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. A key principle to apply: a VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.. 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.
Exhibit
Switch A trunk allowed VLANs: 10,20,30
Switch B trunk allowed VLANs: 10,20
A trunk link between two switches is up, but hosts in VLAN 30 on opposite switches cannot communicate. VLAN 10 works across the same trunk. Which two causes are the most likely?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VLAN 30 is not allowed on the trunk on one side
When one VLAN fails but others work across the same trunk, the problem is likely VLAN-specific. VLAN 30 may not exist on one switch or may not be allowed on the trunk. Option C is incorrect because the native VLAN does not need to be 30; a native VLAN mismatch would typically cause connectivity issues on all VLANs, not just VLAN 30. Option D is incorrect because PortFast only affects the speed at which an access port enters the forwarding state and does not impact communication across an already-up trunk.
Key principle: A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
VLAN 30 is not allowed on the trunk on one side
Why this is correct
If the VLAN is missing from the allowed list on one side, traffic for that VLAN will not traverse the link.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.
✓
VLAN 30 may not exist in the VLAN database on the affected switch
Why this is correct
The VLAN must exist locally to support access ports and forwarding.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.
✗
The trunk native VLAN should always be 30
Why it's wrong here
The native VLAN setting is unrelated to carrying tagged VLAN 30 traffic.
✗
PortFast must be disabled on the access ports in VLAN 30
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.
✓VLAN 30 is not allowed on the trunk on one sideCorrect answer▾
Why this is correct
If the VLAN is missing from the allowed list on one side, traffic for that VLAN will not traverse the link.
✗The trunk native VLAN should always be 30Wrong answer — click to see why▾
Why this is wrong here
The native VLAN is used for untagged traffic on a trunk and is typically VLAN 1 by default. It has no bearing on whether a specific tagged VLAN like VLAN 30 is carried; the allowed VLAN list controls that.
Why candidates choose this
Students often confuse the native VLAN with the concept of a 'management VLAN' or think that setting the native VLAN to the problem VLAN might fix issues, but it does not affect tagged VLAN traffic.
✗PortFast must be disabled on the access ports in VLAN 30Wrong answer — click to see why▾
Why this is wrong here
PortFast is a feature that speeds up the transition of an access port to the forwarding state, bypassing STP listening/learning. It does not affect trunk operation or whether a VLAN is carried on a trunk.
Why candidates choose this
Test-takers may associate PortFast with VLAN issues because it is often used on access ports, but it is unrelated to trunk VLAN filtering.
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
Don't assume trunk issues affect all VLANs equally; check for VLAN-specific settings.
Detailed technical explanation
How to think about this question
A VLAN (Virtual Local Area Network) segments a switched network logically, allowing devices in the same VLAN to communicate as if they were on the same physical network. When switches connect via a trunk link, multiple VLANs can traverse the same physical connection using tagging protocols like IEEE 802.1Q. For hosts in VLAN 30 on different switches to communicate, VLAN 30 must be configured and allowed on the trunk link, and the VLAN must exist in the VLAN database on both switches. Without these conditions, traffic for VLAN 30 will not be forwarded properly.
The trunk link carries traffic for multiple VLANs by tagging frames with VLAN identifiers. If VLAN 30 is not included in the allowed VLAN list on one side of the trunk, frames tagged for VLAN 30 will be dropped, preventing communication. Similarly, if VLAN 30 is not defined in the VLAN database on a switch, that switch cannot forward or recognize traffic for VLAN 30, causing connectivity issues. These two conditions are common causes when one VLAN fails to communicate across a trunk while others succeed.
A common exam trap is confusing the native VLAN setting or PortFast configuration with VLAN trunking issues. The native VLAN only affects untagged frames and does not impact tagged VLAN 30 traffic. PortFast affects port state transitions and does not influence VLAN tagging or trunk membership. Misunderstanding these details can lead to incorrect troubleshooting steps. Practically, ensuring VLAN existence and trunk allowed VLAN lists are correct is the first step in resolving VLAN communication problems across trunks.
KKey Concepts to Remember
A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.
A trunk link carries multiple VLANs by tagging frames, and only VLANs allowed on the trunk are forwarded across it.
If a VLAN is not allowed on one side of a trunk, traffic for that VLAN will be blocked and not traverse the link.
The native VLAN setting on a trunk affects untagged frames but does not impact tagged VLAN traffic like VLAN 30.
PortFast enables immediate port forwarding on access ports but does not influence VLAN tagging or trunk VLAN membership.
Switches drop frames for VLANs that are not configured locally, preventing communication for hosts in that VLAN.
Troubleshooting VLAN communication issues across trunks requires verifying both VLAN existence and allowed VLAN lists on trunks.
VLAN tagging protocols like IEEE 802.1Q enable multiple VLANs to share a single physical trunk link.
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
A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review a VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN., then practise related 200-301 questions on the same topic to reinforce the concept.
Switching and Network Access — This question tests Switching and Network Access — A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN..
What is the correct answer to this question?
The correct answer is: VLAN 30 is not allowed on the trunk on one side — When one VLAN fails but others work across the same trunk, the problem is likely VLAN-specific. VLAN 30 may not exist on one switch or may not be allowed on the trunk. Option C is incorrect because the native VLAN does not need to be 30; a native VLAN mismatch would typically cause connectivity issues on all VLANs, not just VLAN 30. Option D is incorrect because PortFast only affects the speed at which an access port enters the forwarding state and does not impact communication across an already-up trunk.
What should I do if I get this 200-301 question wrong?
Review a VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN., then practise related 200-301 questions on the same topic to reinforce the concept.
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?
A VLAN must exist in the VLAN database on a switch to support forwarding and access ports for that VLAN.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.