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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network administrator connects two enterprise switches via a fiber trunk link on interface GigabitEthernet 0/1. Shortly after enabling the link, console logs on both switches begin displaying periodic warning messages indicating a native VLAN mismatch. Interface status shows the trunk link is active, but untagged management traffic sent from VLAN 10 on Switch-A is being improperly received on VLAN 20 by Switch-B. Which of the following configuration changes will resolve this issue while restoring proper 802.1Q trunking functionality?

  1. Reconfigure the trunk link so that both switch interfaces are configured with matching native VLAN IDs.Answer
  2. B
    Lower the Spanning Tree Protocol (STP) bridge priority on Switch-A to force it to become the root bridge for VLAN 10 and VLAN 20.
  3. C
    Deploy a Layer 3 router subinterface to route untagged frames between VLAN 10 and VLAN 20 across the trunk.
  4. D
    Set both switch interfaces on the trunk link to manual full-duplex mode instead of auto-negotiation.

Answer

Reconfigure the trunk link so that both switch interfaces are configured with matching native VLAN IDs.
On an 802.1Q trunk link, frames belonging to the native VLAN are sent without an 802.1Q VLAN tag. When the switches on opposite ends of the trunk have mismatched native VLAN configurations, untagged frames egressing one switch's native VLAN are ingested into the receiving switch's native VLAN, causing cross-VLAN traffic leakage and generating CDP/syslog mismatch warnings. Reconfiguring both switch interfaces to use the exact same native VLAN ID resolves the mismatch and restores proper frame processing.

Step-by-Step Solution

1
Analyze the log messages and symptoms reported on the switch interfaces.
Identified an 802.1Q native VLAN mismatch where Switch-A considers VLAN 10 native while Switch-B considers VLAN 20 native.
802.1Q trunking sends frames on the native VLAN without adding an explicit 802.1Q VLAN tag.
2
Determine the impact of untagged frame processing across a mismatched native VLAN link.
Untagged frames sent from Switch-A's native VLAN (VLAN 10) are received untagged by Switch-B and incorrectly placed into Switch-B's native VLAN (VLAN 20), causing cross-VLAN traffic leaking and log warnings.
Receiving switchports assume any untagged frame belongs to their locally configured native VLAN.
3
Identify the required remediation step.
Configure matching native VLAN IDs (e.g., switchport trunk native vlan 10 on both switches) to align Layer 2 VLAN tagging expectations.
Matching native VLAN configurations ensure untagged frames map to the same broadcast domain on both ends of the trunk link.

Key Concept

802.1Q Native VLAN Trunk Misconfiguration Troubleshooting
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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