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Difficulty: MediumTroubleshooting VLAN, Trunking, and Switching Issues

A network technician receives a report that hosts on VLAN 10 attached to Switch-1 are unexpectedly receiving untagged traffic from VLAN 20 attached to Switch-2 across a point-to-point trunk link. Console logs on Switch-1 frequently display `%CDP-4-NATIVE_VLAN_MISMATCH` notifications for interface GigabitEthernet 0/1. Which of the following is the root cause of this behavior?

  1. A mismatch in the configured 802.1Q native VLAN between the two interconnecting switch interfaces.Answer
  2. B
    A duplex mismatch on the trunk link causing frames to leak across collision domains.
  3. C
    The lack of an active Layer 3 router to maintain broadcast domain boundaries across the switch ports.
  4. D
    An incorrect Spanning Tree Protocol priority configuration causing the trunk link to operate in a backup blocking state.

Answer

A mismatch in the configured 802.1Q native VLAN between the two interconnecting switch interfaces.
On an 802.1Q trunk, traffic belonging to the designated native VLAN is transmitted without an 802.1Q header tag. If Switch-1 has its native VLAN set to 10 while Switch-2 has its native VLAN set to 20, untagged frames sent from VLAN 20 on Switch-2 will be received by Switch-1 and placed into VLAN 10. This native VLAN mismatch causes cross-VLAN leakage and triggers CDP native VLAN mismatch syslog errors.

Step-by-Step Solution

1
Analyze the log error message and reported symptoms.
The log explicitly shows `%CDP-4-NATIVE_VLAN_MISMATCH` on trunk interface GigabitEthernet 0/1.
Cisco Discovery Protocol (CDP) detects when the local native VLAN ID on a trunk does not match the native VLAN ID configured on the remote switch port.
2
Evaluate how 802.1Q native VLANs process frames across trunk links.
Frames originating on a port's native VLAN are transmitted across an 802.1Q trunk without a VLAN tag (untagged).
When the receiving switch receives an untagged frame, it automatically assigns it to its own configured native VLAN for that trunk port.
3
Determine the impact of differing native VLAN configurations (e.g., VLAN 10 on one end and VLAN 20 on the other).
Untagged frames sent from VLAN 20 on Switch-2 arrive untagged at Switch-1 and are delivered directly into VLAN 10.
This leads to security risks, cross-VLAN traffic leakage, and CDP/STP warning messages.

Key Concept

802.1Q Native VLAN Operation and Mismatch Troubleshooting
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Topics

Troubleshooting VLAN, Trunking, and Switching Issues

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