300-410 Infrastructure Services Practice Question
A network engineer configures a Cisco IOS router as a DHCP relay agent. The router interface connected to the DHCP clients is configured with 'ip helper-address 192.168.1.10'. Which type of traffic will be forwarded to the DHCP server at 192.168.1.10 by default?
⚠ Common exam trap
The trap here is assuming that the helper address only forwards DHCP broadcasts, when in fact it forwards a broader set of UDP services by default.
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
✓
UDP broadcasts for DHCP, TFTP, DNS, NetBIOS, and several other services.
The 'ip helper-address' command forwards UDP broadcasts for a default set of services, including DHCP, TFTP, DNS, NetBIOS, and others. This allows clients to communicate with servers on different subnets. The command is not limited to DHCP, nor does it forward all broadcasts.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Only unicast packets destined to the DHCP server's IP address.
Why it's wrong here
The helper address is used to forward broadcast packets, not unicast. DHCP clients send broadcast DISCOVER messages, which the relay agent converts to unicast and forwards to the helper address. Unicast packets from clients to the server would be routed normally without the helper address.
- ✗
Only UDP broadcasts for DHCP (ports 67 and 68).
Why it's wrong here
The 'ip helper-address' command forwards several types of UDP broadcasts by default, not just DHCP. While DHCP is one of them, other services such as TFTP, DNS, and NetBIOS are also forwarded. Therefore, stating that only DHCP is forwarded is incorrect.
- ✓
UDP broadcasts for DHCP, TFTP, DNS, NetBIOS, and several other services.
Why this is correct
By default, the 'ip helper-address' command forwards UDP broadcasts for a predefined set of services, including DHCP (ports 67/68), TFTP (69), DNS (53), NetBIOS (137/138), and others like BOOTP and TACACS. This allows clients to reach servers on different subnets without additional configuration.
- ✗
All IP broadcasts, including DHCP, TFTP, DNS, and NetBIOS.
Why it's wrong here
The helper address does not forward all IP broadcasts. It forwards a specific set of UDP broadcasts, and only if they match the default list. For example, broadcasts for other services not in the list are not forwarded. Also, it does not forward broadcasts for TCP or other protocols.
Visual reference
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Related to this question
Learn chapter
ACL-Based Traffic Filtering and Policy-Based Routing
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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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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