CCNA Switching and Network Access Practice Question
A network technician notices CDP native VLAN mismatch warnings between switches SW1 and SW2 on their trunk link. The technician runs 'show interfaces trunk' on SW1 and sees native VLAN 1, then on SW2 and sees native VLAN 99. Data traffic is currently passing, but the mismatch can cause broadcast loops. What should the technician do next?
⚠ Common exam trap
Cisco often tests the misconception that data traffic passing means the configuration is fine, but the trap here is that the native VLAN mismatch can still cause serious issues like broadcast loops and security vulnerabilities, even if user data appears to work.
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
✓
Configure the native VLAN to match on both ends of the trunk.
The correct action is to configure the native VLAN to match on both ends of the trunk. CDP reports a native VLAN mismatch when the native VLANs differ on the two sides of a trunk link. Although data traffic may still pass because 802.1Q does not tag frames on the native VLAN, the mismatch can cause broadcast loops and security risks, as frames from one native VLAN may be misinterpreted on the other side. Setting both sides to the same native VLAN (e.g., VLAN 1 or VLAN 99) resolves the mismatch and ensures proper Layer 2 behavior.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Add VLAN 99 to the allowed VLAN list on the trunk interface of SW1.
Why it's wrong here
Even if VLAN 99 were added to the allowed list, the native VLANs would still be mismatched. The CDP warning is about the inconsistency of the untagged VLAN, not about VLAN 99 being absent from the allowed list. Untagged traffic would continue to be placed into different VLANs on each switch, preserving the loop risk.
- ✗
Remove the trunk configuration and set both interfaces as access ports in VLAN 1.
Why it's wrong here
This action is unnecessarily disruptive. The trunk is working (data traffic flows), so removing it would break connectivity across switches. The issue is only a configuration mismatch, which can be fixed without changing the port type. This skips the straightforward corrective step and introduces a network outage.
- ✗
Enable spanning‑tree PortFast on the trunk ports.
Why it's wrong here
PortFast is designed for access ports (edge ports) to bypass listening and learning states. Applying PortFast to a trunk port is inappropriate and would not resolve the native VLAN mismatch. It also risks creating immediate forwarding loops, worsening the problem.
- ✓
Configure the native VLAN to match on both ends of the trunk.
Why this is correct
The root cause is a configured native VLAN mismatch (1 vs 99). Changing one switch’s native VLAN to match the other (or setting both to a common VLAN) immediately resolves the CDP warning and eliminates the potential for broadcast loops caused by the mismatch. This is the most direct and least disruptive next step.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Configure the native VLAN to match on both ends of the trunk.Correct answer▾
Why this is correct
The root cause is a configured native VLAN mismatch (1 vs 99). Changing one switch’s native VLAN to match the other (or setting both to a common VLAN) immediately resolves the CDP warning and eliminates the potential for broadcast loops caused by the mismatch. This is the most direct and least disruptive next step.
✗Add VLAN 99 to the allowed VLAN list on the trunk interface of SW1.Wrong answer — click to see why▾
Why this is wrong here
Common misconception: the warning message implies a VLAN is not allowed, but native VLAN mismatch means the trunk ports disagree on the native VLAN, not that a VLAN is missing from the allowed list.
✗Remove the trunk configuration and set both interfaces as access ports in VLAN 1.Wrong answer — click to see why▾
Why this is wrong here
Over‑reaction: candidates might think a trunk problem requires eliminating the trunk, but the correct approach is to correct the native VLAN parameter on the existing trunk.
✗Enable spanning‑tree PortFast on the trunk ports.Wrong answer — click to see why▾
Why this is wrong here
Wrong feature: PortFast addresses access port convergence, not VLAN mismatches. Candidates may reach for any familiar command, but it targets the wrong layer and port type.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
Key term
Native VLAN mismatch
A Native VLAN mismatch occurs when two connected switches have different Native VLANs configured on the trunk port, causing control traffic like CDP, DTP, and BPDUs to be sent on the wrong VLAN and potentially creating security vulnerabilities or connectivity issues.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.