- A
The ip helper-address command is used on a router interface to forward DHCP broadcasts to a DHCP server on a different subnet.
Correct because ip helper-address converts DHCP broadcast to unicast and forwards it to the specified server.
- B
DHCP relay changes the source IP address of the DHCP packet to the IP address of the relay agent's outgoing interface.
Correct because the relay agent sets the source IP to its own interface IP so the server can reply back to the relay.
- C
The ip helper-address command forwards only DHCP traffic by default.
Why wrong: Incorrect because ip helper-address forwards multiple UDP broadcasts (e.g., TFTP, DNS, NetBIOS) by default, not just DHCP.
- D
DHCP relay inserts the gateway IP address (giaddr) field in the DHCP packet to indicate the subnet of the client.
Correct because the relay agent sets the giaddr field to its own IP address, allowing the server to allocate an IP from the correct subnet.
- E
DHCP relay is required only when the DHCP server is on the same VLAN as the client.
Why wrong: Incorrect because DHCP relay is needed when the server is on a different subnet or VLAN, not the same one.
350-401 NAT and DHCP Practice Question
This 350-401 practice question tests your understanding of nat and dhcp. 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.
Which three statements about DHCP relay are true? (Choose three.)
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 ip helper-address command is used on a router interface to forward DHCP broadcasts to a DHCP server on a different subnet.
This question tests understanding of DHCP relay operation, including the use of ip helper-address, UDP port forwarding, and configuration requirements.
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 ip helper-address command is used on a router interface to forward DHCP broadcasts to a DHCP server on a different subnet.
- ✓
DHCP relay changes the source IP address of the DHCP packet to the IP address of the relay agent's outgoing interface.
Why this is correct
Correct because the relay agent sets the source IP to its own interface IP so the server can reply back to the relay.
Related concept
Access ports place end devices into a single VLAN.
- ✗
The ip helper-address command forwards only DHCP traffic by default.
- ✓
DHCP relay inserts the gateway IP address (giaddr) field in the DHCP packet to indicate the subnet of the client.
Why this is correct
Correct because the relay agent sets the giaddr field to its own IP address, allowing the server to allocate an IP from the correct subnet.
Related concept
Access ports place end devices into a single VLAN.
- ✗
DHCP relay is required only when the DHCP server is on the same VLAN as the client.
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?
NAT and DHCP — This question tests NAT and DHCP — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The ip helper-address command is used on a router interface to forward DHCP broadcasts to a DHCP server on a different subnet. — This question tests understanding of DHCP relay operation, including the use of ip helper-address, UDP port forwarding, and configuration requirements.
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.
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
This 350-401 practice question is part of Courseiva's free Cisco 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 350-401 exam.
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