The answer is to disable Dynamic Trunking Protocol (DTP) on trunk ports and set the native VLAN to an unused VLAN. Disabling DTP prevents an attacker from negotiating a trunk link from an access port, which is the primary vector for switch spoofing attacks. Setting the native VLAN to an unused VLAN mitigates double-tagging attacks, where an attacker sends frames with two 802.1Q tags; the first tag is stripped by the trunk’s native VLAN, and the second tag allows the frame to hop to a different VLAN, but with no hosts on the unused native VLAN, the double-tagged traffic has no target. On the CISSP exam, this tests your understanding of Layer 2 security controls within the Communication and Network Security domain, often appearing as a multi-select question where a common trap is to confuse disabling DTP with disabling trunking entirely. Remember the mnemonic “DUD” for Defense: Disable DTP, Use an Unused native VLAN, and Disable unused ports.
CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 administrator is configuring switches to prevent VLAN hopping attacks. Which TWO of the following measures should be implemented?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Set the native VLAN to an unused VLAN.
Setting the native VLAN to an unused VLAN prevents VLAN hopping via double-tagging attacks. In a double-tagging attack, an attacker sends frames with two 802.1Q tags; the first tag is stripped by the trunk's native VLAN, and the second tag allows the frame to hop to a different VLAN. By using an unused VLAN as the native VLAN, there are no hosts on that VLAN to receive or exploit the double-tagged traffic.
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.
✗
Use private VLANs on all trunk ports.
Why it's wrong here
Private VLANs isolate traffic within a VLAN but do not prevent VLAN hopping attacks.
✓
Set the native VLAN to an unused VLAN.
Why this is correct
This prevents double-tagging attacks by ensuring the native VLAN is not used by any user traffic.
Related concept
Read the scenario before looking for a memorised answer.
Disable Dynamic Trunking Protocol (DTP) on trunk ports.
Why this is correct
DTP can be exploited to negotiate trunks; disabling it prevents unauthorized trunk establishment.
Related concept
Read the scenario before looking for a memorised answer.
✗
Implement port security on all access ports.
Why it's wrong here
Port security limits MAC addresses per port but does not prevent VLAN hopping.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests the distinction between access port security features (like BPDU guard and port security) and trunk-specific controls (like DTP disablement and native VLAN configuration), leading candidates to mistakenly select access port protections for a trunk-based attack.
Detailed technical explanation
How to think about this question
VLAN hopping attacks primarily exploit two mechanisms: DTP negotiation (allowing an attacker to become a trunk neighbor) and double-tagging (where an attacker sends frames with two 802.1Q tags, leveraging the native VLAN to bypass trunk filtering). Disabling DTP on trunk ports (using 'switchport nonegotiate') prevents an attacker from negotiating a trunk link, while setting the native VLAN to an unused VLAN ensures that double-tagged frames are not forwarded to any active host. In real-world scenarios, attackers often connect to an access port and use DTP to automatically negotiate trunking if left enabled, making DTP disablement critical.
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 security team runs a vulnerability scan on a web application and discovers an unpatched SQL injection flaw. The team prioritises remediation by CVSS score — critical flaws are patched within 24 hours, high within 7 days. Questions like this test whether you understand vulnerability management processes, scanning tools, and remediation prioritisation.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Set the native VLAN to an unused VLAN. — Setting the native VLAN to an unused VLAN prevents VLAN hopping via double-tagging attacks. In a double-tagging attack, an attacker sends frames with two 802.1Q tags; the first tag is stripped by the trunk's native VLAN, and the second tag allows the frame to hop to a different VLAN. By using an unused VLAN as the native VLAN, there are no hosts on that VLAN to receive or exploit the double-tagged traffic.
What should I do if I get this CISSP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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