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 trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches.. 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.
A trunk between two switches is up, but users in VLAN 40 cannot communicate across it. The output shows both sides allow VLAN 40. What is another likely trunk-related cause to check next?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Native VLAN mismatch between the two trunk ends
If VLAN 40 is allowed on both ends and the trunk is up, a native VLAN mismatch is still worth checking because trunk problems are not limited to the allowed VLAN list. In plain language, the link may be carrying traffic, but if the two switches disagree on how untagged traffic should be treated, behavior can still become unpredictable. Native VLAN mismatches are a well-known source of warnings and unexpected traffic handling on 802.1Q trunks.
That does not mean every VLAN problem is caused by the native VLAN, but once the obvious allowed-list issue has been ruled out, it becomes a logical next trunk-specific item to verify.
Key principle: A VLANtrunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Native VLAN mismatch between the two trunk ends
Why this is correct
This is correct because a native VLAN mismatch is a common trunk-related issue worth checking after allowed VLANs have been verified.
Related concept
A VLANtrunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches.
✗
Missing router ID in OSPF
Why it's wrong here
This is wrong because OSPF router IDs are unrelated to an inter-switch VLANtrunk problem.
When this WOULD be correct
In a different scenario, if the question were about OSPF routing issues where the router ID is not set, candidates might be asked to troubleshoot OSPF adjacency problems. In that case, a missing router ID would prevent OSPF neighbors from forming, making this option correct.
✗
Incorrect NTP source interface
Why it's wrong here
This is wrong because NTP source settings do not explain VLANtrunk transport issues.
When this WOULD be correct
In a scenario where the question involves troubleshooting time-sensitive applications that rely on accurate timestamps for logging or coordination, a missing or incorrectly configured NTP source interface could lead to discrepancies in time, causing issues in those applications.
✗
Lack of PAT overload on the WAN router
Why it's wrong here
This is wrong because WAN NAT design is unrelated to switch trunkVLAN carriage.
When this WOULD be correct
In a question focused on NAT configurations, if the scenario involves multiple internal networks needing to share a single public IP address, the lack of PAT overload on the WAN router would be a valid concern affecting outbound connectivity.
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.
✓Native VLAN mismatch between the two trunk endsCorrect answer▾
Why this is correct
This is correct because a native VLAN mismatch is a common trunk-related issue worth checking after allowed VLANs have been verified.
✗Missing router ID in OSPFWrong answer — click to see why▾
Why this is wrong here
This option is wrong because the question specifically addresses VLAN communication issues over a trunk link, which are unrelated to OSPF router ID configuration. OSPF router ID affects routing protocols, not VLAN traffic directly.
★ When this WOULD be the correct answer
In a different scenario, if the question were about OSPF routing issues where the router ID is not set, candidates might be asked to troubleshoot OSPF adjacency problems. In that case, a missing router ID would prevent OSPF neighbors from forming, making this option correct.
Why candidates choose this
Candidates might choose this option due to a misunderstanding of how routing protocols interact with VLANs, mistakenly believing that OSPF configuration could impact VLAN communication across a trunk link.
✗Incorrect NTP source interfaceWrong answer — click to see why▾
Why this is wrong here
NTP source interface is unrelated to VLAN communication issues over a trunk link; it primarily affects time synchronization across devices. Since the question focuses on VLAN connectivity, this option does not address the core problem.
★ When this WOULD be the correct answer
In a scenario where the question involves troubleshooting time-sensitive applications that rely on accurate timestamps for logging or coordination, a missing or incorrectly configured NTP source interface could lead to discrepancies in time, causing issues in those applications.
Why candidates choose this
Candidates might confuse the importance of time synchronization in network operations with VLAN functionality, leading them to mistakenly believe that NTP configuration could impact VLAN communication.
✗Lack of PAT overload on the WAN routerWrong answer — click to see why▾
Why this is wrong here
Lack of PAT overload on the WAN router is unrelated to VLAN communication issues between switches. This option pertains to address translation for outbound traffic, not VLAN trunking problems.
★ When this WOULD be the correct answer
In a question focused on NAT configurations, if the scenario involves multiple internal networks needing to share a single public IP address, the lack of PAT overload on the WAN router would be a valid concern affecting outbound connectivity.
Why candidates choose this
Candidates may confuse general connectivity issues with NAT configurations, thinking that any routing or address translation problem could impact VLAN communication, leading them to select this option.
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 that allowed VLANs are the only trunk-related issue; native VLAN mismatches can also disrupt communication.
Detailed technical explanation
How to think about this question
A VLAN trunk is a point-to-point link between two switches that carries traffic for multiple VLANs simultaneously using tagging protocols like IEEE 802.1Q. The trunk encapsulates frames with VLAN tags to identify the VLAN membership of each frame as it traverses the link. The native VLAN is a special VLAN on a trunk port that carries untagged traffic, meaning frames sent on the native VLAN are transmitted without 802.1Q tags. This native VLAN must match on both ends of the trunk to ensure consistent handling of untagged frames.
When troubleshooting VLAN communication issues over a trunk, verifying that the VLAN is allowed on both ends is necessary but not always sufficient. A native VLAN mismatch occurs when the two trunk ports are configured with different native VLAN IDs. This mismatch causes untagged frames sent by one switch to be interpreted as belonging to different VLANs by the other switch, leading to traffic loss or misdelivery. Cisco switches generate native VLAN mismatch warnings in logs, and this issue is a common cause of VLAN communication failures even when the VLAN is explicitly allowed.
The exam trap here is to assume that allowed VLAN lists alone guarantee trunk functionality. Candidates often overlook the native VLAN configuration, which affects untagged traffic on the trunk. In practice, native VLAN mismatches can cause intermittent connectivity problems and unexpected traffic behavior, especially for management VLANs or protocols that send untagged frames. Understanding this subtlety helps avoid misdiagnosis and ensures reliable VLAN trunk operation in Cisco environments.
KKey Concepts to Remember
A VLAN trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches.
The native VLAN on a trunk port carries untagged traffic and must be identical on both trunk ends to prevent traffic misinterpretation.
Allowed VLAN lists on trunk ports control which VLANs can traverse the link but do not affect native VLAN handling.
A native VLAN mismatch causes untagged frames to be assigned to different VLANs on each switch, disrupting communication.
Cisco switches generate native VLAN mismatch warnings in logs to help identify this common trunk configuration error.
Troubleshooting VLAN connectivity issues requires checking both allowed VLANs and native VLAN consistency on trunk ports.
Ignoring native VLAN mismatches can lead to intermittent VLAN communication failures despite trunks being operational.
Proper native VLAN configuration ensures predictable forwarding of untagged traffic and stable VLAN trunk links.
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 trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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 trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches., 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 trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches..
What is the correct answer to this question?
The correct answer is: Native VLAN mismatch between the two trunk ends — If VLAN 40 is allowed on both ends and the trunk is up, a native VLAN mismatch is still worth checking because trunk problems are not limited to the allowed VLAN list. In plain language, the link may be carrying traffic, but if the two switches disagree on how untagged traffic should be treated, behavior can still become unpredictable. Native VLAN mismatches are a well-known source of warnings and unexpected traffic handling on 802.1Q trunks.
That does not mean every VLAN problem is caused by the native VLAN, but once the obvious allowed-list issue has been ruled out, it becomes a logical next trunk-specific item to verify.
What should I do if I get this 200-301 question wrong?
Review a VLAN trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
A VLAN trunk port uses IEEE 802.1Q tagging to carry multiple VLANs over a single physical link between switches.
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.