Question 1hardmultiple choice
Full question →- → Why each wrong option is wrong in this specific scenario
- → When each wrong option would be correct
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Question 860 of 1,052
hardmultiple choiceObjective-mapped
CCNA Practice Question: A network technician is troubleshooting…
This 200-301 practice question tests your understanding of a network technician is troubleshooting…. 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
Router# show running-config | section interface GigabitEthernet0/0 interface GigabitEthernet0/0 no ip address duplex auto speed auto ! interface GigabitEthernet0/0.10 encapsulation dot1Q 10 ip address 10.10.10.1 255.255.255.0 ! interface GigabitEthernet0/0.20 encapsulation dot1Q 20 ip address 10.20.20.1 255.255.255.0 !
A network technician is troubleshooting connectivity between two hosts, Host A (VLAN 10) and Host B (VLAN 20), connected to a single switch. The switch is connected to a router-on-a-stick for inter-VLAN routing. Host A can ping its default gateway (10.10.10.1), but cannot ping Host B (10.20.20.2). The router is configured with subinterfaces for VLANs 10 and 20. Based on the router's running configuration, what 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 router is missing a subinterface for the native VLAN.
The router is configured with subinterfaces for VLANs 10 and 20, but the native VLAN (default VLAN 1) is not explicitly configured on a subinterface. Cisco IOS requires that the native VLAN be configured on a subinterface with the 'encapsulation dot1Q <vlan> native' command to ensure proper handling of untagged frames. Without this, traffic from the native VLAN may be dropped or misrouted, causing inter-VLAN communication to fail. The correct fix is to add a subinterface for the native VLAN (e.g., interface GigabitEthernet0/0.1 with encapsulation dot1Q 1 native) or to change the native VLAN on the trunk to a different VLAN. The other options are not directly related to the symptom.
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.
- ✗
The switch trunk port is not configured with 'switchport trunk allowed vlan 10,20'.
- ✓
The router is missing a subinterface for the native VLAN.
Why this is correct
The router must have a subinterface for the native VLAN (default VLAN 1) with the 'native' keyword to handle untagged frames correctly. Without it, inter-VLAN routing may fail even if other VLANs work.
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 switch port connecting to the router is in access mode instead of trunk mode.
- ✗
The router's subinterface for VLAN 20 has an incorrect IP address.
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 router is missing a subinterface for the native VLAN.Correct answer▾
Why this is correct
The router must have a subinterface for the native VLAN (default VLAN 1) with the 'native' keyword to handle untagged frames correctly. Without it, inter-VLAN routing may fail even if other VLANs work.
✗The switch trunk port is not configured with 'switchport trunk allowed vlan 10,20'.Wrong answer — click to see why▾
Why this is wrong here
The symptom shows partial connectivity, so trunk filtering is not the issue.
✗The switch port connecting to the router is in access mode instead of trunk mode.Wrong answer — click to see why▾
Why this is wrong here
Host A's successful ping to the gateway indicates the trunk is working for VLAN 10.
✗The router's subinterface for VLAN 20 has an incorrect IP address.Wrong answer — click to see why▾
Why this is wrong here
Host A's ability to ping its own gateway shows VLAN 10 routing works, so VLAN 20's IP is likely correct.
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 IP address shown (10.20.20.1) is correct for the gateway of VLAN 20. An incorrect IP would cause Host A to fail to reach the gateway, but it can reach its own gateway.
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 router is missing a subinterface for the native VLAN. — The router is configured with subinterfaces for VLANs 10 and 20, but the native VLAN (default VLAN 1) is not explicitly configured on a subinterface. Cisco IOS requires that the native VLAN be configured on a subinterface with the 'encapsulation dot1Q <vlan> native' command to ensure proper handling of untagged frames. Without this, traffic from the native VLAN may be dropped or misrouted, causing inter-VLAN communication to fail. The correct fix is to add a subinterface for the native VLAN (e.g., interface GigabitEthernet0/0.1 with encapsulation dot1Q 1 native) or to change the native VLAN on the trunk to a different VLAN. The other options are not directly related to the symptom.
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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