The answer is a native VLAN mismatch between the switch and the router, specifically where the switch expects VLAN 1 as the native VLAN but the router’s subinterface is configured to treat VLAN 10 as native. This mismatch causes the router to successfully communicate within VLAN 10 because the switch is sending untagged frames on the native VLAN, which the router interprets as belonging to its VLAN 10 subinterface, while all other VLANs (20 and 30) are tagged and dropped due to the inconsistency in how the native VLAN is handled across the EtherChannel. On the ENCOR 350-401 exam, this scenario tests your understanding of how 802.1Q trunking interacts with router-on-a-stick subinterfaces and EtherChannel—a common trap is assuming all VLANs fail equally, but a native mismatch only breaks non-native VLANs. Remember the mnemonic: “Native is the odd one out; if it matches, all routes shout; if it’s off, only one VLAN is boss.”
350-401 EtherChannel Practice Question
This 350-401 practice question tests your understanding of etherchannel. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 engineer is configuring an EtherChannel between a Cisco switch and a router. The switch ports are configured as trunk ports with allowed VLANs 10, 20, and 30. The router is configured with subinterfaces for each VLAN. After configuring the EtherChannel, the engineer notices that the router can ping devices in VLAN 10 but not in VLAN 20 or 30. 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.
The native VLAN is configured as VLAN 1 on the switch but VLAN 10 on the router.
Correct because a native VLAN mismatch can cause connectivity issues for some VLANs, especially if the native VLAN is used for management or control traffic.
B
The allowed VLAN list on the switch does not include VLAN 20 and 30.
Why wrong: Incorrect because the scenario says the switch ports are configured with allowed VLANs 10, 20, and 30.
C
The router subinterfaces are configured with the wrong encapsulation.
Why wrong: Incorrect because if encapsulation were wrong, none of the VLANs would work.
D
The EtherChannel is configured with 'lacp system-priority' that is too high.
Why wrong: Incorrect because system-priority affects LACP negotiation, not VLAN connectivity.
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 as VLAN 1 on the switch but VLAN 10 on the router.
The correct answer is that the native VLAN mismatch is causing issues. The wrong answers involve misconfigurations that would affect all VLANs or are unrelated.
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 native VLAN is configured as VLAN 1 on the switch but VLAN 10 on the router.
Why this is correct
Correct because a native VLAN mismatch can cause connectivity issues for some VLANs, especially if the native VLAN is used for management or control traffic.
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 allowed VLAN list on the switch does not include VLAN 20 and 30.
Why it's wrong here
Incorrect because the scenario says the switch ports are configured with allowed VLANs 10, 20, and 30.
✗
The router subinterfaces are configured with the wrong encapsulation.
Why it's wrong here
Incorrect because if encapsulation were wrong, none of the VLANs would work.
✗
The EtherChannel is configured with 'lacp system-priority' that is too high.
Why it's wrong here
Incorrect because system-priority affects LACP negotiation, not VLAN connectivity.
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
Scenario analysis trap
Incorrect because the scenario says the switch ports are configured with allowed VLANs 10, 20, and 30.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
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 350-401 questions on switching, trunking, and access-port configuration.
EtherChannel — This question tests EtherChannel — 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 as VLAN 1 on the switch but VLAN 10 on the router. — The correct answer is that the native VLAN mismatch is causing issues. The wrong answers involve misconfigurations that would affect all VLANs or are unrelated.
What should I do if I get this 350-401 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 350-401 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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