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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator attempts to configure Core-Switch-01 to serve as the Spanning Tree Protocol (STP) root bridge for VLAN 50. The administrator executes the command to set the bridge priority for VLAN 50 to 61440. Shortly after, a newly provisioned access switch with factory default settings is connected to the network and unexpectedly assumes the root bridge role for VLAN 50. Which of the following best explains why the access switch became the root bridge instead of Core-Switch-01?

  1. The bridge priority assigned to Core-Switch-01 is numerically higher than the default priority of 32768 on the access switch, and STP prefers the lowest numerical priority value.Answer
  2. B
    A native VLAN mismatch on the trunk link caused BPDU frames to be dropped, forcing an automatic reelection that defaulted to the access switch.
  3. C
    A duplex mismatch on the inter-switch link corrupted BPDU transmissions, preventing Core-Switch-01 from advertising its bridge priority.
  4. D
    Switches operating on VLAN 50 require an active Layer 3 routing protocol interface to exchange root bridge priority election messages across broadcast domains.

Answer

The bridge priority assigned to Core-Switch-01 is numerically higher than the default priority of 32768 on the access switch, and STP prefers the lowest numerical priority value.
In Spanning Tree Protocol (802.1D / 802.1w), the root bridge election is determined by the lowest Bridge ID (BID). The default bridge priority for switches is 32768 (plus the VLAN ID extension). When the administrator set the core switch priority to 61440, the numerical value was increased instead of decreased. As a result, the newly attached switch with its default priority of 32768 had a lower numerical BID and won the root bridge election.

Step-by-Step Solution

1
Analyze Spanning Tree Protocol (STP) root bridge selection criteria.
STP elects the switch with the lowest Bridge Identifier (BID) as the root bridge. The BID consists of Bridge Priority (2 bytes) + System ID Extension/VLAN ID + MAC Address (6 bytes).
Lower numerical priority values take precedence over higher numerical values.
2
Compare the configured priority value against the factory default priority.
Core-Switch-01 priority = 61440. Access switch default priority = 32768.
Because 32768 < 61440, the access switch has a superior (lower) priority value.
3
Determine why the access switch assumed the root bridge role.
Configuring a priority of 61440 made Core-Switch-01 less preferred than any switch running default settings (32768). To guarantee root bridge status, the priority should have been set to a lower value such as 4096 or 0.
Correcting the misconfiguration requires assigning a lower numerical priority to the designated core switch.

Key Concept

Spanning Tree Protocol (STP) Root Bridge Election and Priority Mechanics
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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