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Question 253 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: A network administrator is troubleshooting…
This 200-301 practice question tests your understanding of 200-301 exam topics. 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.
Exhibit
Switch# show running-config interface GigabitEthernet0/1 Building configuration... Current configuration : 120 bytes ! interface GigabitEthernet0/1 switchport trunk encapsulation dot1q switchport mode trunk switchport trunk native vlan 1 end Switch# show vlan brief VLAN Name Status Ports ---- -------------------------------- --------- ------------------------------- 1 default active Gi0/2, Gi0/3 10 VLAN0010 active Gi0/4 20 VLAN0020 active Gi0/5 1002 fddi-default act/unsup 1003 token-ring-default act/unsup 1004 fddinet-default act/unsup 1005 trnet-default act/unsup
A network administrator is troubleshooting connectivity issues between two VLANs on a Cisco switch. Hosts in VLAN 10 (10.10.10.0/24) cannot ping hosts in VLAN 20 (10.10.20.0/24). The switch is connected to a router-on-a-stick using a single trunk link. The administrator checks the switch configuration and sees the following output. What is the most likely cause of the problem?
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 is mismatched with the router
The root cause is that the trunk interface GigabitEthernet0/1 is configured with a native VLAN of 1, but the router connecting to it expects the native VLAN to match. In a router-on-a-stick scenario, the native VLAN must be consistent between the switch and the router; otherwise, traffic on the native VLAN will not be properly tagged or understood. The correct fix is to configure the native VLAN on the trunk to match the router's subinterface configuration, typically changing it to VLAN 1 or ensuring both sides agree. The other options are incorrect because VLANs 10 and 20 are active, the trunk is up, and the native VLAN mismatch is the specific issue.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
VLAN 10 is not allowed on the trunk
- ✓
The native VLAN on the trunk is mismatched with the router
Why this is correct
The switch trunk has native VLAN 1, but the router's subinterface likely expects a different native VLAN, causing traffic to be misidentified. Changing the native VLAN on either side to match resolves the issue.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
- ✗
The trunk encapsulation is not set to dot1q
Why it's wrong here
The output shows 'switchport trunk encapsulation dot1q' is configured, so this is correct. The issue is not with encapsulation.
- ✗
The switch ports for VLAN 10 and 20 are in the wrong access VLAN
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The native VLAN on the trunk is mismatched with the routerCorrect answer▾
Why this is correct
The switch trunk has native VLAN 1, but the router's subinterface likely expects a different native VLAN, causing traffic to be misidentified. Changing the native VLAN on either side to match resolves the issue.
✗VLAN 10 is not allowed on the trunkWrong answer — click to see why▾
Why this is wrong here
The trunk is not explicitly configured to disallow VLAN 10; by default, all VLANs are allowed.
✗The trunk encapsulation is not set to dot1qWrong answer — click to see why▾
Why this is wrong here
The encapsulation is explicitly set to dot1q, so this is not the problem.
✗The switch ports for VLAN 10 and 20 are in the wrong access VLANWrong answer — click to see why▾
Why this is wrong here
The access ports are correctly assigned to their respective VLANs, so this is not the cause.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Trap categories for this question
Command / output trap
The show vlan brief output shows VLAN 10 is active and ports are assigned, but does not indicate trunk allowed VLANs. The trunk is configured with default allowed VLANs, so this is not the issue.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Access ports place end devices into a single VLAN.
What is the correct answer to this question?
The correct answer is: The native VLAN on the trunk is mismatched with the router — The root cause is that the trunk interface GigabitEthernet0/1 is configured with a native VLAN of 1, but the router connecting to it expects the native VLAN to match. In a router-on-a-stick scenario, the native VLAN must be consistent between the switch and the router; otherwise, traffic on the native VLAN will not be properly tagged or understood. The correct fix is to configure the native VLAN on the trunk to match the router's subinterface configuration, typically changing it to VLAN 1 or ensuring both sides agree. The other options are incorrect because VLANs 10 and 20 are active, the trunk is up, and the native VLAN mismatch is the specific issue.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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