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SSCP Practice Question: A company has segmented its network into VLANs…
A company has segmented its network into VLANs for different departments: HR, Finance, and IT. The router interconnecting the VLANs has ACLs configured to block traffic from HR to Finance. However, IT has noticed that traffic from HR VLAN is reaching the Finance VLAN. The network uses managed switches with 802.1Q trunking. All access ports are configured as untagged members of their respective VLANs. What is the most likely cause of this unauthorized traffic flow?
⚠ Common exam trap
ISC2 often tests the distinction between VLAN hopping via double tagging versus switch spoofing; the trap here is that candidates may confuse this with a simple ACL misconfiguration or assume the router is the only point of control, overlooking that the attack occurs at Layer 2 before the router even sees the traffic.
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
✓
VLAN hopping attack via double tagging
The most likely cause is a VLAN hopping attack via double tagging. In this attack, an attacker on the HR VLAN sends a frame with two 802.1Q tags: the outer tag matches the native VLAN of the trunk (often VLAN 1), and the inner tag is the target VLAN (Finance). When the switch receives the frame on an access port, it strips the outer tag (as it is the native VLAN) and forwards the frame with the inner tag over the trunk, allowing the traffic to bypass the ACLs and reach the Finance VLAN.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
DHCP snooping disabled
Why it's wrong here
DHCP snooping prevents rogue DHCP servers, but does not enforce VLAN segmentation.
- ✓
VLAN hopping attack via double tagging
Why this is correct
Double tagging can allow an attacker to send frames to a VLAN other than the source VLAN, potentially bypassing ACLs.
- ✗
Default gateway misconfiguration
Why it's wrong here
A misconfigured default gateway would affect connectivity to other networks, but would not allow traffic to bypass router ACLs.
- ✗
STP misconfiguration
Why it's wrong here
Spanning Tree Protocol misconfiguration could cause loops, not cross-VLAN traffic.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SSCP practice question is part of Courseiva's free ISC2 certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SSCP exam.