- A
The trunk port is in an err-disabled state.
Why wrong: The trunk status is up/up, so it is not err-disabled. An err-disabled port would show as down.
- B
The native VLAN on the trunk port is different on the two switches.
A native VLAN mismatch can cause untagged frames to be placed into different VLANs on each switch. This can prevent communication even if the allowed VLAN list is correct. The administrator should verify that the native VLAN is the same on both ends.
- C
The switchport mode is not set to trunk on one side.
Why wrong: If the trunk link is up/up, both sides are likely configured as trunk ports. The question states the trunk is configured, so this is not the issue.
- D
The spanning-tree protocol is blocking VLAN 10 on the trunk.
Why wrong: Spanning tree can block specific VLANs if there is a loop, but for a simple two-switch topology this is unlikely. Moreover, the symptom is that hosts on the same VLAN cannot communicate, which is more indicative of a native VLAN issue.
Quick Answer
The answer is a native VLAN mismatch on the trunk port. This is correct because when the native VLAN differs between two ends of a trunk, the switches disagree on which frames should remain untagged; one switch may place native VLAN frames into a different VLAN than the other, breaking Layer 2 communication for all VLANs, including VLAN 10. On the CompTIA Network+ N10-009 exam, this scenario tests your understanding of trunking and the 802.1Q standard, often appearing as a tricky question where all other configurations look perfect but connectivity fails. A common trap is assuming that a native VLAN mismatch only affects the native VLAN itself, but in reality it can corrupt the spanning-tree topology and disrupt traffic across the entire trunk. Remember the mnemonic: “Native mismatch? Traffic gets pitched.”
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 a new access switch to the core switch via a trunk port. Both switches have the same VLAN database, and the trunk is configured to allow all VLANs. However, hosts on VLAN 10 connected to the new access switch cannot communicate with hosts on VLAN 10 on the core switch. The administrator verifies that the access ports for VLAN 10 are correctly configured and that the trunk link status is up/up. 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 native VLAN on the trunk port is different on the two switches.
When the native VLAN (typically VLAN 1 by default) is mismatched on a trunk link, the switches will not properly tag frames for that VLAN. Since VLAN 10 is not the native VLAN, a native VLAN mismatch does not directly block VLAN 10 traffic; however, the scenario states that both switches have the same VLAN database and the trunk allows all VLANs, so the most likely cause is a native VLAN mismatch that can cause control plane issues or miscommunication. In practice, a native VLAN mismatch can lead to VLAN 10 hosts being unable to communicate because the switches may place the native VLAN frames into different VLANs, disrupting Layer 2 connectivity for all VLANs including VLAN 10.
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 trunk port is in an err-disabled state.
Why it's wrong here
The trunk status is up/up, so it is not err-disabled. An err-disabled port would show as down.
- ✓
The native VLAN on the trunk port is different on the two switches.
Why this is correct
A native VLAN mismatch can cause untagged frames to be placed into different VLANs on each switch. This can prevent communication even if the allowed VLAN list is correct. The administrator should verify that the native VLAN is the same on both ends.
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.
- ✗
The switchport mode is not set to trunk on one side.
Why it's wrong here
If the trunk link is up/up, both sides are likely configured as trunk ports. The question states the trunk is configured, so this is not the issue.
- ✗
The spanning-tree protocol is blocking VLAN 10 on the trunk.
Why it's wrong here
Spanning tree can block specific VLANs if there is a loop, but for a simple two-switch topology this is unlikely. Moreover, the symptom is that hosts on the same VLAN cannot communicate, which is more indicative of a native VLAN issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a native VLAN mismatch only affects the native VLAN itself, but it actually disrupts Layer 2 communication for all VLANs because the switches misclassify untagged frames and can cause spanning-tree inconsistencies.
Trap categories for this question
Command / output trap
The trunk status is up/up, so it is not err-disabled. An err-disabled port would show as down.
Detailed technical explanation
How to think about this question
On Cisco switches, the native VLAN is the only VLAN that is not tagged with an 802.1Q header on a trunk. If the native VLAN differs on each end, the switches will receive untagged frames and assign them to their respective native VLANs, causing a VLAN mismatch that can break spanning-tree and CDP, and prevent hosts in the same VLAN from communicating. This is often discovered by seeing 'native VLAN mismatch' in the logs or via 'show interfaces trunk' output.
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 native VLAN on the trunk port is different on the two switches. — When the native VLAN (typically VLAN 1 by default) is mismatched on a trunk link, the switches will not properly tag frames for that VLAN. Since VLAN 10 is not the native VLAN, a native VLAN mismatch does not directly block VLAN 10 traffic; however, the scenario states that both switches have the same VLAN database and the trunk allows all VLANs, so the most likely cause is a native VLAN mismatch that can cause control plane issues or miscommunication. In practice, a native VLAN mismatch can lead to VLAN 10 hosts being unable to communicate because the switches may place the native VLAN frames into different VLANs, disrupting Layer 2 connectivity for all VLANs including VLAN 10.
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 11, 2026
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