CCNP Infrastructure Practice Question
A network engineer is configuring a new Cisco Catalyst 9300 switch stack. The company requires that the native VLAN for all 802.1Q trunk ports be changed from the default to VLAN 99. The engineer enters the global configuration command vlan dot1q tag native and then configures the trunk ports with switchport trunk native vlan 99. After applying the configuration, the engineer notices that untagged frames received on the trunk are being dropped. What is the most likely cause?
⚠ Common exam trap
The trap here is assuming that vlan dot1q tag native only affects outbound tagging and not inbound frame processing.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The vlan dot1q tag native command causes all native VLAN frames to be tagged, so untagged frames are dropped.
Enabling vlan dot1q tag native globally causes the switch to tag all frames on the native VLAN for 802.1Q trunks. As a result, any untagged frames received on those trunks are dropped because they are not expected. This behavior is by design and explains the observed frame drops.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The vlan dot1q tag native command must be configured on all switches in the topology, not just the local switch.
Why it's wrong here
The vlan dot1q tag native command is a global configuration that affects only the local switch. It does not need to be configured on all switches. However, if configured on one side only, it can cause interoperability issues, but the immediate dropping of untagged frames is due to the local switch tagging native VLAN frames.
- ✗
The native VLAN must be the same on both ends of the trunk; otherwise, untagged frames are dropped.
Why it's wrong here
While a native VLAN mismatch can cause problems, it typically results in VLAN hopping or connectivity issues, not immediate dropping of all untagged frames. The scenario does not indicate a mismatch on the other end. The vlan dot1q tag native command is the direct cause of dropping untagged frames.
- ✗
The switchport trunk native vlan 99 command is only valid if the native VLAN is VLAN 1.
Why it's wrong here
The switchport trunk native vlan command can be used to set any valid VLAN as the native VLAN, not just VLAN 1. The command is valid for VLAN 99. The issue is not with the command's validity but with the global tagging behavior.
- ✓
The vlan dot1q tag native command causes all native VLAN frames to be tagged, so untagged frames are dropped.
Why this is correct
With vlan dot1q tag native enabled, the switch tags all frames on the native VLAN. Any untagged frames arriving on the trunk are considered invalid and dropped. This is the expected behavior when this global command is used, and it explains why untagged frames are being dropped.
Visual reference
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.