CCNA Network Services and Security Practice Question
Exhibit
interface vlan 30 ip address 10.30.30.1 255.255.255.0 no shutdown DHCP server: 10.99.99.20
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?
⚠ Common exam trap
A frequent exam trap is selecting ip directed-broadcast or default-router as the solution for DHCP relay issues. ip directed-broadcast only enables forwarding of directed broadcasts but does not relay DHCP requests to servers on other subnets. default-router is a DHCP pool parameter that assigns a gateway to clients but does not affect how DHCP broadcasts are forwarded. Another trap is assuming service dhcp-server is an interface command for relay, which it is not. These distractors test your understanding of DHCP relay mechanisms and Cisco IOS command usage.
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
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.
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 directed-broadcast
Why it's wrong here
The `ip directed-broadcast` command, when configured on an interface, permits the router to forward IP packets that are specifically addressed to a subnet's broadcast address, such as 10.99.99.255. DHCP clients, however, send packets to 255.255.255.255, a limited broadcast that is never forwarded by routers, and DHCP relay requires the router to unicast the packet to the server address. Enabling directed broadcast does not create a relay agent nor does it pass DHCP traffic to 10.99.99.20; it only affects how hops are handled for a different type of traffic, making it irrelevant to the client's inability to receive an address.
When this WOULD be correct
In a different scenario where a network administrator needs to allow directed broadcasts for a specific application that requires it, such as a legacy system that relies on broadcast messages, this option would be appropriate.
- ✓
ip helper-address 10.99.99.20
Why this is correct
On the VLAN 30 SVI, the DHCP client's broadcast is not forwarded by default because routers do not forward link-local broadcasts. The command `ip helper-address 10.99.99.20` instructs the router to convert that DHCPDISCOVER broadcast into a unicast packet and send it to the DHCP server at 10.99.99.20, while also inserting the SVI's IP address as the giaddr field. This allows the server to know which subnet the client is on and assign an address from the correct pool. Without this command, the client's broadcast remains confined to VLAN 30 and the server never receives the request.
- ✗
service dhcp-server 10.99.99.20
Why it's wrong here
The `service dhcp-server 10.99.99.20` command is not a recognized Cisco IOS command; the actual global command is `service dhcp` (or `service dhcpv6` for IPv6), which the router can use to enable DHCP server and relay functionality, but it does not accept an IP address. There is also `ip dhcp relay address` for a global DHCP relay destination, but that is not `service dhcp-server`. Configuring an invalid command would be rejected by the CLI, leaving the interface with no relay configuration, so DHCP broadcasts from VLAN 30 are never sent to the server. This option is both syntactically incorrect and conceptually wrong for an SVI-level relay configuration.
When this WOULD be correct
In a different scenario where the question asks about configuring a DHCP server directly on a router, 'service dhcp-server 10.99.99.20' could be correct if the router is intended to provide DHCP services to clients on the same subnet.
- ✗
default-router 10.99.99.20
Why it's wrong here
The `default-router` command is a mandatory parameter within the DHCP address-pool configuration on the server, specified under `ip dhcp pool <name>` to assign the default gateway (option 3) to clients. It is not an interface command and cannot be applied to the VLAN 30 SVI; therefore, it would not cause an SVI to relay DHCP broadcasts. Even if it were globally configured, it only informs clients which gateway to use after they receive an IP address, and it does nothing to deliver the DHCPDISCOVER message to the server at 10.99.99.20. The missing relay is the actual issue, and `default-router` cannot solve it.
When this WOULD be correct
In a different scenario where the question asks for the configuration of a VLAN interface to provide a default gateway for devices within that VLAN, specifying 'default-router 10.99.99.20' would be correct if 10.99.99.20 is indeed the gateway for that VLAN.
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 10.99.99.20Correct answer▾
Why this is correct
On the VLAN 30 SVI, the DHCP client's broadcast is not forwarded by default because routers do not forward link-local broadcasts. The command `ip helper-address 10.99.99.20` instructs the router to convert that DHCPDISCOVER broadcast into a unicast packet and send it to the DHCP server at 10.99.99.20, while also inserting the SVI's IP address as the giaddr field. This allows the server to know which subnet the client is on and assign an address from the correct pool. Without this command, the client's broadcast remains confined to VLAN 30 and the server never receives the request.
✗ip directed-broadcastWrong answer — click to see why▾
Why this is wrong here
ip directed-broadcast enables forwarding of directed broadcasts on the interface but does not relay DHCP broadcasts to a specific server on a different subnet.
★ When this WOULD be the correct answer
In a different scenario where a network administrator needs to allow directed broadcasts for a specific application that requires it, such as a legacy system that relies on broadcast messages, this option would be appropriate.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of how DHCP operates across subnets, mistakenly believing that enabling directed broadcasts would solve the issue of clients not receiving IP addresses.
✗service dhcp-server 10.99.99.20Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because 'service dhcp-server' is not a valid command for enabling DHCP relay or configuring a DHCP server on a router or switch. The correct command to forward DHCP requests is 'ip helper-address'.
★ When this WOULD be the correct answer
In a different scenario where the question asks about configuring a DHCP server directly on a router, 'service dhcp-server 10.99.99.20' could be correct if the router is intended to provide DHCP services to clients on the same subnet.
Why candidates choose this
Candidates might choose this option because it contains 'dhcp-server', which could mislead them into thinking it relates to DHCP functionality, especially if they are familiar with DHCP server configurations.
✗default-router 10.99.99.20Wrong answer — click to see why▾
Why this is wrong here
The 'default-router' command is used to specify the default gateway for a VLAN interface, not to facilitate DHCP relay. Therefore, it does not address the issue of the DHCP client not receiving an address from a server on a different subnet.
★ When this WOULD be the correct answer
In a different scenario where the question asks for the configuration of a VLAN interface to provide a default gateway for devices within that VLAN, specifying 'default-router 10.99.99.20' would be correct if 10.99.99.20 is indeed the gateway for that VLAN.
Why candidates choose this
Candidates may be tempted by this option because they might confuse the need for a default gateway with the requirement for DHCP address assignment, leading them to think that specifying a default router could help with DHCP issues.
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
DHCP relay
A network device or feature that forwards DHCP broadcast messages between clients and servers on different subnets so that IP address assignment works across multiple network segments without a separate DHCP server on each one.
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.
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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.