mediumMultiple Choice
350-401 Practice Question: Review the following DHCP relay configuration:…
Review the following DHCP relay configuration: ```
interface Vlan10 ip address 192.168.10.1 255.255.255.0 ip helper-address 172.16.1.100
``` What is the effect of the 'ip helper-address' command?
⚠ Common exam trap
Cisco often tests the misconception that 'ip helper-address' translates the source IP address of the DHCP request, when in fact it modifies the giaddr field to enable the server to reply correctly.
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
✓
It forwards DHCP requests from VLAN 10 to the DHCP server at 172.16.1.100.
The 'ip helper-address' command configures the router to act as a DHCP relay agent. It intercepts DHCP broadcast requests from clients on VLAN 10 (subnet 192.168.10.0/24) and unicasts them to the specified DHCP server at 172.16.1.100, allowing the server to assign an IP address from a different subnet. This is defined in RFC 1542 and is essential for centralized DHCP services across multiple VLANs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
It forwards DHCP requests from VLAN 10 to the DHCP server at 172.16.1.100.
Why this is correct
The ip helper-address command on the VLAN 10 SVI enables DHCP relay: the router intercepts the client's Layer 3 broadcast DHCPDISCOVER on interface gi0/0.10, rewrites it as a unicast packet, and forwards it to the configured server at 172.16.1.100. It also populates the giaddr (gateway IP address) field with its own interface IP (192.168.10.1), which tells the server exactly which subnet the client lies on so the correct scope is selected. The server's DHCPOFFER is then routed back to the relay agent, which broadcasts it to the client on VLAN 10.
- ✗
It configures the router as a DHCP server for VLAN 10.
Why it's wrong here
This command does not enable DHCP server functionality. A router configured as a DHCP server uses the 'ip dhcp pool' and 'network' commands to define address scopes, leases, and options; no scope or pool is being defined here. Instead, 'ip helper-address' simply forwards existing DHCP packets from clients to an external server, leaving all address assignment decisions to that remote server. The presence of a helper address on the SVI is a relay/forwarding configuration, not a server configuration.
- ✗
It translates the source IP of DHCP requests to 192.168.10.1.
Why it's wrong here
The helper-address command does not perform any IP address translation, and it is not a NAT operation. While the relay agent does use its own interface IP as the source IP for the unicast forwarded DHCP packet (per RFC 1542), the original client's MAC address and the giaddr field are what the server uses to reply; there is no rewriting or translation of the client's source IP address in the DHCP payload. If NAT were intended, you would use an 'ip nat' policy, which is a separate concept entirely and would be configured with different commands. The command itself only configures forwarding to a specific unicast address.
- ✗
It blocks DHCP traffic from VLAN 10.
Why it's wrong here
The helper-address command causes the router to actively forward DHCP broadcasts, not block or filter them. In fact, without this command, DHCP client broadcasts would still be received by the router but would simply be dropped because routers do not forward Layer 3 broadcasts by default. The command specifically enables the router to act as a relay, so it is the opposite of blocking; it expands the reach of the client's broadcast to a remote subnet. Any blocking would be done by an ACL or a DHCP snooping feature, not by a helper address.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 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 350-401 exam.