The correct answer is to implement an ACL that denies traffic from the guest VLAN to the corporate VLAN. This is necessary because, while separate VLANs logically segment broadcast domains, routers and layer-3 switches still perform inter-VLAN routing by default, meaning traffic can flow between the guest and corporate networks unless explicitly blocked. An Access Control List applied to the VLAN interface or firewall rule set acts as a filter, inspecting packets and dropping any that originate from the guest VLAN and are destined for the corporate subnet, thereby enforcing the isolation policy. On the CompTIA A+ Core 1 220-1201 exam, this concept tests your understanding that VLANs alone do not guarantee security—they require ACLs or firewall rules to prevent unauthorized inter-VLAN traffic. A common trap is assuming a separate SSID and VLAN are sufficient for complete isolation, but without an ACL, guest devices could still reach internal resources through the router. Remember the mnemonic: "VLANs split the road, ACLs block the load."
220-1101 Network Configuration Concepts Practice Question
This 220-1201 practice question tests your understanding of network configuration concepts. 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 company's IT policy requires that all network traffic from the guest Wi-Fi be isolated from the internal corporate network. The guest network uses a separate SSID and VLAN. Which additional configuration is necessary to ensure complete isolation?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Implement an ACL that denies traffic from the guest VLAN to the corporate VLAN
Access Control Lists (ACLs) on the router or firewall can block traffic between VLANs. Even with separate VLANs, inter-VLAN routing must be explicitly denied to prevent guest devices from reaching the corporate network.
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.
✗
Enable DHCP snooping on the guest VLAN
Why it's wrong here
DHCP snooping prevents rogue DHCP servers but does not isolate traffic between VLANs; ACLs are needed for that.
✗
Configure a router-on-a-stick with subinterfaces
Why it's wrong here
This allows routing between VLANs, which is the opposite of isolation; without ACLs, it would actually enable communication.
✓
Implement an ACL that denies traffic from the guest VLAN to the corporate VLAN
Why this is correct
An ACL on the router or Layer 3 switch can block all traffic from the guest VLAN to the corporate VLAN, ensuring isolation.
Related concept
Access ports place end devices into a single VLAN.
✗
Set the guest network to use a different subnet mask
Why it's wrong here
Different subnet masks alone do not prevent routing; the router will still forward traffic unless explicitly blocked by ACLs.
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.
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 220-1201 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 220-1201 questions on switching, trunking, and access-port configuration.
Network Configuration Concepts — This question tests Network Configuration Concepts — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: Implement an ACL that denies traffic from the guest VLAN to the corporate VLAN — Access Control Lists (ACLs) on the router or firewall can block traffic between VLANs. Even with separate VLANs, inter-VLAN routing must be explicitly denied to prevent guest devices from reaching the corporate network.
What should I do if I get this 220-1201 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 220-1201 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
About these practice questions
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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 technician is configuring a new wireless router for a home office. The customer wants to ensure that guests can access the internet but cannot see other devices on the main network. Which feature should the technician enable on the router?
easy
A.MAC address filtering
B.Port forwarding
✓ C.Guest network
D.WPS (Wi-Fi Protected Setup)
Why C: A guest network creates a separate SSID and VLAN that isolates guest traffic from the main network, allowing internet access while preventing access to local devices. This is a common feature on modern routers for security and privacy.
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Question Discussion
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