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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

Router# show running-config interface gigabitethernet 0/0
!
interface GigabitEthernet0/0
 no ip address
 duplex auto
 speed auto
!
interface GigabitEthernet0/0.10
 encapsulation dot1Q 10 native
 ip address 10.10.10.1 255.255.255.0
!
interface GigabitEthernet0/0.20
 encapsulation dot1Q 20
 ip address 10.10.20.1 255.255.255.0
!

A network administrator is troubleshooting connectivity between two hosts on different VLANs. Host A (10.10.10.10/24, VLAN 10) and Host B (10.10.20.20/24, VLAN 20) are connected to the same access switch. The access switch is connected via a trunk to a router-on-a-stick running IOS-XE. Host A can ping its default gateway (10.10.10.1), but cannot ping Host B. The router has subinterfaces for VLAN 10 and VLAN 20. 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.

Question 1hardmultiple choice
Full question →

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 is configured on the router subinterface but the switch trunk expects a different native VLAN.

The root cause is that the native VLAN on the trunk is misconfigured. The router subinterface for VLAN 10 is configured as the native VLAN with the 'native' keyword, but the access switch trunk likely expects the native VLAN to be VLAN 1 (default) or another VLAN. This mismatch causes the router to drop frames from VLAN 10 because it expects them untagged, but the switch sends them tagged (or vice versa). Correcting the native VLAN configuration on either the router or the switch to match will restore connectivity. The other options are incorrect because the subinterface IPs are correct, the trunk is up, and the router has inter-VLAN routing enabled by default.

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 router's subinterface for VLAN 10 is missing the 'encapsulation dot1Q 10' command.

    Why it's wrong here

    The exhibit shows that encapsulation dot1Q 10 native is configured on subinterface G0/0.10, so it is present.

  • ✓

    The native VLAN is configured on the router subinterface but the switch trunk expects a different native VLAN.

    Why this is correct

    The router subinterface G0/0.10 has 'native' keyword, indicating VLAN 10 is the native VLAN. If the switch trunk has a different native VLAN (e.g., VLAN 1), frames from VLAN 10 will be mishandled.

    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 router's GigabitEthernet0/0 main interface is missing the 'no shutdown' command.

    Why it's wrong here

    The main interface is up (implicitly) because subinterfaces are operational; the exhibit does not show a shutdown state.

  • ✗

    The router is not configured with 'ip routing' globally, preventing inter-VLAN routing.

    Why it's wrong here

    Router-on-a-stick works with default routing; 'ip routing' is enabled by default on Cisco routers and is not shown disabled.

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 is configured on the router subinterface but the switch trunk expects a different native VLAN.Correct answer▾

Why this is correct

The router subinterface G0/0.10 has 'native' keyword, indicating VLAN 10 is the native VLAN. If the switch trunk has a different native VLAN (e.g., VLAN 1), frames from VLAN 10 will be mishandled.

✗The router's subinterface for VLAN 10 is missing the 'encapsulation dot1Q 10' command.Wrong answer — click to see why▾

Why this is wrong here

The command is present; the issue is the mismatched native VLAN.

✗The router's GigabitEthernet0/0 main interface is missing the 'no shutdown' command.Wrong answer — click to see why▾

Why this is wrong here

No evidence of shutdown; the issue is with native VLAN mismatch.

✗The router is not configured with 'ip routing' globally, preventing inter-VLAN routing.Wrong answer — click to see why▾

Why this is wrong here

IP routing is enabled by default; the problem is at Layer 2, not Layer 3.

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 exhibit shows that encapsulation dot1Q 10 native is configured on subinterface G0/0.10, so it is present.

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 is configured on the router subinterface but the switch trunk expects a different native VLAN. — The root cause is that the native VLAN on the trunk is misconfigured. The router subinterface for VLAN 10 is configured as the native VLAN with the 'native' keyword, but the access switch trunk likely expects the native VLAN to be VLAN 1 (default) or another VLAN. This mismatch causes the router to drop frames from VLAN 10 because it expects them untagged, but the switch sends them tagged (or vice versa). Correcting the native VLAN configuration on either the router or the switch to match will restore connectivity. The other options are incorrect because the subinterface IPs are correct, the trunk is up, and the router has inter-VLAN routing enabled by default.

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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