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 interfaces without specific 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
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
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.
Key principle: DHCP clients send broadcast messages that do not cross router interfaces without specific 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.
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
The helper address forwards DHCP broadcasts as unicast to the server.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
DHCP clients send broadcast messages that do not cross router interfaces without specific relay configuration.
✗
service dhcp-server 10.99.99.20
Why it's wrong here
That is not the Cisco IOS interface command for relay.
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
That is a DHCP pool parameter, not an SVI relay command.
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
The helper address forwards DHCP broadcasts as unicast to the server.
✗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?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Trap categories for this question
Command / output trap
That is not the Cisco IOS interface command for relay.
Detailed technical explanation
How to think about this question
Dynamic Host Configuration Protocol (DHCP) clients initially send DHCP Discover messages as broadcasts to locate available DHCP servers. Because routers do not forward broadcast traffic by default, clients on one VLAN or subnet cannot directly communicate with DHCP servers on another subnet. This behavior ensures broadcast containment but requires a relay mechanism for DHCP to function across routed boundaries.
To enable DHCP clients on VLAN 30 to receive IP addresses from a DHCP server on a different subnet, the router interface connected to VLAN 30 must be configured with the ip helper-address command pointing to the DHCP server's IP address. This command instructs the router to intercept DHCP broadcast messages and forward them as unicast packets to the specified DHCP server, effectively bridging the broadcast domain gap. The SVI configuration must include this command to relay DHCP requests properly.
A common exam trap is confusing ip helper-address with other interface commands like ip directed-broadcast or DHCP pool parameters such as default-router. While ip directed-broadcast enables forwarding of directed broadcasts, it does not relay DHCP requests. Similarly, default-router is a DHCP pool setting that provides clients with a gateway address but does not influence relay behavior. Understanding this distinction is critical for correctly configuring DHCP relay in Cisco environments and ensuring clients receive IP addresses across VLANs.
KKey Concepts to Remember
DHCP clients send broadcast messages that do not cross router interfaces without specific relay configuration.
The ip helper-address command on an SVI or routed interface forwards DHCP broadcasts as unicast packets to a DHCP server on a different subnet.
Without ip helper-address, DHCP Discover messages from clients on VLANs cannot reach DHCP servers located on other subnets.
The ip directed-broadcast command enables forwarding directed broadcasts but does not facilitate DHCP relay functionality.
DHCP pool parameters like default-router configure client options but do not affect DHCP relay behavior on interfaces.
Cisco IOS does not use a service dhcp-server command on interfaces to enable DHCP relay; ip helper-address is required.
An SVI (Switched Virtual Interface) acts as a Layer 3 interface for VLANs and must be configured with ip helper-address to relay DHCP requests.
DHCP relay via ip helper-address converts client broadcasts into unicast packets directed to the DHCP server IP address.
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 interfaces without specific 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 interfaces without specific 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 interfaces without specific relay configuration..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Review dHCP clients send broadcast messages that do not cross router interfaces without specific 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: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
What is the key concept behind this question?
DHCP clients send broadcast messages that do not cross router interfaces without specific relay configuration.
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