Question 484 of 1,020

Common Networking Hardware →hardMultiple ChoiceObjective-mapped

Inter-VLAN Routing Misconfiguration: Trunk vs Access Port

This 220-1201 practice question tests your understanding of common networking hardware. 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 company's network uses a managed switch with VLANs. Users in VLAN 10 can ping the default gateway but cannot communicate with users in VLAN 20. The switch is connected to a router that handles inter-VLAN routing. What is the most likely misconfiguration?

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.

A
The router's firewall is blocking traffic between VLANs.
Why wrong: While possible, the question implies the router is set up for inter-VLAN routing. A firewall block would be a deliberate policy, not a common misconfiguration.
B
The switch port connecting to the router is configured as an access port instead of a trunk.
An access port carries traffic for only one VLAN. For the router to receive traffic from multiple VLANs, the port must be a trunk port that tags frames with VLAN IDs.
C
The default gateway for VLAN 20 is misconfigured.
Why wrong: Users can ping the default gateway, so the gateway is reachable. The issue is communication between VLANs, which requires the router to receive traffic from both.
D
The switch has Spanning Tree Protocol enabled, blocking the link.
Why wrong: STP would block redundant links, not a single link to the router. If it were blocking, users in VLAN 10 might not even reach the gateway.

Full breakdown with real-world context →

Quick Answer

The answer is that the switch port connecting to the router is configured as an access port instead of a trunk. For inter-VLAN routing to function, the router must receive traffic tagged with multiple VLAN IDs, which requires the switch port to operate in trunk mode, allowing all VLAN frames to pass. An access port, by contrast, strips VLAN tags and only carries traffic for a single VLAN, effectively isolating VLAN 20 from the router and breaking communication between VLANs. On the CompTIA A+ Core 1 220-1201 exam, this scenario tests your grasp of VLAN tagging and the physical link between a router and a managed switch—a common trap is assuming any port can route between VLANs. Remember the memory tip: “Trunk for trucking traffic between VLANs; access for a single address.”

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 switch port connecting to the router is configured as an access port instead of a trunk.

The most likely misconfiguration is that the switch port connecting to the router is configured as an access port instead of a trunk. For inter-VLAN routing to work, the router must receive traffic from multiple VLANs on the same physical interface. A trunk port (using 802.1Q tagging) allows frames from VLAN 10 and VLAN 20 to be forwarded to the router, which then routes between them. An access port can only carry traffic for a single VLAN, so frames from VLAN 20 would be dropped or misdirected, preventing communication.

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 router's firewall is blocking traffic between VLANs.
Why it's wrong here
While possible, the question implies the router is set up for inter-VLAN routing. A firewall block would be a deliberate policy, not a common misconfiguration.
✓
The switch port connecting to the router is configured as an access port instead of a trunk.
Why this is correct
An access port carries traffic for only one VLAN. For the router to receive traffic from multiple VLANs, the port must be a trunk port that tags frames with VLAN IDs.
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 default gateway for VLAN 20 is misconfigured.
Why it's wrong here
Users can ping the default gateway, so the gateway is reachable. The issue is communication between VLANs, which requires the router to receive traffic from both.
✗
The switch has Spanning Tree Protocol enabled, blocking the link.
Why it's wrong here
STP would block redundant links, not a single link to the router. If it were blocking, users in VLAN 10 might not even reach the gateway.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that inter-VLAN routing can work with an access port if the router's interface is configured with an IP address, but the trap is that an access port strips VLAN tags, so the router cannot differentiate between VLANs, breaking communication.

Detailed technical explanation

How to think about this question

Inter-VLAN routing requires a router-on-a-stick configuration where the switch port is set to trunk mode (e.g., 'switchport mode trunk') and the router subinterfaces are configured with encapsulation dot1Q and IP addresses matching each VLAN's subnet. Without 802.1Q tagging, frames from VLAN 20 are either untagged or tagged incorrectly, so the router cannot distinguish them from VLAN 10 traffic. In real-world scenarios, this misconfiguration often occurs when a technician assumes a single VLAN can be carried over an access port to the router, forgetting that trunking is mandatory for multiple VLANs.

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.

Visual reference

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 220-1201 question test?

Common Networking Hardware — This question tests Common Networking Hardware — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The switch port connecting to the router is configured as an access port instead of a trunk. — The most likely misconfiguration is that the switch port connecting to the router is configured as an access port instead of a trunk. For inter-VLAN routing to work, the router must receive traffic from multiple VLANs on the same physical interface. A trunk port (using 802.1Q tagging) allows frames from VLAN 10 and VLAN 20 to be forwarded to the router, which then routes between them. An access port can only carry traffic for a single VLAN, so frames from VLAN 20 would be dropped or misdirected, preventing communication.

What should I do if I get this 220-1201 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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Same concept, more angles

1 more ways this is tested on 220-1201

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A user reports that their VoIP phone works but the connected PC cannot access the network. The phone is connected to the wall jack, and the PC is connected to the phone's passthrough port. What is the most likely cause?

medium

A.The PC's network cable is faulty.
B.The VoIP phone's passthrough port is disabled.
✓ C.The switch port is not configured for the data VLAN, so the PC cannot get an IP address.
D.The PC's network adapter is set to a static IP address that conflicts with the phone.

Why C: The VoIP phone and PC share a single physical switch port via the phone's passthrough port. For the PC to access the network, the switch port must be configured as a trunk (or access port with voice VLAN) carrying both the voice VLAN (for the phone) and the data VLAN (for the PC). If the switch port is not configured for the data VLAN, the PC will not receive an IP address from the DHCP server and cannot communicate on the network, even though the phone works because it uses its own voice VLAN.

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