This N10-009 practice question tests your understanding of network implementation. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Two switches are connected via a trunk link configured with allowed VLANs 10, 20, and 30. Hosts on VLAN 20 can communicate across the trunk, but hosts on VLAN 30 cannot. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
D
Option D is correct because the most likely cause is that VLAN 30 is not allowed on the trunk link. Even though the trunk is configured with allowed VLANs 10, 20, and 30, if VLAN 30 is not present in the allowed VLAN list on one of the switch ports (e.g., due to a missing 'switchport trunk allowed vlan add 30' command or a pruning issue), frames from VLAN 30 will be dropped at the trunk. This explains why hosts on VLAN 20 can communicate while those on VLAN 30 cannot.
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.
✗
A
Why it's wrong here
A native VLAN mismatch would cause issues for untagged traffic, but the problem is specific to VLAN 30, not all untagged traffic.
✗
B
Why it's wrong here
Trunk encapsulation mismatches (e.g., ISL vs. 802.1Q) would affect all VLANs, not just one.
✗
C
Why it's wrong here
If VLAN 30 were not allowed on the trunk, traffic would not pass, but since it is in the allowed list, this is not the issue.
✓
D
Why this is correct
If VLAN 30 is only created on one switch, the other switch will not have the VLAN in its database and will drop frames from that VLAN.
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
The trap here is that candidates often confuse trunk allowed VLAN configuration with native VLAN settings or assume a spanning-tree issue is VLAN-specific, when in fact spanning-tree blocks the entire trunk, not individual VLANs.
Detailed technical explanation
How to think about this question
Trunk links use IEEE 802.1Q tagging to carry multiple VLANs, and the 'switchport trunk allowed vlan' command defines which VLANs are permitted. If VLAN 30 is not explicitly added to the allowed list on one side, frames tagged with VLAN 30 are discarded at the egress port. Additionally, VTP pruning can dynamically remove VLANs from trunks if they are not active on downstream switches, which could also cause this symptom without manual misconfiguration.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
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: D — Option D is correct because the most likely cause is that VLAN 30 is not allowed on the trunk link. Even though the trunk is configured with allowed VLANs 10, 20, and 30, if VLAN 30 is not present in the allowed VLAN list on one of the switch ports (e.g., due to a missing 'switchport trunk allowed vlan add 30' command or a pruning issue), frames from VLAN 30 will be dropped at the trunk. This explains why hosts on VLAN 20 can communicate while those on VLAN 30 cannot.
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.
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Question Discussion
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