- A
The DHCP pool is missing the 'default-router' command.
Why wrong: Incorrect because the DHCP pool configuration is verified correct, and missing default-router would not prevent DISCOVER messages from reaching the server.
- B
The router needs the 'ip dhcp relay information option' command to insert Option 82.
Why wrong: Incorrect because Option 82 is not required for basic DHCP relay; it is used for security and policy, not for basic forwarding.
- C
The 'ip helper-address' command must be configured under the interface with the VRF name using 'ip helper-address vrf <vrf-name> <server-ip>'.
Correct because when the interface is in a VRF, the helper address must specify the VRF to ensure the DHCP broadcast is forwarded into the correct routing table.
- D
The DHCP server is not configured with a scope for the client subnet.
Why wrong: Incorrect because the issue is that DISCOVER messages never reach the server, so the server's scope configuration is irrelevant at this point.
Quick Answer
The answer is that the 'ip helper-address' command must include the VRF name using the syntax 'ip helper-address vrf <vrf-name> <server-ip>'. This is required because when a router interface belongs to a VRF, standard DHCP relay broadcasts are not automatically forwarded across VRF boundaries; the relay agent must be explicitly told which VRF to use for the forwarded unicast packets, otherwise the DHCP DISCOVER messages are silently dropped. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of VRF-aware services and DHCP relay configuration, often appearing as a trick where the engineer correctly configures the helper address but forgets the VRF keyword, leading to a failure that is not obvious from a simple 'show run'. A common memory tip is to think of the VRF as a separate routing table—the helper address needs a "passport" (the VRF keyword) to leave its local table and reach the server. Remember: no VRF keyword, no relay.
300-410 DHCP (IPv4 and IPv6) Practice Question
This 300-410 practice question tests your understanding of dhcp (ipv4 and ipv6). The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network engineer is troubleshooting a DHCPv4 issue on a Cisco router configured as a DHCP server. Clients on VLAN 10 are unable to obtain IP addresses. The engineer verifies that the DHCP pool is correctly configured and that the router interface facing the clients has 'ip helper-address 192.168.1.1' pointing to the DHCP server. However, the DHCP server is on a different subnet and the router's interface is in a VRF. The DHCP DISCOVER messages are not reaching the server. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 'ip helper-address' command must be configured under the interface with the VRF name using 'ip helper-address vrf <vrf-name> <server-ip>'.
The DHCP relay agent (the router) must be configured to forward DHCP broadcasts across VRFs using the 'ip helper-address vrf <name>' command, otherwise the packets are dropped because the interface is in a VRF and the helper address is not VRF-aware.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 pool is missing the 'default-router' command.
Why it's wrong here
Incorrect because the DHCP pool configuration is verified correct, and missing default-router would not prevent DISCOVER messages from reaching the server.
- ✗
The router needs the 'ip dhcp relay information option' command to insert Option 82.
Why it's wrong here
Incorrect because Option 82 is not required for basic DHCP relay; it is used for security and policy, not for basic forwarding.
- ✓
The 'ip helper-address' command must be configured under the interface with the VRF name using 'ip helper-address vrf <vrf-name> <server-ip>'.
Why this is correct
Correct because when the interface is in a VRF, the helper address must specify the VRF to ensure the DHCP broadcast is forwarded into the correct routing table.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
- ✗
The DHCP server is not configured with a scope for the client subnet.
Why it's wrong here
Incorrect because the issue is that DISCOVER messages never reach the server, so the server's scope configuration is irrelevant at this point.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
- Access ports place end devices into a single VLAN.
- Trunk ports carry multiple VLANs between switches.
- Allowed VLAN lists decide which VLANs can cross a trunk.
- Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
- Use show vlan brief to verify access VLANs.
- Use show interfaces trunk to verify trunk state and allowed VLANs.
- Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.
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FAQ
Questions learners often ask
What does this 300-410 question test?
DHCP (IPv4 and IPv6) — This question tests DHCP (IPv4 and IPv6) — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The 'ip helper-address' command must be configured under the interface with the VRF name using 'ip helper-address vrf <vrf-name> <server-ip>'. — The DHCP relay agent (the router) must be configured to forward DHCP broadcasts across VRFs using the 'ip helper-address vrf <name>' command, otherwise the packets are dropped because the interface is in a VRF and the helper address is not VRF-aware.
What should I do if I get this 300-410 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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Last reviewed: Jun 19, 2026
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