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300-410 Practice Question: Is troubleshooting a DHCPv4 issue where a router…
A network engineer is troubleshooting a DHCPv4 issue where a router configured as a DHCP server is not assigning addresses to clients on a subnet that is reachable via a different router (relay). The relay router (R2) has 'ip helper-address 10.1.1.1' on its client-facing interface, and the DHCP server is at 10.1.1.1 (R1). The engineer sees that R2 is sending DHCP DISCOVER messages with giaddr set to the client-facing interface IP, but R1 is not responding. R1 has a DHCP pool for the client subnet. The engineer pings 10.1.1.1 from R2 successfully. What is the most likely cause?
⚠ Common exam trap
The trap is focusing on relay-side configuration (Option 82, helper-address) when the actual failure is server-side return routing to the giaddr subnet — candidates forget that DHCP is a bidirectional exchange.
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 to the client subnet (the giaddr subnet).
For DHCP relay to work, the DHCP server must have a route back to the giaddr subnet (the client subnet) so it can send the DHCP OFFER to the relay agent. Since R1 has no route to the client subnet, it drops the request and never responds.
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 does not have a route to the client subnet (the giaddr subnet).
Why this is correct
R1 replies to the DISCOVER by unicasting to the giaddr (the client-facing interface on R2). Without a route back to that subnet, R1 drops the reply, so no offer is sent. R2's successful ping to R1 proves only forward reachability.
- ✗
The relay agent R2 is missing the 'ip dhcp relay information option' command.
Why it's wrong here
The relay information option inserts circuit and remote IDs for DHCP snooping and option 82 handling; it is not required for basic DISCOVER forwarding. It is tempting because option 82 relates to relayed DHCP, but R1's failure to respond stems from pool or giaddr matching, not missing option 82.
- ✗
The DHCP pool on R1 is missing the 'default-router' command.
Why it's wrong here
A missing default-router only omits the gateway address in the DHCP offer; it does not stop R1 from selecting a pool and replying. It is tempting because the client needs a gateway, but the symptom is no response at all, indicating pool selection failure.
- ✗
The 'ip helper-address' on R2 should point to the server's loopback address, not the interface IP.
Why it's wrong here
The helper address correctly targets the server's reachable interface, and giaddr is already being set, so redirecting to a loopback changes nothing. It is tempting because loopback addresses are stable, but reachability is proven by the successful ping.
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Related to this question
Learn chapter
BGP Route Selection and Policy
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.
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.
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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
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