- A
There is a native VLAN mismatch between the two switches.
Why wrong: A native VLAN mismatch does not prevent a trunk from forming. DTP negotiates trunking irrespective of native VLAN assignment. A mismatch will allow the trunk to come up but cause CDP/STP notifications and possible untagged traffic leakage, yet the trunk interface appears in show interfaces trunk.
- B
The connecting cable is a straight-through Ethernet cable rather than a crossover cable.
Why wrong: Modern Cisco switches support auto-MDIX, which automatically adjusts the interface to work with either a straight-through or crossover cable. A cable type mismatch would not prevent the link from coming up or forming a trunk. The link is already up, so this is not the cause.
- C
One switch lacks a VLAN that exists on the other switch.
Why wrong: Trunk formation does not require identical VLAN databases. Trunk negotiation uses DTP, not VLAN presence. A missing VLAN would result in traffic for that VLAN being discarded or not forwarded, but the trunk itself would still be established and shown as a trunk port.
- D
Both ports are set to dynamic auto, so neither switch initiates DTP negotiation.
In dynamic auto mode, a switch port passively waits for DTP negotiation requests. When both ends are dynamic auto, no side initiates the negotiation, so the ports default to access mode. This perfectly matches the symptom: the link is up, but no trunk appears under show interfaces trunk, and multi-VLAN traffic is not passing.
Quick Answer
The answer is that both ports are set to dynamic auto, so neither switch initiates DTP negotiation. This is because the dynamic auto mode is passive—it listens for and responds to Dynamic Trunking Protocol (DTP) requests but never sends them. With both ends configured this way, no DTP frames are exchanged, so the ports remain in access mode, explaining why the link is up but no trunk appears in the show interfaces trunk output. On the CCNA 200-301 v2 exam, this question tests your understanding of DTP negotiation states and the critical difference between dynamic auto and dynamic desirable. A common trap is assuming any dynamic setting will form a trunk, but the key is that dynamic auto requires the other side to initiate. Remember the memory tip: “Auto is afraid to ask; Desirable will propose.”
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
A network engineer notices that a newly connected switch-to-switch link is up, but traffic from multiple VLANs is not passing. When issuing the show interfaces trunk command, no trunk ports are listed. Both switch ports are configured with switchport mode dynamic auto. 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 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
Both ports are set to dynamic auto, so neither switch initiates DTP negotiation.
When both ends of a link are set to switchport mode dynamic auto, neither switch will initiate DTP negotiation. Dynamic auto responds only to DTP requests; it never sends them. Therefore, no DTP messages are exchanged, and both ports silently remain in access mode. This explains why the link is up and passing traffic only for the access VLAN, and no trunk ports appear in the output. Other issues (native VLAN mismatch, straight-through cable, missing VLANs) do not prevent trunk formation—they affect operational behavior or management reporting but would still show a trunk interface if DTP negotiation succeeded.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both 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.
- ✗
There is a native VLAN mismatch between the two switches.
Why it's wrong here
A native VLAN mismatch does not prevent a trunk from forming. DTP negotiates trunking irrespective of native VLAN assignment. A mismatch will allow the trunk to come up but cause CDP/STP notifications and possible untagged traffic leakage, yet the trunk interface appears in show interfaces trunk.
- ✗
The connecting cable is a straight-through Ethernet cable rather than a crossover cable.
Why it's wrong here
Modern Cisco switches support auto-MDIX, which automatically adjusts the interface to work with either a straight-through or crossover cable. A cable type mismatch would not prevent the link from coming up or forming a trunk. The link is already up, so this is not the cause.
- ✗
One switch lacks a VLAN that exists on the other switch.
Why it's wrong here
Trunk formation does not require identical VLAN databases. Trunk negotiation uses DTP, not VLAN presence. A missing VLAN would result in traffic for that VLAN being discarded or not forwarded, but the trunk itself would still be established and shown as a trunk port.
- ✓
Both ports are set to dynamic auto, so neither switch initiates DTP negotiation.
Why this is correct
In dynamic auto mode, a switch port passively waits for DTP negotiation requests. When both ends are dynamic auto, no side initiates the negotiation, so the ports default to access mode. This perfectly matches the symptom: the link is up, but no trunk appears under show interfaces trunk, and multi-VLAN traffic is not passing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
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.
✓Both ports are set to dynamic auto, so neither switch initiates DTP negotiation.Correct answer▾
Why this is correct
In dynamic auto mode, a switch port passively waits for DTP negotiation requests. When both ends are dynamic auto, no side initiates the negotiation, so the ports default to access mode. This perfectly matches the symptom: the link is up, but no trunk appears under show interfaces trunk, and multi-VLAN traffic is not passing.
✗There is a native VLAN mismatch between the two switches.Wrong answer — click to see why▾
Why this is wrong here
Confusing DTP trunk negotiation with the operational consequences of a native VLAN mismatch.
✗The connecting cable is a straight-through Ethernet cable rather than a crossover cable.Wrong answer — click to see why▾
Why this is wrong here
Assuming that a crossover cable is mandatory for switch-to-switch connections, ignoring auto-MDIX.
✗One switch lacks a VLAN that exists on the other switch.Wrong answer — click to see why▾
Why this is wrong here
Mistaking the ability to forward traffic for a specific VLAN with the ability to negotiate a trunk.
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: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Command / output trap
A native VLAN mismatch does not prevent a trunk from forming. DTP negotiates trunking irrespective of native VLAN assignment. A mismatch will allow the trunk to come up but cause CDP/STP notifications and possible untagged traffic leakage, yet the trunk interface appears in show interfaces trunk.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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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 — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Both ports are set to dynamic auto, so neither switch initiates DTP negotiation. — When both ends of a link are set to switchport mode dynamic auto, neither switch will initiate DTP negotiation. Dynamic auto responds only to DTP requests; it never sends them. Therefore, no DTP messages are exchanged, and both ports silently remain in access mode. This explains why the link is up and passing traffic only for the access VLAN, and no trunk ports appear in the output. Other issues (native VLAN mismatch, straight-through cable, missing VLANs) do not prevent trunk formation—they affect operational behavior or management reporting but would still show a trunk interface if DTP negotiation succeeded.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 14, 2026
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