The answer is to set the native VLAN to an unused VLAN ID on all trunk ports and to disable Dynamic Trunking Protocol (DTP) on all access ports. These two methods directly close the attack vectors used in VLAN hopping: the first prevents an attacker from exploiting the native VLAN to hop between VLANs via double-tagging, while the second stops a switch port from automatically negotiating a trunk, which an attacker could spoof to gain access to multiple VLANs. On the ENCOR 350-401 exam, this topic tests your understanding of Layer 2 security fundamentals, often appearing in a multiple-select question where a common trap is to confuse disabling DTP on trunk ports (which is not needed) versus access ports. A reliable memory tip is to think of the native VLAN as the “default key” to all VLANs—if you leave it as VLAN 1, you’re handing out that key; moving it to an unused ID locks the door.
350-401 Security Practice Question
This 350-401 practice question tests your understanding of 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.
Which TWO of the following are valid methods to mitigate VLAN hopping attacks?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Disable Dynamic Trunking Protocol (DTP) on all access ports.
Option B is correct because disabling Dynamic Trunking Protocol (DTP) on all access ports prevents a switch port from automatically negotiating a trunk, which is the primary vector for VLAN hopping attacks. An attacker can spoof DTP messages to force a port into trunking mode, gaining access to multiple VLANs; disabling DTP eliminates this risk.
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.
✗
Configure switchport mode dynamic auto on all ports.
Read the scenario before looking for a memorised answer.
✗
Use 802.1Q trunking instead of ISL.
Why it's wrong here
Both are susceptible; switching encapsulation is not a mitigation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that simply using 802.1Q trunking (Option E) or setting the native VLAN to VLAN 1 (Option C) provides security, when in fact the key mitigations are disabling DTP on access ports and using an unused native VLAN on trunk ports.
Detailed technical explanation
How to think about this question
VLAN hopping attacks exploit DTP to negotiate a trunk (DTP spoofing) or use double-tagging to send frames with two 802.1Q tags, where the outer tag is stripped by the first switch and the inner tag allows the frame to reach a different VLAN. Disabling DTP on access ports with 'switchport nonegotiate' or setting the port to 'switchport mode access' stops trunk negotiation, while setting the native VLAN to an unused VLAN ID (Option D) prevents double-tagging attacks because the attacker's frame would be tagged with the native VLAN, which is not used for any real traffic.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Security — This question tests Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Disable Dynamic Trunking Protocol (DTP) on all access ports. — Option B is correct because disabling Dynamic Trunking Protocol (DTP) on all access ports prevents a switch port from automatically negotiating a trunk, which is the primary vector for VLAN hopping attacks. An attacker can spoof DTP messages to force a port into trunking mode, gaining access to multiple VLANs; disabling DTP eliminates this risk.
What should I do if I get this 350-401 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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