- A
The native VLAN is different on each switch
Why wrong: While a native VLAN mismatch can cause problems with untagged traffic, it does not prevent all VLAN traffic from passing; the trunk would still carry tagged frames for other VLANs.
- B
VLAN 1 has been deleted on one of the switches
Why wrong: VLAN 1 is the default VLAN and cannot be deleted on most Cisco switches.
- C
Spanning Tree Protocol is blocking the trunk link
Why wrong: If the link is up and trunking is configured, STP would typically not block a trunk port unless there is a loop. The scenario states the link is up, so STP blocking is unlikely.
- D
The trunk encapsulation is mismatched
If one switch is configured for ISL encapsulation and the other for 802.1Q, the trunk will not pass traffic because the encapsulation methods are incompatible.
Quick Answer
The answer is a trunk encapsulation mismatch, which is the most likely cause when a trunk link is up but all VLAN traffic stops. This occurs because both ends of a trunk must agree on the same encapsulation protocol—either 802.1Q or the older Cisco-proprietary ISL—to properly tag and interpret frames; if one switch uses dot1q while the other uses ISL, the frames are dropped or misread, halting all VLAN traffic even though the physical link remains active. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of trunking fundamentals and the common oversight that encapsulation must match, as the administrator already verified mode and allowed VLANs. A frequent trap is assuming a working link means correct configuration, but encapsulation is a separate setting that can auto-negotiate incorrectly. Remember the memory tip: “Encapsulation first, trunk mode second—match the tag to avoid the lag.”
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 administrator configures a trunk link between two switches. The link is up, but no traffic from any VLAN is passed between the switches. The administrator verifies that the trunk port is configured correctly on both switches with 'switchport mode trunk' and allowed VLANs. Which of the following 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
The trunk encapsulation is mismatched
Option D is correct because a trunk link requires both ends to use the same encapsulation protocol—either 802.1Q or ISL. If one switch is set to 'switchport trunk encapsulation dot1q' and the other uses 'isl' (or auto-negotiates to a different type), the frames will be dropped or misinterpreted, preventing any VLAN traffic from passing. The administrator verified 'switchport mode trunk' and allowed VLANs, but encapsulation mismatch is a common oversight that stops all VLAN traffic while keeping the link up.
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 different on each switch
Why it's wrong here
While a native VLAN mismatch can cause problems with untagged traffic, it does not prevent all VLAN traffic from passing; the trunk would still carry tagged frames for other VLANs.
- ✗
VLAN 1 has been deleted on one of the switches
Why it's wrong here
VLAN 1 is the default VLAN and cannot be deleted on most Cisco switches.
- ✗
Spanning Tree Protocol is blocking the trunk link
Why it's wrong here
If the link is up and trunking is configured, STP would typically not block a trunk port unless there is a loop. The scenario states the link is up, so STP blocking is unlikely.
- ✓
The trunk encapsulation is mismatched
Why this is correct
If one switch is configured for ISL encapsulation and the other for 802.1Q, the trunk will not pass traffic because the encapsulation methods are incompatible.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between 'switchport mode trunk' (which sets the mode) and 'switchport trunk encapsulation' (which sets the protocol), leading candidates to assume that setting the mode alone is sufficient for trunking to work.
Trap categories for this question
Scenario analysis trap
If the link is up and trunking is configured, STP would typically not block a trunk port unless there is a loop. The scenario states the link is up, so STP blocking is unlikely.
Detailed technical explanation
How to think about this question
Trunk encapsulation mismatch is often caused by Cisco switches defaulting to Dynamic Trunking Protocol (DTP) negotiation, which can result in one side using 802.1Q and the other using ISL if not explicitly configured. The 'show interfaces trunk' command displays the encapsulation type on each side; a mismatch will show 'not trunking' or inconsistent encapsulation values. In real-world scenarios, this is a frequent issue when mixing older Cisco switches that support ISL with newer ones that only support 802.1Q.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
Network Implementation — study guide chapter
Learn the concepts, then practise the questions
- →
Network Implementation practice questions
Targeted practice on this topic area only
- →
All N10-009 questions
520 questions across all exam domains
- →
CompTIA Network+ N10-009 study guide
Full concept coverage aligned to exam objectives
- →
N10-009 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related N10-009 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Networking Concepts practice questions
Practise N10-009 questions linked to Networking Concepts.
Network Implementation practice questions
Practise N10-009 questions linked to Network Implementation.
Network Operations practice questions
Practise N10-009 questions linked to Network Operations.
Network Security practice questions
Practise N10-009 questions linked to Network Security.
Network Troubleshooting practice questions
Practise N10-009 questions linked to Network Troubleshooting.
Network+ network fundamentals practice questions
Practise N10-009 questions linked to Network+ network fundamentals.
Practice this exam
Start a free N10-009 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this N10-009 question test?
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The trunk encapsulation is mismatched — Option D is correct because a trunk link requires both ends to use the same encapsulation protocol—either 802.1Q or ISL. If one switch is set to 'switchport trunk encapsulation dot1q' and the other uses 'isl' (or auto-negotiates to a different type), the frames will be dropped or misinterpreted, preventing any VLAN traffic from passing. The administrator verified 'switchport mode trunk' and allowed VLANs, but encapsulation mismatch is a common oversight that stops all VLAN traffic while keeping the link up.
What should I do if I get this N10-009 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.
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 →
Keep practising
More N10-009 practice questions
- Which layer of the OSI model is responsible for logical addressing and routing of packets between networks?
- Users in VLAN 10 cannot obtain IP addresses from a DHCP server located in VLAN 20. The router has an ip helper-address c…
- Which of the following is a characteristic of a Layer 2 network switch?
- Which of the following network devices operates primarily at Layer 2 of the OSI model and uses MAC addresses to forward…
- Which of the following is a characteristic of UDP when compared to TCP?
- Which of the following IPv6 addresses is a valid link-local address?
Last reviewed: Jun 30, 2026
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 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.