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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

An engineer adds a new access switch to an established enterprise network. Shortly after connecting the switch to the core layer, traffic across multiple VLANs experiences severe latency and suboptimal path routing. Investigation reveals that the new access switch has unexpectedly taken over as the Spanning Tree Protocol (STP) root bridge for all VLANs. Which configuration error on the new access switch is the primary cause of this network issue?

  1. The switch was configured with a numerical STP bridge priority value lower than that of the core switch.Answer
  2. B
    The switch was configured with a numerical STP bridge priority value higher than that of the core switch.
  3. C
    The switch trunk interface was configured with a native VLAN mismatch relative to the core switch.
  4. D
    The switch port connecting to the host devices was set up as a separate VLAN broadcast domain without a Layer 3 router.

Answer

The switch was configured with a numerical STP bridge priority value lower than that of the core switch.
In Spanning Tree Protocol (STP), the root bridge is elected based on the lowest Bridge ID (BID). The BID is composed of a bridge priority and the switch's MAC address. By default, switches have a priority of 32768. If a newly introduced access switch is configured with a lower numerical priority value (such as 4096 or 0), it wins the root bridge election, causing network traffic to shift paths toward the access layer.

Step-by-Step Solution

1
Analyze STP root bridge selection criteria
STP selects the root bridge using the Bridge ID (BID), which combines Bridge Priority (2 bytes) and MAC Address (6 bytes).
The switch with the lowest numerical Bridge ID in the broadcast domain is elected as the root bridge.
2
Evaluate the impact of setting a lower numerical priority value
Setting a lower numerical priority value (e.g., 4096 instead of the default 32768) makes that switch the preferred candidate for root bridge.
Lower numerical values take precedence in STP decision vectors.
3
Correlate with the scenario symptoms
When an access switch with lower priority becomes the root bridge, all Layer 2 traffic paths recalculate toward it, bottlenecking traffic over slower access links.
This explains why network performance degraded across all VLANs after the new switch was added.

Key Concept

Spanning Tree Protocol (STP) Root Bridge Election and Priority Mechanics
Estimated Time:1m 15s
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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