- A
The switch port is configured as a trunk port instead of an access port
A trunk port expects 802.1Q tagged frames; the server sends untagged frames, so the switch may not associate them with VLAN 20, causing communication failure within the VLAN.
- B
VLAN 20 is not allowed on the trunk to the router
Why wrong: This would affect communication to other VLANs, not within the same VLAN. Intra-VLAN communication does not go through the router.
- C
The server does not have a default gateway configured
Why wrong: A default gateway is only required for communication outside the local subnet. It is not needed for devices in the same VLAN.
- D
The port is administratively down
Why wrong: If the port were down, there would be no connectivity at all, but the question implies the server is connected (just unable to communicate).
Quick Answer
The answer is a trunk port misconfiguration, because the switch port is actually set to trunk mode instead of access mode, causing a VLAN mismatch that prevents server communication. When a switch port is configured as a trunk, it expects all incoming frames to carry a VLAN tag; an untagged frame from the server is automatically assigned to the native VLAN, typically VLAN 1, not VLAN 20. This means the server’s traffic is isolated from other devices in VLAN 20, even though the server has a correct static IP address. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of how access and trunk ports handle VLAN tagging, and it’s a common trap where candidates overlook the native VLAN behavior. Remember the key distinction: access ports accept untagged traffic for a single VLAN, while trunk ports expect tagged frames and shunt untagged traffic to the native VLAN. A simple memory tip is “access accepts, trunk tags”—if the port expects tags but gets none, the VLAN will be wrong.
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 adds a new server to VLAN 20. The switch port is configured as an access port in VLAN 20, and the server has a correct static IP address in that subnet. However, the server cannot communicate with other devices in the same VLAN. The VLAN exists on the switch and other devices in VLAN 20 are working. What is the most likely cause of this issue?
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 switch port is configured as a trunk port instead of an access port
The scenario states the switch port is configured as an access port in VLAN 20, but the server cannot communicate with other devices in the same VLAN. If the port were actually configured as a trunk port, it would expect frames to be tagged with a VLAN ID. An untagged frame from the server would be placed into the native VLAN (typically VLAN 1), not VLAN 20, causing a mismatch. This explains why the server, despite having a correct static IP in VLAN 20's subnet, cannot reach other devices in VLAN 20.
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 switch port is configured as a trunk port instead of an access port
Why this is correct
A trunk port expects 802.1Q tagged frames; the server sends untagged frames, so the switch may not associate them with VLAN 20, causing communication failure within the 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.
- ✗
VLAN 20 is not allowed on the trunk to the router
Why it's wrong here
This would affect communication to other VLANs, not within the same VLAN. Intra-VLAN communication does not go through the router.
- ✗
The server does not have a default gateway configured
Why it's wrong here
A default gateway is only required for communication outside the local subnet. It is not needed for devices in the same VLAN.
- ✗
The port is administratively down
Why it's wrong here
If the port were down, there would be no connectivity at all, but the question implies the server is connected (just unable to communicate).
Common exam traps
Common exam trap: answer the scenario, not the keyword
CompTIA often tests the distinction between access and trunk ports by presenting a scenario where a device has correct IP settings but cannot communicate within its VLAN, tempting candidates to blame routing or gateway issues when the real problem is a layer 2 VLAN mismatch caused by trunk mode on an access port.
Detailed technical explanation
How to think about this question
On a Cisco switch, an access port sends and receives untagged frames and implicitly belongs to a single VLAN. A trunk port, by default, expects 802.1Q-tagged frames for all VLANs except the native VLAN, which is untagged. If a port is accidentally left in trunk mode (or configured as a trunk), the server's untagged frames are associated with the native VLAN, not VLAN 20, isolating the server from VLAN 20 hosts. This is a common misconfiguration when a port is intended for an end device but inherits a trunk configuration from a previous deployment or template.
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
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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: The switch port is configured as a trunk port instead of an access port — The scenario states the switch port is configured as an access port in VLAN 20, but the server cannot communicate with other devices in the same VLAN. If the port were actually configured as a trunk port, it would expect frames to be tagged with a VLAN ID. An untagged frame from the server would be placed into the native VLAN (typically VLAN 1), not VLAN 20, causing a mismatch. This explains why the server, despite having a correct static IP in VLAN 20's subnet, cannot reach other devices in VLAN 20.
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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Last reviewed: Jun 30, 2026
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