- A
The native VLAN is mismatched on the two switches.
Why wrong: A native VLAN mismatch would cause issues for untagged frames, but would affect all VLANs if configured incorrectly, not just VLAN 20. This is not the most likely cause.
- B
VLAN 20 is not created on the second switch.
Correct. If VLAN 20 does not exist on the second switch, it will discard frames received on the trunk tagged with VLAN 20. Creating the VLAN on both switches is essential for inter-switch communication.
- C
The trunk uses ISL instead of 802.1Q.
Why wrong: Both ISL and 802.1Q work with multiple VLANs. If both switches support ISL and it is configured correctly, this would not cause a selective problem for VLAN 20.
- D
The default gateway is missing for VLAN 20.
Why wrong: A default gateway is required for inter-VLAN routing, not for communication within the same VLAN across switches. Since VLAN 20 is a single broadcast domain, a gateway is not needed for intra-VLAN traffic.
N10-009 Network Implementation Practice Question
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.
A network administrator connects two switches with a trunk link that is configured to allow all VLANs. Workstations in VLAN 10 can communicate across the switches, but workstations in VLAN 20 cannot. Both VLANs are configured on the first switch. What is the most likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"first"Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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
VLAN 20 is not created on the second switch.
VLAN 20 is configured on the first switch but not on the second switch. Even though the trunk link allows all VLANs, the second switch must have VLAN 20 created in its VLAN database for traffic to be forwarded. Without the VLAN existing on the second switch, frames tagged for VLAN 20 are dropped at the receiving switch because there is no corresponding VLAN interface or forwarding table entry.
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 mismatched on the two switches.
Why it's wrong here
A native VLAN mismatch would cause issues for untagged frames, but would affect all VLANs if configured incorrectly, not just VLAN 20. This is not the most likely cause.
- ✓
VLAN 20 is not created on the second switch.
Why this is correct
Correct. If VLAN 20 does not exist on the second switch, it will discard frames received on the trunk tagged with VLAN 20. Creating the VLAN on both switches is essential for inter-switch communication.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The trunk uses ISL instead of 802.1Q.
Why it's wrong here
Both ISL and 802.1Q work with multiple VLANs. If both switches support ISL and it is configured correctly, this would not cause a selective problem for VLAN 20.
- ✗
The default gateway is missing for VLAN 20.
Why it's wrong here
A default gateway is required for inter-VLAN routing, not for communication within the same VLAN across switches. Since VLAN 20 is a single broadcast domain, a gateway is not needed for intra-VLAN traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a trunk configured to 'allow all VLANs' automatically makes every VLAN functional across the link, when in fact each switch must have the VLAN defined in its local database for traffic to be processed.
Detailed technical explanation
How to think about this question
On Cisco switches, VLANs must be created globally on each switch before they can forward traffic for that VLAN. The 'show vlan brief' command on the second switch would reveal that VLAN 20 is missing. In a real-world scenario, using VTP (VLAN Trunking Protocol) could propagate VLAN definitions automatically, but if VTP is not configured or is in transparent mode, each switch must have the VLAN manually created.
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.
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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: VLAN 20 is not created on the second switch. — VLAN 20 is configured on the first switch but not on the second switch. Even though the trunk link allows all VLANs, the second switch must have VLAN 20 created in its VLAN database for traffic to be forwarded. Without the VLAN existing on the second switch, frames tagged for VLAN 20 are dropped at the receiving switch because there is no corresponding VLAN interface or forwarding table entry.
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: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
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Last reviewed: Jun 11, 2026
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