The answer is a switchport mode mismatch, where one side is configured as a trunk and the peer as an access port. This is the most likely cause because the two ends fundamentally disagree on how to handle VLAN traffic: the trunk side expects 802.1Q tagging and the ability to carry multiple VLANs, while the access side treats the link as a single-VLAN, untagged endpoint. On the CCNA 200-301 v2 exam, this tests your understanding of Layer 2 interface states and the fact that a physical link can be up even when logical forwarding fails—a common trap where candidates assume the link must be down. The key insight is that DTP (Dynamic Trunking Protocol) negotiations fail, but the interface still comes up, leading to unpredictable behavior like dropped frames or VLAN leakage. A solid memory tip is "Trunk tags, Access strips"—if one side tags and the other doesn’t, traffic gets lost or misdirected.
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 trunk port carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation.. 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 switchport on one side of a link is configured as a trunk, but the peer side is configured as an access port. The physical link is up, but VLAN traffic behaves unexpectedly. What is the most likely cause?
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
✓
The two ends disagree on whether the link is a trunk or an access port.
The most likely cause is a switchport mode mismatch. In practical terms, one side expects the link to carry multiple VLANs with tagging behavior, while the other side treats it as a normal one-VLAN endpoint-style access connection. The physical interface can still come up, but the two ends do not agree on how the traffic should be handled.
This is a classic Layer 2 troubleshooting pattern. The link may not be fully down, but the configuration disagreement causes logical forwarding problems.
Key principle: A trunk port carries multiple VLANs by tagging frames with VLANIDs using IEEE 802.1Q encapsulation.
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 two ends disagree on whether the link is a trunk or an access port.
Why this is correct
This is correct because trunk/access mismatch causes VLAN handling problems.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A trunk port carries multiple VLANs by tagging frames with VLANIDs using IEEE 802.1Q encapsulation.
✗
The switches must both use the same hostname.
Why it's wrong here
This is wrong because hostnames do not affect trunk versus access behavior.
When this WOULD be correct
In a different scenario where a question asks about the importance of hostname consistency for management or monitoring purposes in a network, option B could be correct. For example, if the exam question focused on network management tools that rely on hostname resolution, having the same hostname could be crucial.
✗
The native VLAN must be set to 1 on both sides first.
Why it's wrong here
This is wrong because the deeper issue is the trunk/access role mismatch itself.
When this WOULD be correct
In a different question, if the scenario involved two switches configured as trunks but with different native VLANs, then specifying that the native VLAN must be set to 1 on both sides could be correct. This would be relevant if the question focused on ensuring consistent native VLAN settings for proper communication.
✗
The ports need OSPF enabled.
Why it's wrong here
This is wrong because OSPF is not relevant to this Layer 2 switchport mismatch.
When this WOULD be correct
In a different scenario where the question asks about routing protocols and their necessity for inter-VLAN communication, option D could be correct if the exam context involves enabling OSPF on interfaces to facilitate routing between VLANs across different switches.
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 two ends disagree on whether the link is a trunk or an access port.Correct answer▾
Why this is correct
This is correct because trunk/access mismatch causes VLAN handling problems.
✗The switches must both use the same hostname.Wrong answer — click to see why▾
Why this is wrong here
OSPF is a Layer 3 routing protocol used for exchanging routes between routers, not for resolving Layer 2 switchport mismatches. This issue is purely about trunk/access configuration, which is unrelated to OSPF.
★ When this WOULD be the correct answer
In a different scenario where a question asks about the importance of hostname consistency for management or monitoring purposes in a network, option B could be correct. For example, if the exam question focused on network management tools that rely on hostname resolution, having the same hostname could be crucial.
Why candidates choose this
Students may confuse Layer 2 and Layer 3 concepts, thinking that enabling a routing protocol could fix connectivity issues. However, OSPF has no effect on VLAN tagging or trunk negotiation.
✗The native VLAN must be set to 1 on both sides first.Wrong answer — click to see why▾
Why this is wrong here
While native VLAN mismatch can cause issues on a trunk link, the primary problem here is that one side is configured as access, not trunk. Even if native VLAN is set to 1 on both sides, the access port will still not process tagged frames correctly.
★ When this WOULD be the correct answer
In a different question, if the scenario involved two switches configured as trunks but with different native VLANs, then specifying that the native VLAN must be set to 1 on both sides could be correct. This would be relevant if the question focused on ensuring consistent native VLAN settings for proper communication.
Why candidates choose this
Native VLAN is a common source of trunk problems, so test-takers may focus on that detail. However, the root cause is the trunk/access role mismatch, not the native VLAN value.
✗The ports need OSPF enabled.Wrong answer — click to see why▾
Why this is wrong here
Hostnames are purely for identification and have no impact on switchport operation or VLAN tagging. The trunk/access mismatch is a Layer 2 configuration issue independent of hostnames.
★ When this WOULD be the correct answer
In a different scenario where the question asks about routing protocols and their necessity for inter-VLAN communication, option D could be correct if the exam context involves enabling OSPF on interfaces to facilitate routing between VLANs across different switches.
Why candidates choose this
Some students might think that matching hostnames is required for switch interoperability, but this is incorrect. Hostnames are only used for CLI identification and logging.
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
Be cautious of assuming all VLAN issues are due to allowed lists or STP. Consider mode mismatches when the link is physically up but traffic is disrupted.
Detailed technical explanation
How to think about this question
A switchport configured as a trunk is designed to carry traffic for multiple VLANs by tagging frames with VLAN identifiers using the IEEE 802.1Q standard. This allows a single physical link to transport traffic from many VLANs simultaneously. Conversely, an access port is assigned to a single VLAN and does not tag frames, treating all traffic as belonging to that VLAN. When one side of a link is set as a trunk and the other as an access port, the two ends have incompatible expectations about frame tagging and VLAN handling.
Cisco switches enforce strict VLAN tagging rules based on the port mode configuration. If one side expects tagged frames (trunk) but the other side sends untagged frames (access), VLAN traffic will not be properly forwarded or segregated. The physical link may still come up because Layer 1 and basic Layer 2 link integrity checks succeed, but the logical Layer 2 VLAN separation fails, causing unexpected traffic behavior and connectivity issues.
This mismatch is a common exam trap because the link appears operational, misleading candidates to suspect other issues like native VLAN mismatches or routing protocols. However, the root cause is the fundamental disagreement on port mode. In practice, this misconfiguration can cause VLAN leaks, broadcast storms, or dropped traffic, making it critical to verify that both ends of a link agree on trunk or access mode to ensure proper VLAN traffic forwarding.
KKey Concepts to Remember
A trunk port carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation.
An access port belongs to a single VLAN and sends untagged frames without VLAN tags.
Switchports must be configured consistently on both ends as either trunk or access to ensure proper VLAN traffic handling.
A trunk/access mismatch causes VLAN traffic to be misinterpreted, leading to connectivity and forwarding problems despite the physical link being up.
Cisco switches do not automatically resolve trunk/access mismatches, so manual configuration alignment is required.
The native VLAN setting affects untagged frames on trunk ports but does not fix trunk/access mode mismatches.
Layer 2 protocols like STP operate independently of trunk/access mismatches but rely on correct VLAN tagging for proper topology.
Physical link status does not guarantee correct VLAN forwarding; logical configuration consistency is essential for network stability.
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 trunk port carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation.
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 trunk port carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation., 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 trunk port carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation..
What is the correct answer to this question?
The correct answer is: The two ends disagree on whether the link is a trunk or an access port. — The most likely cause is a switchport mode mismatch. In practical terms, one side expects the link to carry multiple VLANs with tagging behavior, while the other side treats it as a normal one-VLAN endpoint-style access connection. The physical interface can still come up, but the two ends do not agree on how the traffic should be handled.
This is a classic Layer 2 troubleshooting pattern. The link may not be fully down, but the configuration disagreement causes logical forwarding problems.
What should I do if I get this 200-301 question wrong?
Review a trunk port carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation., 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 trunk port carries multiple VLANs by tagging frames with VLAN IDs using IEEE 802.1Q encapsulation.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A switchport on one side of a link is configured as a trunk, but the peer side is configured as an access port. What is the most likely result?
hard
✓ A.The link roles are mismatched, so VLAN traffic will not be handled as intended.
B.The access side automatically becomes a routed port.
C.Both switches automatically synchronize their switchport settings.
D.The mismatch forces OSPF to change router IDs.
Why A: The most likely result is a logical mismatch that prevents VLAN traffic from crossing the link as intended. In practical terms, one side is trying to carry multiple VLANs with tagging behavior, while the other side is treating the connection as a normal one-VLAN endpoint port. That disagreement usually leads to unexpected or failed traffic behavior.
This is a classic switching mismatch scenario. The link may still be physically up, but the two sides do not agree on how the traffic should be handled.
Last reviewed: May 17, 2026
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