- A
The native VLAN is mismatched between SW1 and SW2, and VLAN 30 is the native VLAN on one side.
A native VLAN mismatch causes one switch to send untagged frames for VLAN 30 while the other expects tagged frames, so the receiving switch cannot associate the untagged traffic with VLAN 30, leading to a communication failure only for that VLAN.
- B
The trunk encapsulation is set to ISL on one switch and 802.1Q on the other.
Why wrong: A trunk with mismatched encapsulations would fail to come up entirely for all VLANs, not selectively for one VLAN. Modern Cisco switches auto-negotiate trunk encapsulation, making this scenario unlikely.
- C
Spanning Tree Protocol has placed VLAN 30 in a blocking state on the trunk link.
Why wrong: STP operates per VLAN, but there is no inherent reason for STP to block only one VLAN while others forward, especially without a topology change. The scenario gives no indication of a loop or STP change.
- D
The switched virtual interface (SVI) for VLAN 30 on SW1 is administratively down.
Why wrong: SVIs are used for inter-VLAN routing, not for bridging traffic within the same VLAN across a trunk. Disabling the SVI does not affect Layer 2 forwarding of VLAN 30 frames between switches.
Quick Answer
The answer is a native VLAN mismatch, specifically where VLAN 30 is the native VLAN on one switch but not the other. This is correct because on an 802.1Q trunk, the native VLAN carries traffic without an 802.1Q tag; when the native VLAN is mismatched, frames from VLAN 30 on one switch arrive untagged and are placed into the other switch’s configured native VLAN—say VLAN 1—instead of VLAN 30, breaking communication for that specific VLAN. On the CCNA 200-301 v2 exam, this scenario tests your understanding of trunking behavior and the common trap that a native VLAN mismatch only disrupts traffic for the mismatched native VLAN, while all other tagged VLANs (like 10 and 20) work fine. A key memory tip: “Untagged frames follow the native VLAN—if they don’t match, they land in the wrong VLAN.”
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An administrator notices that hosts in VLAN 30 on SW1 cannot communicate with hosts in VLAN 30 on SW2, even though both switches are connected via an 802.1Q trunk. Traffic for VLANs 10 and 20 passes without issues across the same trunk. The trunk is configured to allow all VLANs, and the allowed VLAN list explicitly includes VLAN 30. What is the most likely cause of the problem?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 native VLAN is mismatched between SW1 and SW2, and VLAN 30 is the native VLAN on one side.
The most likely cause is a native VLAN mismatch. When the native VLAN is mismatched on an 802.1Q trunk, traffic for the native VLAN is not tagged, so frames from VLAN 30 on one switch are received as untagged frames on the other switch and placed into the switch's configured native VLAN. If the native VLAN on one side is VLAN 30 and on the other side is a different VLAN (e.g., VLAN 1), the hosts in VLAN 30 cannot communicate because the frames are interpreted as belonging to different VLANs. Traffic for VLANs 10 and 20 passes because they are not the native VLAN and are properly tagged.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 native VLAN is mismatched between SW1 and SW2, and VLAN 30 is the native VLAN on one side.
Why this is correct
A native VLAN mismatch causes one switch to send untagged frames for VLAN 30 while the other expects tagged frames, so the receiving switch cannot associate the untagged traffic with VLAN 30, leading to a communication failure only for that VLAN.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The trunk encapsulation is set to ISL on one switch and 802.1Q on the other.
- ✗
Spanning Tree Protocol has placed VLAN 30 in a blocking state on the trunk link.
- ✗
The switched virtual interface (SVI) for VLAN 30 on SW1 is administratively down.
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.
✓The native VLAN is mismatched between SW1 and SW2, and VLAN 30 is the native VLAN on one side.Correct answer▾
Why this is correct
A native VLAN mismatch causes one switch to send untagged frames for VLAN 30 while the other expects tagged frames, so the receiving switch cannot associate the untagged traffic with VLAN 30, leading to a communication failure only for that VLAN.
✗The trunk encapsulation is set to ISL on one switch and 802.1Q on the other.Wrong answer — click to see why▾
Why this is wrong here
This would cause a complete trunk failure, not a failure limited to a single VLAN.
✗Spanning Tree Protocol has placed VLAN 30 in a blocking state on the trunk link.Wrong answer — click to see why▾
Why this is wrong here
STP would not randomly block one VLAN on a point-to-point trunk while the rest are forwarding; this is not a typical behavior.
✗The switched virtual interface (SVI) for VLAN 30 on SW1 is administratively down.Wrong answer — click to see why▾
Why this is wrong here
Layer 2 switching within the same VLAN does not require an SVI; an SVI is only needed for routing between VLANs or management.
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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the native VLAN mismatch scenario by describing a trunk that works for most VLANs but fails for one specific VLAN, leading candidates to incorrectly suspect STP blocking or SVI issues instead of recognizing the native VLAN mismatch.
Trap categories for this question
Scenario analysis trap
A trunk with mismatched encapsulations would fail to come up entirely for all VLANs, not selectively for one VLAN. Modern Cisco switches auto-negotiate trunk encapsulation, making this scenario unlikely.
Detailed technical explanation
How to think about this question
In 802.1Q trunking, the native VLAN (default VLAN 1) is the only VLAN whose frames are sent untagged across the trunk. If the native VLAN is mismatched, frames from the native VLAN on one switch are received as untagged frames on the other switch and are mapped to that switch's native VLAN, causing a VLAN mismatch. This is a common misconfiguration that can be verified with the 'show interfaces trunk' command, which displays the native VLAN on each side; the native VLAN must match on both ends for proper communication.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Switching and Network Access — This question tests Switching and Network Access — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The native VLAN is mismatched between SW1 and SW2, and VLAN 30 is the native VLAN on one side. — The most likely cause is a native VLAN mismatch. When the native VLAN is mismatched on an 802.1Q trunk, traffic for the native VLAN is not tagged, so frames from VLAN 30 on one switch are received as untagged frames on the other switch and placed into the switch's configured native VLAN. If the native VLAN on one side is VLAN 30 and on the other side is a different VLAN (e.g., VLAN 1), the hosts in VLAN 30 cannot communicate because the frames are interpreted as belonging to different VLANs. Traffic for VLANs 10 and 20 passes because they are not the native VLAN and are properly tagged.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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.
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