The answer is to configure the ip helper-address 10.99.99.20 command under interface Vlan30. This is correct because DHCP clients in VLAN 30 send broadcast discovery messages that cannot cross a Layer 3 boundary to reach the DHCP server at 10.99.99.20, which resides on a different subnet. The SVI for VLAN 30 acts as the local gateway, and applying the ip helper-address there instructs the router to convert those broadcasts into unicast requests forwarded directly to the server. On the CCNA 200-301 v2 exam, this scenario tests your understanding of DHCP relay configuration and the critical role of the gateway interface—a common trap is placing the helper address on the server’s VLAN interface instead of the client’s. Remember the key rule: the ip helper-address always goes on the interface facing the DHCP clients, not the server. A useful memory tip is “helper on the client side, server on the other side.”
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: a VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet.. 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 Vlan30
ip address 10.30.30.1 255.255.255.0
no shutdown
interface Vlan99
ip address 10.99.99.1 255.255.255.0
no shutdown
Remote DHCP server: 10.99.99.20
Based on the exhibit, which configuration should be added to restore DHCP service for clients in VLAN 30?
interface Vlan30
ip address 10.30.30.1 255.255.255.0
no shutdown
interface Vlan99
ip address 10.99.99.1 255.255.255.0
no shutdown
Remote DHCP server: 10.99.99.20
A
ip helper-address 10.99.99.20 under interface Vlan30
This is correct because the VLAN 30 SVI must relay DHCP requests toward the remote server.
B
switchport mode trunk under interface Vlan30
Why wrong: This is wrong because an SVI is not a Layer 2 switchport.
C
ip default-gateway 10.99.99.20 under interface Vlan30
Why wrong: This is wrong because that is not how DHCP relay is configured.
D
spanning-tree portfast under interface Vlan30
Why wrong: This is wrong because PortFast is unrelated to DHCP relay on an SVI.
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 10.99.99.20 under interface Vlan30
The correct fix is to add an IP helper address pointing to the remote DHCP server on the Layer 3 interface for VLAN 30. In practical terms, the clients are sending DHCP discovery as a broadcast, and the server is on another subnet. The SVI for VLAN 30 is the local gateway that must relay those requests toward the server.
This is one of the most exam-realistic campus troubleshooting scenarios because it tests both subnet boundaries and the role of the local gateway interface.
Key principle: A VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet.
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 helper-address 10.99.99.20 under interface Vlan30
Why this is correct
This is correct because the VLAN 30 SVI must relay DHCP requests toward the remote server.
Related concept
A VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet.
✗
switchport mode trunk under interface Vlan30
Why it's wrong here
This is wrong because an SVI is not a Layer 2 switchport.
When this WOULD be correct
In a different scenario where the question asks about configuring a switch port to allow multiple VLANs to pass through, 'switchport mode trunk' would be the correct answer. For example, if the question specified that VLAN 30 needs to communicate with other VLANs via a trunk link, this option would be appropriate.
✗
ip default-gateway 10.99.99.20 under interface Vlan30
Why it's wrong here
This is wrong because that is not how DHCP relay is configured.
When this WOULD be correct
In a scenario where the question asks for configuring a Layer 2 switch to communicate with a router for routing purposes, 'ip default-gateway 10.99.99.20' would be the correct answer if the switch needs to reach the DHCP server for clients on a different VLAN.
✗
spanning-tree portfast under interface Vlan30
Why it's wrong here
This is wrong because PortFast is unrelated to DHCP relay on an SVI.
When this WOULD be correct
In a different scenario where the question asks about optimizing switch port configurations for end devices connected to VLAN 30, 'spanning-tree portfast' would be correct to reduce the time it takes for ports to transition to the forwarding state, improving connectivity for clients.
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 10.99.99.20 under interface Vlan30Correct answer▾
Why this is correct
This is correct because the VLAN 30 SVI must relay DHCP requests toward the remote server.
✗switchport mode trunk under interface Vlan30Wrong answer — click to see why▾
Why this is wrong here
The 'switchport mode trunk' command is used on Layer 2 switch ports to allow multiple VLANs, not on SVIs. An SVI is a virtual Layer 3 interface and does not support the switchport command.
★ When this WOULD be the correct answer
In a different scenario where the question asks about configuring a switch port to allow multiple VLANs to pass through, 'switchport mode trunk' would be the correct answer. For example, if the question specified that VLAN 30 needs to communicate with other VLANs via a trunk link, this option would be appropriate.
Why candidates choose this
Students may confuse SVIs with physical switch ports and think that trunking is needed to carry VLAN 30 traffic, but SVIs are already associated with a VLAN and do not require trunk configuration.
✗ip default-gateway 10.99.99.20 under interface Vlan30Wrong answer — click to see why▾
Why this is wrong here
The 'ip default-gateway' command is used on end devices to set a default gateway, not on a router or switch SVI. On an SVI, the correct command to relay DHCP is 'ip helper-address'.
★ When this WOULD be the correct answer
In a scenario where the question asks for configuring a Layer 2 switch to communicate with a router for routing purposes, 'ip default-gateway 10.99.99.20' would be the correct answer if the switch needs to reach the DHCP server for clients on a different VLAN.
Why candidates choose this
The term 'default-gateway' might be associated with routing, and a student might think it helps forward DHCP requests, but it does not perform DHCP relay.
✗spanning-tree portfast under interface Vlan30Wrong answer — click to see why▾
Why this is wrong here
The 'spanning-tree portfast' command is applied to Layer 2 access ports to speed up convergence, not to SVIs. It has no effect on DHCP relay or forwarding.
★ When this WOULD be the correct answer
In a different scenario where the question asks about optimizing switch port configurations for end devices connected to VLAN 30, 'spanning-tree portfast' would be correct to reduce the time it takes for ports to transition to the forwarding state, improving connectivity for clients.
Why candidates choose this
PortFast is a common feature that students learn, and they might incorrectly assume it helps with DHCP by reducing delays, but it does not address the need to forward DHCP broadcasts across subnets.
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 common exam trap is confusing the ip helper-address command with ip default-gateway or Layer 2 commands like switchport mode trunk. Candidates might incorrectly apply switchport commands to an SVI, which is a Layer 3 interface, or think setting ip default-gateway will relay DHCP requests. These mistakes cause DHCP broadcasts to fail reaching the remote server, leading to no IP address assignment for clients. Understanding that ip helper-address is the DHCP relay mechanism on Layer 3 interfaces is critical to avoid this trap.
Detailed technical explanation
How to think about this question
DHCP relay is essential in multi-VLAN environments where the DHCP server resides on a different subnet than the clients. Clients send DHCPDISCOVER messages as broadcasts, which routers do not forward by default. To enable DHCP communication across subnets, the router’s VLAN interface (SVI) must be configured with the ip helper-address command, which forwards these broadcasts as unicast packets to the DHCP server’s IP address. This process allows clients to obtain IP addresses even when the DHCP server is remote.
The ip helper-address command acts as a relay agent on the Layer 3 interface associated with the VLAN. When a DHCP broadcast is received on the SVI, the router converts it into a unicast message directed at the DHCP server. This relay function is critical because routers block broadcasts between subnets to reduce unnecessary traffic. Without this configuration, clients in VLAN 30 cannot reach the DHCP server on a different subnet, resulting in failed IP address assignment.
A frequent source of confusion is treating the SVI like a Layer 2 interface and applying commands such as switchport mode trunk or spanning-tree portfast, which are invalid on SVIs. Another mistake is using ip default-gateway, which only sets the management gateway on Layer 2 devices and does not relay DHCP requests. Understanding the SVI’s role as a Layer 3 gateway and the necessity of ip helper-address for DHCP relay is crucial for both exam success and practical network troubleshooting.
KKey Concepts to Remember
A VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet.
DHCP clients send broadcast messages that cannot cross Layer 3 boundaries without a relay agent like the ip helper-address feature on an SVI.
The ip helper-address command forwards UDP broadcasts for DHCP and other services from clients to a specified remote server IP address.
An SVI is a Layer 3 interface and does not support Layer 2 commands such as switchport mode trunk or spanning-tree portfast.
The ip default-gateway command is used on Layer 2 devices to specify a gateway for management traffic and does not relay DHCP requests.
Without the ip helper-address on the VLAN interface, DHCP requests from clients in that VLAN will not reach the DHCP server on another subnet.
The local gateway interface (SVI) acts as the DHCP relay agent, forwarding client broadcasts to the DHCP server using the ip helper-address.
Configuring ip helper-address on the correct VLAN interface ensures proper DHCP service restoration for clients in that VLAN.
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
A VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet.
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 a VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet., 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 — A VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet..
What is the correct answer to this question?
The correct answer is: ip helper-address 10.99.99.20 under interface Vlan30 — The correct fix is to add an IP helper address pointing to the remote DHCP server on the Layer 3 interface for VLAN 30. In practical terms, the clients are sending DHCP discovery as a broadcast, and the server is on another subnet. The SVI for VLAN 30 is the local gateway that must relay those requests toward the server.
This is one of the most exam-realistic campus troubleshooting scenarios because it tests both subnet boundaries and the role of the local gateway interface.
What should I do if I get this 200-301 question wrong?
Review a VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
A VLAN interface (SVI) must have an ip helper-address configured to relay DHCP broadcasts to a DHCP server on a different subnet.
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. A router is configured as follows:
interface g0/1
ip address 172.16.1.1 255.255.255.0
ip helper-address 10.20.20.10
Hosts on 172.16.1.0/24 are not receiving addresses from the DHCP server at 10.20.20.10. The server is reachable by ping from the router.
What is the purpose of the ip helper-address command in this scenario?
medium
A.It converts DHCP unicast replies into broadcasts on the client segment
✓ B.It forwards certain UDP broadcasts, including DHCP requests, to a remote server
C.It provides DNS resolution for DHCP clients before they receive an address
D.It creates a static route to the DHCP server
Why B: The ip helper-address command exists to solve a broadcast-boundary problem. DHCP clients begin by sending broadcast traffic because they do not yet have a valid IP configuration. Routers normally do not forward broadcasts between subnets, so if the DHCP server lives on a different network, the client request would stop at the router. The helper-address function listens for that local broadcast and relays it as unicast traffic to the remote DHCP server. In plain language, it lets a client on one VLAN ask a DHCP server on another VLAN for an address. The command is not a routing statement and it is not a DNS feature. It is a relay mechanism for broadcast-based UDP services such as DHCP.
Variation 2. A DHCP client on VLAN 30 is not receiving an IP address from a DHCP server (10.99.99.20) on another subnet. The SVI for VLAN 30 is configured with an IP address and is up, but the DHCP relay command is missing. Which command should be added to the SVI configuration?
hard
A.ip directed-broadcast
✓ B.ip helper-address 10.99.99.20
C.service dhcp-server 10.99.99.20
D.default-router 10.99.99.20
Why B: DHCP Discover messages are broadcasts and do not cross routers by default. On an SVI or routed interface facing the clients, an ip helper-address relays those broadcasts to the DHCP server on another subnet.
Last reviewed: May 17, 2026
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