CompTIA Network+ (N10-009)Network TroubleshootingHard
A network administrator configures a trunk link between two switches. On Switch A, the native VLAN is set to VLAN 1, while on Switch B, it is mistakenly set to VLAN 99. Devices on VLAN 1 begin experiencing intermittent connectivity issues, and the switches log CDP native VLAN mismatch warnings. What is the most likely root cause of the reported issue?
- AThe trunk port's speed and duplex settings do not match on both switches
- BThe trunk link is using an unsupported encapsulation protocol
- CSpanning Tree Protocol has placed the trunk port into a blocking state
- DA native VLAN mismatch is causing untagged traffic to be misclassified between VLANs
Show answer & explanationAnswer & explanation
Correct answer: D. A native VLAN mismatch is causing untagged traffic to be misclassified between VLANs
On an 802.1Q trunk, untagged frames are associated with each switch's configured native VLAN. If the two ends disagree on the native VLAN, untagged traffic intended for VLAN 1 on one switch can be delivered into VLAN 99 on the other, causing leakage between VLANs and the exact CDP warning described.
Why the other options are wrong
- A. Speed/duplex mismatches cause performance degradation, not VLAN-specific traffic leakage.
- B. Encapsulation mismatches (e.g., ISL vs 802.1Q) would prevent the trunk from forming at all, not cause selective VLAN leakage.
- C. STP blocking would stop all traffic on the port, not cause selective VLAN issues.
Native VLAN Mismatch
Occurs when two ends of an 802.1Q trunk are configured with different native VLANs, causing untagged traffic to be misdirected between VLANs.
- Untagged frames on a trunk are assigned to the native VLAN.
- Mismatched native VLANs cause security and connectivity issues.
- Cisco switches generate CDP native VLAN mismatch warnings automatically.
Memory trick: 'Untagged frames go home to whichever VLAN is native — make sure both ends agree.'