explicitly lowering the priority value of a switch (for example, to 2457624576 or 40964096) ensures it possesses the lowest BID in the network and becomes the root bridge.

Step-by-Step Solution

1
Analyze how STP selects the root bridge.
STP compares Bridge IDs across all switches in the Layer 2 domain.
The switch with the lowest Bridge ID is elected as the root bridge.
2
Examine the composition of the Bridge ID.
The Bridge ID consists of a 2-byte Bridge Priority combined with a 6-byte System MAC Address.
Bridge Priority is evaluated first before MAC address tiebreakers.
3
Determine the necessary configuration change.
Lowering the bridge priority below the default value of 32768 creates a smaller Bridge ID value than all competing switches.
A smaller numerical priority guarantees winning the root bridge election.

Key Concept

STP Root Bridge Election Mechanics
Estimated Time:45s
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Topics

Spanning Tree Protocol Operations

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