CCNA Network Services and Security Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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
The VLAN 30 SVI is the Layer 3 gateway for that subnet, so DHCP client broadcasts must be relayed as unicast to the server at 10.99.99.20. The ip helper-address command enables this relay function and is correctly placed on the SVI that receives the clients' requests. Without it, the broadcast would be dropped by the router.
- ✗
switchport mode trunk under interface Vlan30
Why it's wrong here
The switchport mode trunk command is a Layer 2 interface configuration used only on physical or aggregated switchports that carry multiple VLANs, but an SVI is a virtual Layer 3 interface created with the interface vlan command. Applying switchport mode trunk to an SVI would be rejected because SVIs do not operate as switchports. This command has no relationship to the DHCP relay mechanism needed here.
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
The command ip default-gateway is a global configuration command used on an end host to reach an external network, not a per-interface command. On a Layer 3 switch, a global default gateway for management traffic is configured with ip default-gateway, but it does not provide any DHCP relay functionality. DHCP relay requires ip helper-address on the SVI to forward client broadcasts to the server.
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
The spanning-tree portfast feature is meant for edge access ports that connect to end devices, automatically transitioning them directly to the forwarding state to avoid the listening/learning delay. It does not influence DHCP operations, and the command is invalid on an SVI because an SVI is a Layer 3 VLAN interface, not an access port. The only way to relay this DHCP traffic is with ip helper-address on the 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
The VLAN 30 SVI is the Layer 3 gateway for that subnet, so DHCP client broadcasts must be relayed as unicast to the server at 10.99.99.20. The ip helper-address command enables this relay function and is correctly placed on the SVI that receives the clients' requests. Without it, the broadcast would be dropped by the router.
✗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?”
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Key term
SVI
A Switch Virtual Interface (SVI) is a logical, software-based interface on a multilayer switch that provides Layer 3 routing capabilities for a VLAN, allowing devices in different VLANs to communicate without an external router.
Key term
VLAN
A VLAN logically segments a physical network into isolated broadcast domains without requiring separate physical switches.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.