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300-410 Practice Question: Is troubleshooting a DHCPv4 relay scenario where…

A network engineer is troubleshooting a DHCPv4 relay scenario where clients on subnet 10.1.1.0/24 are unable to obtain IP addresses from a DHCP server at 192.168.1.10. The router interface Gi0/0 (10.1.1.1/24) has 'ip helper-address 192.168.1.10' configured. The engineer captures packets and sees DHCP DISCOVER messages sourced from 10.1.1.1 being sent to 192.168.1.10, but no replies are seen. The server is reachable via ping from the router. What is the most likely cause?

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 is not configured with a scope for subnet 10.1.1.0/24.

The DHCP server may not have a route back to the client subnet (10.1.1.0/24) or the server's default gateway does not have a route. The relay agent sets the gateway IP address (giaddr) to the interface IP (10.1.1.1), and the server sends the OFFER back to that giaddr. If the server cannot reach 10.1.1.1, the reply is lost. However, the engineer confirms the server is reachable via ping, so the issue is likely that the server is not configured with a scope for the 10.1.1.0/24 subnet, causing it to drop the DISCOVER.

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 10.1.1.0/24.

    Why it's wrong here

    Incorrect because the server is reachable via ping from the router, indicating routing is functional; the issue is at the application layer.

  • The DHCP server is not configured with a scope for subnet 10.1.1.0/24.

    Why this is correct

    Correct because if the server has no scope for the client subnet, it will ignore the DISCOVER message and not send any reply, even though the relayed packet reaches the server.

  • The 'ip helper-address' command should be configured on the server-facing interface, not the client-facing interface.

    Why it's wrong here

    Incorrect because the helper address must be on the interface facing the clients to intercept their DHCP broadcasts.

  • The router needs the 'ip dhcp relay information option' command.

    Why it's wrong here

    Incorrect because Option 82 is optional and not required for basic relay; its absence does not prevent the server from receiving the DISCOVER.

Visual reference

Client DHCP Server 1 Discover (broadcast) 2 Offer (IP: 192.168.1.10) 3 Request (I accept) 4 Acknowledge (lease confirmed) DORA — the four-step DHCP lease process

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Written by Johnson Ajibi, MSc IT Security

Senior Network & Security Engineer · founder of Courseiva

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