CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: dHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server.. 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.
Exhibit
interface Vlan20
ip address 10.20.20.1 255.255.255.0
ip helper-address 10.50.0.100
interface Vlan30
ip address 10.30.30.1 255.255.255.0
ip helper-address 10.50.0.10
DHCP server address: 10.50.0.10
Users on VLAN 20 are not receiving IPv4 addresses from the centralized DHCP server at 10.50.0.10. Users in other VLANs are working normally. Based on the exhibit, which change should fix the issue for VLAN 20 clients?
interface Vlan20
ip address 10.20.20.1 255.255.255.0
ip helper-address 10.50.0.100
interface Vlan30
ip address 10.30.30.1 255.255.255.0
ip helper-address 10.50.0.10
DHCP server address: 10.50.0.10
A
Change the helper address on interface Vlan20 to 10.50.0.10.
That points DHCP relay to the actual DHCP server.
B
Convert the VLAN 20 user ports to trunk mode.
Why wrong: User access ports should stay in access mode.
C
Configure a default gateway on the user PCs manually.
Why wrong: That does not solve the DHCP relay problem.
D
Disable DHCP snooping on VLAN 20.
Why wrong: Nothing in the exhibit suggests DHCP snooping is the cause.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Change the helper address on interface Vlan20 to 10.50.0.10.
The SVI for VLAN 20 is forwarding DHCP requests to the wrong helper address. DHCP relay depends on the Layer 3 interface for that VLAN sending client broadcasts to the correct server. Trunks, access ports, and the DHCP pool name on the server are not the first issue shown here. DHCP snooping can filter DHCP server replies on untrusted ports, but since other VLANs are working and no trust misconfiguration is indicated, the root cause is the incorrect ip helper-address on Vlan20.
Key principle: DHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Change the helper address on interface Vlan20 to 10.50.0.10.
DHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server.
✗
Convert the VLAN 20 user ports to trunk mode.
Why it's wrong here
User access ports should stay in access mode.
When this WOULD be correct
In a different scenario, if the question stated that VLAN 20 clients were connected to a switch port configured as an access port and needed to communicate with multiple VLANs, converting the user ports to trunk mode could be the correct solution to allow DHCP traffic to traverse between VLANs.
✗
Configure a default gateway on the user PCs manually.
In a different scenario where the question states that VLAN 20 clients are unable to communicate with the DHCP server due to incorrect routing or subnetting, manually configuring a default gateway on the user PCs could resolve the issue by ensuring they can reach the DHCP server's subnet.
✗
Disable DHCP snooping on VLAN 20.
Why it's wrong here
Nothing in the exhibit suggests DHCP snooping is the cause.
When this WOULD be correct
In a scenario where a question asks about troubleshooting DHCP issues and indicates that DHCP snooping is incorrectly blocking legitimate DHCP traffic from a server, disabling DHCP snooping could be the correct action to allow clients to receive IP addresses. This would be applicable if the configuration was preventing legitimate DHCP responses.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Change the helper address on interface Vlan20 to 10.50.0.10.Correct answer▾
Why this is correct
That points DHCP relay to the actual DHCP server.
✗Convert the VLAN 20 user ports to trunk mode.Wrong answer — click to see why▾
Why this is wrong here
Converting VLAN 20 user ports to trunk mode is incorrect because it does not address the issue of DHCP packets not being forwarded to the centralized DHCP server. Trunking is used for carrying multiple VLANs, but the problem lies in the lack of a helper address for DHCP relay.
★ When this WOULD be the correct answer
In a different scenario, if the question stated that VLAN 20 clients were connected to a switch port configured as an access port and needed to communicate with multiple VLANs, converting the user ports to trunk mode could be the correct solution to allow DHCP traffic to traverse between VLANs.
Why candidates choose this
Candidates may choose this option because they associate trunking with VLAN communication and believe that enabling trunking would resolve connectivity issues, overlooking the specific requirement for DHCP relay configuration.
✗Configure a default gateway on the user PCs manually.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because configuring a default gateway on user PCs does not address the issue of VLAN 20 clients not receiving DHCP addresses. The problem lies in the DHCP relay configuration, not the individual PC settings.
★ When this WOULD be the correct answer
In a different scenario where the question states that VLAN 20 clients are unable to communicate with the DHCP server due to incorrect routing or subnetting, manually configuring a default gateway on the user PCs could resolve the issue by ensuring they can reach the DHCP server's subnet.
Why candidates choose this
Candidates may choose this option because they recognize that a default gateway is essential for network communication and might assume that configuring it on user PCs could resolve connectivity issues without considering the specific context of DHCP relay configurations.
✗Disable DHCP snooping on VLAN 20.Wrong answer — click to see why▾
Why this is wrong here
Disabling DHCP snooping on VLAN 20 would not resolve the issue of clients not receiving IP addresses, as DHCP snooping is a security feature that prevents unauthorized DHCP servers from providing IP addresses. The problem lies in the lack of a proper helper address for DHCP requests, not in the snooping configuration.
★ When this WOULD be the correct answer
In a scenario where a question asks about troubleshooting DHCP issues and indicates that DHCP snooping is incorrectly blocking legitimate DHCP traffic from a server, disabling DHCP snooping could be the correct action to allow clients to receive IP addresses. This would be applicable if the configuration was preventing legitimate DHCP responses.
Why candidates choose this
Candidates might choose this option because they recognize DHCP snooping as a common security feature that can interfere with DHCP operations, leading them to believe that disabling it could resolve connectivity issues without fully understanding the specific context of the problem.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that user ports must be trunks or that disabling DHCP snooping will fix DHCP address assignment issues. In reality, user ports should remain in access mode to maintain VLAN membership, and DHCP snooping is unrelated to this specific forwarding problem because the exhibit shows a misconfigured helper address while other VLANs function normally. Another common mistake is thinking that manually configuring a default gateway on clients solves DHCP problems, but DHCP discovery requires proper relay configuration on the Layer 3 interface. Misconfiguring or omitting the ip helper-address on the VLAN interface causes DHCP requests to fail, which is the core issue here.
Detailed technical explanation
How to think about this question
DHCP relay is a critical function in multi-VLAN networks where a centralized DHCP server provides IP addresses to clients across different subnets. Since DHCP clients initially send broadcast messages to discover a DHCP server, these broadcasts cannot cross Layer 3 boundaries by default. To solve this, Cisco devices use the ip helper-address command on the VLAN interface (SVI) to forward DHCP broadcasts as unicast packets to the DHCP server’s IP address. This mechanism ensures clients in remote VLANs receive IP addresses without requiring a DHCP server in each VLAN.
The decision process for DHCP relay involves verifying that each VLAN’s SVI has the correct helper address configured. If the helper address is missing or incorrect, DHCP requests from clients in that VLAN will not reach the DHCP server, causing address assignment failures. In this question, VLAN 20 clients fail to get addresses because the helper address on interface Vlan20 does not point to the DHCP server at 10.50.0.10. Correcting this configuration directs DHCP requests properly, restoring service.
A common exam trap is confusing port modes or unrelated features with DHCP relay issues. For example, converting user ports to trunk mode or disabling DHCP snooping does not fix DHCP relay problems. DHCP snooping controls DHCP message security but does not forward requests. Similarly, manually setting a default gateway on clients does not influence DHCP relay. Understanding the role of the ip helper-address command on SVIs is essential to avoid these pitfalls and ensure DHCP functions correctly in routed VLAN environments.
KKey Concepts to Remember
DHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server.
Each VLAN’s switched virtual interface (SVI) must have the correct helper address configured to ensure DHCP requests reach the DHCP server.
DHCP broadcasts are not routed by default, so the helper address enables forwarding of DHCP requests across different IP subnets.
Access ports connected to end devices should remain in access mode to maintain proper VLAN membership and avoid DHCP issues.
Manually configuring a default gateway on clients does not resolve DHCP relay problems because DHCP discovery relies on broadcast forwarding.
DHCP snooping is a security feature that filters DHCP messages but is unrelated to DHCP relay configuration on SVIs.
Incorrect or missing helper addresses on SVIs cause DHCP requests from clients in that VLAN to fail reaching the DHCP server.
The ip helper-address command points the router or Layer 3 switch to the correct DHCP server IP, enabling address assignment.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
DHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review dHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Services and Security — This question tests Network Services and Security — DHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server..
What is the correct answer to this question?
The correct answer is: Change the helper address on interface Vlan20 to 10.50.0.10. — The SVI for VLAN 20 is forwarding DHCP requests to the wrong helper address. DHCP relay depends on the Layer 3 interface for that VLAN sending client broadcasts to the correct server. Trunks, access ports, and the DHCP pool name on the server are not the first issue shown here. DHCP snooping can filter DHCP server replies on untrusted ports, but since other VLANs are working and no trust misconfiguration is indicated, the root cause is the incorrect ip helper-address on Vlan20.
What should I do if I get this 200-301 question wrong?
Review dHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
DHCP relay uses the ip helper-address command on a Layer 3 interface to forward client DHCP broadcasts to a centralized DHCP server.
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