300-410 Infrastructure Services Practice Question
A network engineer is configuring a Cisco router to act as a DHCP server for a remote subnet. The router's interface connected to the remote subnet is configured with the `ip helper-address` command pointing to the DHCP server. However, clients on the remote subnet are not receiving IP addresses. The engineer verifies that the DHCP server is operational and has a valid pool for the remote subnet. What is the most likely cause of the problem?
⚠ Common exam trap
The trap here is focusing on the router configuration and overlooking the need for a return route on the DHCP server.
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 DHCP server does not have a route back to the remote subnet.
When using `ip helper-address` to forward DHCP requests to a server on a different subnet, the DHCP server must have a route back to the client subnet. The server uses this route to send DHCPOFFER and DHCPACK messages. Without it, the server cannot reach the clients, and they will not receive IP addresses. Ensuring the server has a route to the remote subnet resolves the issue.
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 to the wrong interface.
Why it's wrong here
The `ip helper-address` should be applied to the interface facing the clients (the remote subnet interface). If it were applied to the wrong interface, the DHCP requests would not be forwarded. However, the scenario states it is applied to the interface connected to the remote subnet, which is correct. So this is not the cause.
- ✗
The router's interface connected to the remote subnet is down.
Why it's wrong here
If the interface were down, clients would not be able to send DHCPDISCOVER messages to the router, and the `ip helper-address` would not function. However, the scenario implies the interface is up because the engineer is troubleshooting client addressing. Typically, if the interface were down, the engineer would notice immediately. The most likely cause is a missing route on the server.
- ✗
The DHCP server is configured with a different subnet mask than the clients.
Why it's wrong here
The subnet mask in the DHCP pool must match the subnet configured on the router interface. If it doesn't, clients might receive incorrect subnet information, but they would still receive an IP address. The issue described is that clients are not receiving addresses at all, which points to a communication problem rather than a mask mismatch.
- ✓
The DHCP server does not have a route back to the remote subnet.
Why this is correct
For DHCP to work across subnets, the DHCP server must have a route to the remote subnet to send the DHCPOFFER and DHCPACK messages. If the server lacks a route, it cannot respond to the client's request. The `ip helper-address` forwards the initial DHCPDISCOVER, but the server's reply must be routable back to the client's subnet. Thus, a missing route on the server is a common cause.
Visual reference
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Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
DHCP Client and Server
A DHCP client is any device that requests network configuration settings from a DHCP server, which automatically assigns IP addresses and other parameters to devices on a network.
Key term
DHCP Relay Configuration
A DHCP relay configuration allows a router to forward DHCP broadcast requests from clients on one subnet to a DHCP server on a different subnet, enabling centralized IP address management across multiple networks.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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