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Difficulty: HardSwitching Concepts and VLANs

In an enterprise network running Per-VLAN Spanning Tree Plus (PVST+), two switches (Switch A and Switch B) are interconnected. Switch A has a base MAC address of 00:11:22:33:44:55 and is configured with an STP base priority of 28672 for VLAN 50. Switch B has a base MAC address of 00:11:22:33:44:AA and currently operates with the default STP base priority of 32768 for VLAN 50. A network administrator wants Switch B to become the Root Bridge for VLAN 50. Which priority value must be configured on Switch B for VLAN 50 to guarantee it becomes the Root Bridge?

  1. 24576Answer
  2. B
    28672
  3. C
    32768
  4. D
    36864

Answer

Setting the base priority of Switch B to 24576 ensures it achieves a lower Bridge ID than Switch A and becomes the Root Bridge for VLAN 50.
In Spanning Tree Protocol, the switch with the lowest Bridge ID (BID) is elected as the Root Bridge. BID consists of Bridge Priority + System ID Extension (VLAN ID) + Base MAC Address. Configured in steps of 4096, setting Switch B's priority to 24576 results in a total priority of 24626, lower than Switch A's total priority of 28722, securing the Root Bridge election for Switch B.

Step-by-Step Solution

1
Calculate Switch A's total Bridge ID priority for VLAN 50
28672 (base priority) + 50 (System ID Extension for VLAN 50) = 28722
PVST+ combines the 4-bit priority field with the 12-bit VLAN ID extension to form the total priority portion of the Bridge ID.
2
Evaluate the outcome if Switch B matches Switch A's base priority of 28672
Total priority tie (28722 vs 28722). Switch A wins based on lower MAC address (00:11:22:33:44:55 < 00:11:22:33:44:AA).
When total priorities are equal, Spanning Tree Protocol uses the switch base MAC address as the ultimate tiebreaker, where lower numerical value wins.
3
Determine the required priority increment to guarantee Switch B wins
Select the next lower valid STP priority increment of 4096 below 28672, which is 24576.
Bridge priorities must be configured in multiples of 4096. A base priority of 24576 gives Switch B a total priority of 24626, which is lower than Switch A's 28722.

Key Concept

STP Root Bridge Selection and Priority Tie-Breaking Logic
Estimated Time:2m 0s
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Topics

Switching Concepts and VLANs

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