CompTIA Network+ (N10-009)Network ImplementationEasy
A technician configures a trunk link between two switches. Switch A has VLAN 1 set as the native VLAN, while Switch B has VLAN 99 set as the native VLAN on the same trunk. Which of the following is the MOST likely result of this misconfiguration?
- AUntagged frames will be misclassified, potentially leaking traffic between VLANs and triggering CDP/native VLAN mismatch warnings
- BThe trunk will fail to negotiate and go into an err-disabled state immediately
- CAll tagged VLAN traffic will be dropped by both switches
- DThe switches will automatically renegotiate a common native VLAN using DTP
Show answer & explanationAnswer & explanation
Correct answer: A. Untagged frames will be misclassified, potentially leaking traffic between VLANs and triggering CDP/native VLAN mismatch warnings
When native VLANs differ across a trunk, untagged frames sent by one switch are interpreted as belonging to the other switch's native VLAN, which can leak traffic between VLANs and generate mismatch warnings via protocols like CDP. This does not automatically err-disable the port or drop all tagged traffic.
Why the other options are wrong
- B. Native VLAN mismatch alone does not automatically disable the port on most platforms.
- C. Tagged VLAN traffic is unaffected; only the untagged (native) traffic is misinterpreted.
- D. DTP negotiates trunk mode, not native VLAN assignment.
Native VLAN Mismatch
Occurs when two ends of an 802.1Q trunk are configured with different native (untagged) VLANs, causing untagged frames to be placed in the wrong VLAN.
- Native VLAN carries untagged traffic on a trunk
- Mismatches can leak traffic between VLANs
- Best practice: set native VLAN to an unused VLAN, and match on both ends
Memory trick: No tag, no home — untagged frames go wherever the native VLAN says.