Question 1,215 of 2,015

NAT and DHCP →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the router lacks a return route to the client subnet, causing the DHCP server’s reply to be dropped. When you configure the ip helper-address command on a VLAN interface, the router converts the client’s DHCP broadcast into a unicast and forwards it to the specified server. However, the DHCP server sends its offer back to the router’s IP address as the source, and if the router has no route back to the client’s subnet, that reply is discarded—leaving the client without an address. On the ENCOR 350-401 exam, this scenario tests your understanding that DHCP relay is a two-way process: forwarding the request is only half the job; the router must also be able to route the server’s response back to the client. A common trap is assuming reachability to the server alone is sufficient, but the missing piece is the return route. Remember the memory tip: “Helper forwards, but route returns.”

350-401 NAT and DHCP Practice Question

This 350-401 practice question tests your understanding of nat and dhcp. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 configuring a Cisco router as a DHCP relay agent to forward DHCP requests from a client VLAN to a centralized DHCP server located in a different subnet. The engineer configures the ip helper-address command on the VLAN interface. However, clients in the VLAN are not receiving IP addresses. The DHCP server is reachable from the router. 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.

Question 1mediummultiple choice

Open the full VLAN trunking answer →

A
The ip helper-address command is applied on the wrong interface (e.g., the interface facing the DHCP server).
Why wrong: Incorrect because the command must be applied on the interface that receives the client broadcast, not on the server-facing interface.
B
The DHCP server is not configured with a scope for the client subnet.
Why wrong: Incorrect because the DHCP server would still send a reply (possibly with no offer) but the client would not receive it; however, the most common issue is routing of the reply.
C
The router does not have a return route to the client subnet, so the DHCP server's reply is dropped.
Correct because the DHCP server sends the reply to the relay agent (router), which then forwards it as a broadcast to the client. If the router cannot reach the client subnet, the reply is lost.
D
The DHCP client is using DHCPv6 instead of DHCPv4.
Why wrong: Incorrect because the scenario implies IPv4 DHCP; the ip helper-address command works for DHCPv4 broadcasts.

Full breakdown with real-world context →

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 router does not have a return route to the client subnet, so the DHCP server's reply is dropped.

The ip helper-address command forwards DHCP broadcasts as unicasts to the specified server. If the DHCP server receives the request but the reply cannot be routed back to the client, the client will not get an address. This often happens when the router does not have a route back to the client subnet.

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 applied on the wrong interface (e.g., the interface facing the DHCP server).
Why it's wrong here
Incorrect because the command must be applied on the interface that receives the client broadcast, not on the server-facing interface.
✗
The DHCP server is not configured with a scope for the client subnet.
Why it's wrong here
Incorrect because the DHCP server would still send a reply (possibly with no offer) but the client would not receive it; however, the most common issue is routing of the reply.
✓
The router does not have a return route to the client subnet, so the DHCP server's reply is dropped.
Why this is correct
Correct because the DHCP server sends the reply to the relay agent (router), which then forwards it as a broadcast to the client. If the router cannot reach the client subnet, the reply is lost.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
The DHCP client is using DHCPv6 instead of DHCPv4.
Why it's wrong here
Incorrect because the scenario implies IPv4 DHCP; the ip helper-address command works for DHCPv4 broadcasts.

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.

Trap categories for this question

Command / output trap
Incorrect because the command must be applied on the interface that receives the client broadcast, not on the server-facing interface.
Scenario analysis trap
Incorrect because the scenario implies IPv4 DHCP; the ip helper-address command works for DHCPv4 broadcasts.

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 router does not have a return route to the client subnet, so the DHCP server's reply is dropped. — The ip helper-address command forwards DHCP broadcasts as unicasts to the specified server. If the DHCP server receives the request but the reply cannot be routed back to the client, the client will not get an address. This often happens when the router does not have a route back to the client subnet.

What should I do if I get this 350-401 question wrong?

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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