The answer is the ip helper-address 10.99.99.10 command on the VLAN 20 SVI. This is correct because DHCP Discover messages are Layer 2 broadcasts, and by default, routers do not forward broadcasts across subnets. The ip helper-address configuration on the client’s gateway interface converts those broadcasts into unicast requests directed to the remote DHCP server, allowing the relay agent to forward them and return the offer. On the CCNA 200-301 v2 exam, this concept tests your understanding of DHCP relay and how to solve “no IP address” issues when the server is on a different subnet. A common trap is forgetting that the ip helper-address must be applied to the interface facing the DHCP clients, not the server-facing interface. Remember the mnemonic: “Helper on the client side, server address inside.”
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. A key principle to apply: dHCP clients send broadcast messages that do not cross router boundaries without relay configuration.. 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
VLAN 20 clients: 10.20.20.0/24
DHCP server: 10.99.99.10
Clients and server are in different subnets
Exhibit: PCs in VLAN 20 are not receiving addresses from a DHCP server in another subnet. The switch SVI for VLAN 20 is up, and routing is working. Which configuration is most likely missing on the gateway for VLAN 20?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ip helper-address 10.99.99.10
DHCP Discover messages are broadcasts and do not cross routers by default. An ip helper-address on the client gateway interface relays those requests to a remote DHCP server.
Key principle: DHCP clients send broadcast messages that do not cross router boundaries without relay configuration.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
ip default-gateway 10.20.20.1
Why it's wrong here
That is for a Layer 2 switch's own management, not DHCP relay for routed clients.
When this WOULD be correct
In a different question setup where the focus is on configuring a standalone host or a router that needs to reach a default gateway for local traffic, the option 'ip default-gateway 10.20.20.1' would be correct. For example, if the question asked about configuring a PC in VLAN 20 to communicate with a router in the same subnet, this command would be appropriate.
✓
ip helper-address 10.99.99.10
Why this is correct
That forwards DHCP broadcasts to the remote server.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
DHCP clients send broadcast messages that do not cross router boundaries without relay configuration.
✗
switchport trunk allowed vlan 20
Why it's wrong here
The issue is inter-subnet DHCP, not trunk allowance in the exhibit.
When this WOULD be correct
In a scenario where the question asks about ensuring that a trunk port is correctly configured to allow VLAN 20 traffic to pass between switches, 'switchport trunk allowed vlan 20' would be the correct answer. This would be relevant if the question focused on VLAN propagation across trunk links.
✗
spanning-tree portfast default
Why it's wrong here
PortFast is unrelated to relaying DHCP requests to another subnet.
When this WOULD be correct
In a scenario where the question asks about optimizing the DHCP response time for end devices connected to access ports in a VLAN, 'spanning-tree portfast default' would be correct. This would apply if the focus was on reducing the time it takes for devices to start communicating on the network after being powered on.
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.
✓ip helper-address 10.99.99.10Correct answer▾
Why this is correct
That forwards DHCP broadcasts to the remote server.
✗ip default-gateway 10.20.20.1Wrong answer — click to see why▾
Why this is wrong here
The option 'ip default-gateway 10.20.20.1' is incorrect because it is used for configuring the default gateway on a host, not on a router or switch interface. In this scenario, the issue is related to DHCP relay, which requires the 'ip helper-address' command instead.
★ When this WOULD be the correct answer
In a different question setup where the focus is on configuring a standalone host or a router that needs to reach a default gateway for local traffic, the option 'ip default-gateway 10.20.20.1' would be correct. For example, if the question asked about configuring a PC in VLAN 20 to communicate with a router in the same subnet, this command would be appropriate.
Why candidates choose this
Candidates may find this option tempting because it relates to gateway configuration, which is a common task in network setups. They might mistakenly believe that any gateway-related command would be relevant to the DHCP issue presented.
✗switchport trunk allowed vlan 20Wrong answer — click to see why▾
Why this is wrong here
The option 'switchport trunk allowed vlan 20' is incorrect because it pertains to configuring trunk ports on switches, not directly related to enabling DHCP relay for VLAN 20. The issue described is about DHCP address assignment across subnets, which is not resolved by trunking configurations.
★ When this WOULD be the correct answer
In a scenario where the question asks about ensuring that a trunk port is correctly configured to allow VLAN 20 traffic to pass between switches, 'switchport trunk allowed vlan 20' would be the correct answer. This would be relevant if the question focused on VLAN propagation across trunk links.
Why candidates choose this
Candidates may choose this option because they recognize the importance of VLAN configurations in switch operations, leading them to mistakenly associate trunking with the DHCP issue instead of focusing on the need for a helper address.
✗spanning-tree portfast defaultWrong answer — click to see why▾
Why this is wrong here
This option is wrong because 'spanning-tree portfast default' is related to enabling PortFast on access ports, which does not address the issue of DHCP relay for VLAN 20. The problem lies in the DHCP relay configuration, not spanning-tree settings.
★ When this WOULD be the correct answer
In a scenario where the question asks about optimizing the DHCP response time for end devices connected to access ports in a VLAN, 'spanning-tree portfast default' would be correct. This would apply if the focus was on reducing the time it takes for devices to start communicating on the network after being powered on.
Why candidates choose this
Candidates might choose this option due to a misunderstanding of the role of spanning-tree in network configurations, believing that it directly impacts DHCP functionality without recognizing the specific need for DHCP relay settings.
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 selecting the ip default-gateway command as the solution for DHCP relay issues. This command only applies to Layer 2 switches for their own management traffic and does not forward DHCP broadcasts across routed interfaces. Candidates may also mistakenly focus on VLAN trunking or spanning-tree settings, which do not affect DHCP relay functionality. The key is to recognize that DHCP broadcasts must be explicitly forwarded by the router or Layer 3 switch interface using ip helper-address to reach a DHCP server in another subnet.
Detailed technical explanation
How to think about this question
DHCP (Dynamic Host Configuration Protocol) uses broadcast messages to discover available DHCP servers and obtain IP addresses. When clients reside in a different subnet than the DHCP server, broadcasts do not cross router boundaries by default. To enable clients in remote VLANs or subnets to receive DHCP addresses, the router interface acting as the default gateway must relay these DHCP broadcast requests to the DHCP server using a helper function.
The Cisco IOS command ip helper-address is configured on the Layer 3 interface (SVI or routed port) serving as the gateway for the client VLAN. This command specifies the IP address of the DHCP server to which the router forwards DHCP broadcast messages as unicast packets. Without this configuration, DHCP Discover messages from clients in VLAN 20 will not reach the DHCP server in another subnet, causing clients to fail to obtain IP addresses.
A common exam trap is confusing the ip default-gateway command with DHCP relay functionality. The ip default-gateway command is used only on Layer 2 switches for management traffic and does not relay DHCP requests. Understanding the role of ip helper-address in forwarding UDP broadcasts such as DHCP is critical for inter-VLAN DHCP operations and avoiding misconfigurations that block client IP assignment.
KKey Concepts to Remember
DHCP clients send broadcast messages that do not cross router boundaries without relay configuration.
The ip helper-address command on a Layer 3 interface forwards DHCP broadcasts to a specified remote DHCP server.
A switch virtual interface (SVI) for a VLAN acts as the default gateway and must have ip helper-address for DHCP relay.
The ip default-gateway command configures a management gateway on Layer 2 switches and does not relay DHCP requests.
Routing must be operational between VLANs for DHCP relay to function correctly across subnets.
Without ip helper-address, DHCP Discover messages from clients in remote VLANs will not reach the DHCP server.
The ip helper-address command forwards several UDP services, including DHCP, to the specified server IP address.
DHCP relay enables centralized DHCP servers to serve multiple VLANs or subnets without local DHCP servers.
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 clients send broadcast messages that do not cross router boundaries without relay configuration.
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 clients send broadcast messages that do not cross router boundaries without relay configuration., 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 clients send broadcast messages that do not cross router boundaries without relay configuration..
What is the correct answer to this question?
The correct answer is: ip helper-address 10.99.99.10 — DHCP Discover messages are broadcasts and do not cross routers by default. An ip helper-address on the client gateway interface relays those requests to a remote DHCP server.
What should I do if I get this 200-301 question wrong?
Review dHCP clients send broadcast messages that do not cross router boundaries without relay configuration., then practise related 200-301 questions on the same topic to reinforce the concept.
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?
DHCP clients send broadcast messages that do not cross router boundaries without relay configuration.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
medium
✓ A.Change the helper address on interface Vlan20 to 10.50.0.10.
B.Convert the VLAN 20 user ports to trunk mode.
C.Configure a default gateway on the user PCs manually.
D.Disable DHCP snooping on VLAN 20.
Why A: 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.
Variation 2. PCs in VLAN 40 are not receiving addresses from the centralized DHCP server at 172.16.1.10. What should be configured on the VLAN 40 default gateway interface?
hard
A.ip dhcp excluded-address 10.40.40.1 10.40.40.10
✓ B.ip helper-address 172.16.1.10
C.service dhcp
D.ip default-gateway 172.16.1.10
Why B: When DHCP clients and the DHCP server are on different subnets, the router interface serving the client subnet must relay broadcasts to the server with the ip helper-address command.
Last reviewed: May 17, 2026
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