CCNA Network Services and Security Practice Question
A network engineer is troubleshooting a Cisco IOS router that should forward DHCP requests from a LAN segment to a centralized DHCP server at 10.1.1.10. The router's LAN interface is GigabitEthernet0/1 with IP address 192.168.20.1/24. The engineer enters the command ip helper-address 10.1.1.10 on interface GigabitEthernet0/1. However, clients on the LAN are not receiving IP addresses. Which condition would cause the DHCP relay to fail?
⚠ Common exam trap
The trap here is assuming that because the DHCP server is on a different subnet, relay cannot work, when in fact relay is designed for that exact situation and the real issue is often routing.
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 route to reach the DHCP server at 10.1.1.10, so it cannot forward the relayed packets.
DHCP relay requires the router to have a route to the DHCP server so it can unicast the client's request. Without a route, the relayed packets are dropped, and clients receive no address. While other conditions like ACLs or disabled service dhcp could interfere, the absence of a route is a fundamental and sufficient cause for relay failure in this scenario.
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 DHCP server is on a different subnet than the LAN interface, so relay is not possible.
Why it's wrong here
DHCP relay is specifically designed to forward requests to a server on a different subnet. The ip helper-address command allows the router to unicast the client's broadcast to the server's IP address. The server being on a different subnet is the normal use case and does not cause failure.
- ✗
The router's LAN interface is configured with the ip helper-address command but the global configuration is missing the service dhcp command.
Why it's wrong here
The service dhcp command enables the DHCP server and relay agent functionality on the router. It is enabled by default on most Cisco IOS versions, but if it were disabled, the relay would not function. However, this is not the most common cause and is not directly indicated as missing in the scenario. The scenario states the command was entered, implying basic configuration was attempted.
- ✗
An access list applied to the LAN interface is blocking inbound UDP port 67 traffic from the DHCP clients.
Why it's wrong here
DHCP clients send from UDP port 68 to destination UDP port 67. An inbound access list blocking UDP port 67 would block server-to-server or relay-to-server traffic, not the initial client broadcast. However, if an ACL blocks UDP 67 inbound on the LAN interface, it would prevent the router from receiving the client's DHCPDISCOVER, which is sent to destination port 67. This could cause failure, but it is a less direct cause than a missing route to the DHCP server. The scenario does not mention ACLs, so this is speculative.
- ✓
The router does not have a route to reach the DHCP server at 10.1.1.10, so it cannot forward the relayed packets.
Why this is correct
For the router to relay DHCP requests, it must have a route to the DHCP server's IP address. If no route exists, the router cannot forward the unicast packets, and clients will not receive leases. This is a common and definitive cause of relay failure. The scenario implies connectivity may be missing, and this condition directly prevents the relay from working.
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Related to this question
Learn chapter
Lab: Configure DHCP Relay
Key term
CAN
A CAN (Controller Area Network) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
Key term
DHCP
Dynamic Host Configuration Protocol (DHCP) automatically assigns IP addresses and network settings to devices on a network, so they can communicate without manual configuration.
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JA
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
This 200-301 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 200-301 exam.