- A
The switch does not have an 'ip helper-address' configured to forward DHCP requests to the server.
Correct because the DHCP server is in VLAN 10, but clients may be in a different VLAN, requiring a helper address.
- B
The interface GigabitEthernet0/1 should be configured as an untrusted port for DHCP snooping.
Why wrong: Incorrect because the server port must be trusted to allow DHCP replies.
- C
The switch has DHCP snooping rate limiting enabled, which is dropping all DHCP packets.
Why wrong: Incorrect because rate limiting is not mentioned and would not prevent all DHCP packets.
- D
The DHCP server is connected to a port in a different VLAN, and DHCP snooping only works within the same VLAN.
Why wrong: Incorrect because DHCP snooping works across VLANs with a helper address.
CCNP Infrastructure Security Practice Question
This 350-401 practice question tests your understanding of infrastructure security. 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 network engineer is implementing DHCP snooping on a Cisco switch to prevent rogue DHCP servers. The switch has multiple VLANs, and the DHCP server is connected to interface GigabitEthernet0/1 in VLAN 10. The engineer enables DHCP snooping globally and for VLAN 10, then configures 'ip dhcp snooping trust' on GigabitEthernet0/1. However, clients in VLAN 10 are not receiving IP addresses. The engineer checks the DHCP snooping binding table and sees no entries. What is the most likely cause?
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.
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 does not have an 'ip helper-address' configured to forward DHCP requests to the server.
DHCP snooping requires the DHCP server port to be trusted. If the server is on a different VLAN than the clients, the switch must also have IP routing enabled or use a DHCP relay. However, the scenario does not mention a relay. The most likely cause is that the DHCP server is not on the same subnet as the clients, and no IP helper address is configured. Option A is correct because without a helper address, DHCP broadcasts are not forwarded to the server. Option B is incorrect because the trust configuration is correct. Option C is incorrect because rate limiting is not configured. Option D is incorrect because DHCP snooping does not require a specific VLAN for the server port.
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.
- ✓
The switch does not have an 'ip helper-address' configured to forward DHCP requests to the server.
- ✗
The interface GigabitEthernet0/1 should be configured as an untrusted port for DHCP snooping.
Why it's wrong here
Incorrect because the server port must be trusted to allow DHCP replies.
- ✗
The switch has DHCP snooping rate limiting enabled, which is dropping all DHCP packets.
Why it's wrong here
Incorrect because rate limiting is not mentioned and would not prevent all DHCP packets.
- ✗
The DHCP server is connected to a port in a different VLAN, and DHCP snooping only works within the same VLAN.
Why it's wrong here
Incorrect because DHCP snooping works across VLANs with a helper address.
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.
What to study next
Got this wrong? Here's your next step.
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 350-401 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 350-401 question test?
Infrastructure Security — This question tests Infrastructure Security — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The switch does not have an 'ip helper-address' configured to forward DHCP requests to the server. — DHCP snooping requires the DHCP server port to be trusted. If the server is on a different VLAN than the clients, the switch must also have IP routing enabled or use a DHCP relay. However, the scenario does not mention a relay. The most likely cause is that the DHCP server is not on the same subnet as the clients, and no IP helper address is configured. Option A is correct because without a helper address, DHCP broadcasts are not forwarded to the server. Option B is incorrect because the trust configuration is correct. Option C is incorrect because rate limiting is not configured. Option D is incorrect because DHCP snooping does not require a specific VLAN for the server port.
What should I do if I get this 350-401 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 350-401 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 18, 2026
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