The correct answer is to add the `ip helper-address 10.99.99.20` command under interface Vlan30. This is necessary because DHCP client requests are broadcast messages, and broadcasts are not forwarded across Layer 3 boundaries by default; the router or Layer 3 switch must act as a relay agent to unicast those requests to the DHCP server on a different subnet. On the CCNA 200-301 v2 exam, this scenario tests your understanding of DHCP relay configuration for cross-VLAN communication, a common troubleshooting point when clients fail to obtain IP addresses. A frequent trap is confusing `ip helper-address` with `ip default-gateway` or assuming DHCP snooping trust alone solves the issue—remember, the helper-address is what bridges the VLAN gap. Memory tip: think of the helper-address as a "phone number" you give the router so it can call the DHCP server on behalf of the shouting clients.
CCNA Switching and Network Access Practice Question
This 200-301 practice question tests your understanding of switching and network access. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: a Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets.. 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
DHCP server IP: 10.99.99.20
Clients in VLAN 30 are not receiving addresses from the DHCP server located in VLAN 99. Which configuration change should be made on the Layer 3 interface for 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
DHCP server IP: 10.99.99.20
A
Add ip dhcp snooping trust under interface Vlan30.
Why wrong: This is wrong because DHCP snooping trust helps identify trusted ports but does not relay client broadcasts across subnets.
B
Add switchport mode trunk under interface Vlan30.
Why wrong: This is wrong because an SVI is a Layer 3 virtual interface, not a Layer 2 switch port. The missing function is DHCP relay on the client VLAN gateway.
C
Add ip default-gateway 10.99.99.20 under interface Vlan30.
Why wrong: This is wrong because `ip default-gateway` is not used on a Layer 3 SVI to relay DHCP traffic.
D
Add ip helper-address 10.99.99.20 under interface Vlan30.
This is correct because the SVI for VLAN 30 is the interface that receives the client DHCP broadcasts. By adding `ip helper-address 10.99.99.20`, the Layer 3 device forwards the request as a unicast packet to the DHCP server in VLAN 99.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add ip helper-address 10.99.99.20 under interface Vlan30.
The DHCP server is on a different subnet, so the client broadcast messages from VLAN 30 will not naturally cross the Layer 3 boundary. In simple terms, the clients are asking for an address by shouting on their own floor of the building, but the server lives on another floor and cannot hear that broadcast directly. The router or Layer 3 switch must relay the request for them. On Cisco devices, that relay function is usually configured with `ip helper-address` on the interface that receives the client broadcasts.
Here, that receiving interface is Vlan30, because that is the default gateway for the clients in VLAN 30. Pointing `ip helper-address` to 10.99.99.20 tells the Layer 3 device to forward DHCP requests to the remote server. DHCP snooping trust is a separate security feature, trunk mode is unrelated to an SVI, and `ip default-gateway` is not the correct solution for relaying DHCP across subnets.
Key principle: A Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Add ip dhcp snooping trust under interface Vlan30.
Why it's wrong here
This is wrong because DHCP snooping trust helps identify trusted ports but does not relay client broadcasts across subnets.
When this WOULD be correct
In a different scenario where the question asks about securing a network with multiple VLANs and preventing unauthorized DHCP servers, adding 'ip dhcp snooping trust' under interface Vlan30 would be correct to allow trusted DHCP responses from a legitimate server.
✗
Add switchport mode trunk under interface Vlan30.
Why it's wrong here
This is wrong because an SVI is a Layer 3 virtual interface, not a Layer 2 switch port. The missing function is DHCP relay on the client VLANgateway.
When this WOULD be correct
In a different scenario where the question asks about configuring a Layer 2 switch port that connects to another switch, and the requirement is to allow multiple VLANs to traverse that link, 'switchport mode trunk' would be the correct configuration to enable trunking on that interface.
✗
Add ip default-gateway 10.99.99.20 under interface Vlan30.
Why it's wrong here
This is wrong because `ip default-gateway` is not used on a Layer 3 SVI to relay DHCP traffic.
When this WOULD be correct
In a scenario where the question asks about configuring a Layer 2 switch that needs to communicate with a router for routing purposes, using 'ip default-gateway 10.99.99.20' would be correct to ensure the switch can reach the router for management traffic.
✓
Add ip helper-address 10.99.99.20 under interface Vlan30.
Why this is correct
This is correct because the SVI for VLAN 30 is the interface that receives the client DHCP broadcasts. By adding `ip helper-address 10.99.99.20`, the Layer 3 device forwards the request as a unicast packet to the DHCP server in VLAN 99.
Related concept
A Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets.
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.
✓Add ip helper-address 10.99.99.20 under interface Vlan30.Correct answer▾
Why this is correct
This is correct because the SVI for VLAN 30 is the interface that receives the client DHCP broadcasts. By adding `ip helper-address 10.99.99.20`, the Layer 3 device forwards the request as a unicast packet to the DHCP server in VLAN 99.
✗Add ip dhcp snooping trust under interface Vlan30.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because enabling DHCP snooping trust on VLAN 30 does not facilitate communication with the DHCP server in VLAN 99; it only protects against rogue DHCP servers.
★ When this WOULD be the correct answer
In a different scenario where the question asks about securing a network with multiple VLANs and preventing unauthorized DHCP servers, adding 'ip dhcp snooping trust' under interface Vlan30 would be correct to allow trusted DHCP responses from a legitimate server.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of DHCP snooping, believing that it directly relates to the DHCP process rather than its role in securing the network against rogue servers.
✗Add switchport mode trunk under interface Vlan30.Wrong answer — click to see why▾
Why this is wrong here
Adding 'switchport mode trunk' under interface Vlan30 is incorrect because VLAN 30 is already configured as a Layer 3 interface, and trunking is not applicable to Layer 3 interfaces. This command is used for Layer 2 interfaces to allow multiple VLANs over a single link.
★ When this WOULD be the correct answer
In a different scenario where the question asks about configuring a Layer 2 switch port that connects to another switch, and the requirement is to allow multiple VLANs to traverse that link, 'switchport mode trunk' would be the correct configuration to enable trunking on that interface.
Why candidates choose this
Candidates may choose this option because they might confuse Layer 3 interfaces with Layer 2 configurations, thinking that trunking is necessary for VLAN communication, especially if they are familiar with VLAN setups in general.
✗Add ip default-gateway 10.99.99.20 under interface Vlan30.Wrong answer — click to see why▾
Why this is wrong here
This option is wrong because the command 'ip default-gateway' is used to set a default gateway for a Layer 2 device, not for enabling DHCP relay on a Layer 3 interface. Clients in VLAN 30 need a helper address to reach the DHCP server in VLAN 99.
★ When this WOULD be the correct answer
In a scenario where the question asks about configuring a Layer 2 switch that needs to communicate with a router for routing purposes, using 'ip default-gateway 10.99.99.20' would be correct to ensure the switch can reach the router for management traffic.
Why candidates choose this
Candidates may choose this option because they recognize the need for a default gateway for devices in a VLAN, mistakenly believing it applies to enabling DHCP services instead of understanding the specific role of DHCP relay.
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
Remember that DHCP snooping and trunk mode do not facilitate DHCP relay. Focus on the purpose of `ip helper-address` for relaying requests across VLANs.
Detailed technical explanation
How to think about this question
DHCP (Dynamic Host Configuration Protocol) allows clients to obtain IP addresses dynamically. When clients send DHCP requests, they use broadcast messages because they do not yet have an IP address. Broadcasts are limited to the local subnet and do not cross Layer 3 boundaries such as VLAN interfaces or routers. Therefore, when a DHCP server resides on a different VLAN or subnet, the client broadcasts cannot reach it directly.
To solve this, Cisco devices use the ip helper-address command on the Layer 3 interface (SVI) that serves as the default gateway for the client VLAN. This command configures the router or Layer 3 switch to forward DHCP broadcast requests as unicast packets to the specified DHCP server IP address. The interface receiving the broadcast converts it and relays it, enabling clients in VLAN 30 to receive addresses from the DHCP server in VLAN 99.
Common exam traps include confusing DHCP snooping trust with DHCP relay, or misapplying switchport commands on SVIs. DHCP snooping trust secures DHCP traffic but does not forward broadcasts across VLANs. Similarly, switchport mode trunk is a Layer 2 port configuration and irrelevant for Layer 3 VLAN interfaces. Understanding these distinctions is critical for correctly configuring DHCP relay and ensuring clients receive IP addresses across VLAN boundaries.
KKey Concepts to Remember
A Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets.
DHCP client broadcasts do not cross Layer 3 boundaries without a relay agent configured on the VLAN interface.
The ip helper-address command on a Layer 3 interface forwards UDP broadcasts, including DHCP requests, to a specified DHCP server IP.
DHCP snooping trust configures trusted ports for DHCP security but does not relay DHCP requests across VLANs or subnets.
Switchport mode trunk applies only to physical Layer 2 switch ports and is irrelevant for Layer 3 VLAN interfaces (SVIs).
The ip default-gateway command is used on Layer 2 switches for management traffic and does not enable DHCP relay on Layer 3 interfaces.
DHCP relay agents convert client broadcast DHCP requests into unicast messages directed to the DHCP server on a different subnet.
Without ip helper-address, clients in one VLAN cannot obtain IP addresses from a DHCP server located in another VLAN or subnet.
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 Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets.
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 Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets., then practise related 200-301 questions on the same topic to reinforce the concept.
Switching and Network Access — This question tests Switching and Network Access — A Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets..
What is the correct answer to this question?
The correct answer is: Add ip helper-address 10.99.99.20 under interface Vlan30. — The DHCP server is on a different subnet, so the client broadcast messages from VLAN 30 will not naturally cross the Layer 3 boundary. In simple terms, the clients are asking for an address by shouting on their own floor of the building, but the server lives on another floor and cannot hear that broadcast directly. The router or Layer 3 switch must relay the request for them. On Cisco devices, that relay function is usually configured with `ip helper-address` on the interface that receives the client broadcasts.
Here, that receiving interface is Vlan30, because that is the default gateway for the clients in VLAN 30. Pointing `ip helper-address` to 10.99.99.20 tells the Layer 3 device to forward DHCP requests to the remote server. DHCP snooping trust is a separate security feature, trunk mode is unrelated to an SVI, and `ip default-gateway` is not the correct solution for relaying DHCP across subnets.
What should I do if I get this 200-301 question wrong?
Review a Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
A Layer 3 interface (SVI) for a VLAN acts as the default gateway and forwards traffic between VLANs and subnets.
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