CCNA Network Services and Security Practice Question
Network Topology
You are connected to SW1, a multilayer switch. Configure DHCP snooping and an IP helper-address so that clients in VLAN 20 receive IP addresses from the DHCP server at 10.0.0.2. The DHCP server is already configured with a pool for 192.168.20.0/24, but clients are not getting addresses. Identify and correct the issues in the current configuration.
⚠ Common exam trap
A common trap is to trust all ports or to forget that the ip helper-address must match the actual DHCP server IP. Also, candidates may confuse which port should be trusted: only the port facing the legitimate DHCP server should be trusted, not client-facing ports.
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
✓
Enable DHCP snooping globally and on VLAN 20, configure interface G0/0 as trusted, and change the ip helper-address on the SVI for VLAN 20 from 10.0.0.3 to 10.0.0.2.
The DHCP relay helper-address was pointing to 10.0.0.3 instead of the actual server at 10.0.0.2. Also, DHCP snooping was not enabled. After enabling DHCP snooping globally and on VLAN 20, configure the uplink to the DHCP server as a trusted port (G0/0) and the access port (G0/1) as untrusted (default). Finally, correct the helper-address to 10.0.0.2. These steps allow DHCP broadcasts from VLAN 20 to be relayed to the server and prevent rogue DHCP attacks.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Enable DHCP snooping globally and on VLAN 20, configure interface G0/0 as trusted, and change the ip helper-address on the SVI for VLAN 20 from 10.0.0.3 to 10.0.0.2.
Why this is correct
This is correct because DHCP snooping must be enabled globally and per VLAN to filter untrusted DHCP messages. The uplink to the DHCP server (G0/0) must be trusted to allow server replies, while access ports remain untrusted. The ip helper-address must point to the actual DHCP server IP (10.0.0.2) to relay broadcasts correctly.
- ✗
Enable DHCP snooping globally and on VLAN 20, configure interface G0/1 as trusted, and change the ip helper-address on the SVI for VLAN 20 from 10.0.0.3 to 10.0.0.2.
Why it's wrong here
This is incorrect because the access port (G0/1) should remain untrusted, not trusted. Trusting the access port would allow rogue DHCP servers connected to that port to send replies, defeating the purpose of DHCP snooping.
- ✗
Enable DHCP snooping globally and on VLAN 20, configure interface G0/0 as trusted, and keep the ip helper-address as 10.0.0.3 because that is the correct server address.
Why it's wrong here
Although trusting the uplink G0/0 is correct, the ip helper-address remains incorrectly set to 10.0.0.3. The DHCP server actually resides at 10.0.0.2, so the relay agent will forward client broadcasts to the wrong destination, and no DHCP replies will return. The command 'ip helper-address' must point to the real server IP; otherwise DHCP snooping alone does not fix the broken relay path.
- ✗
Enable DHCP snooping globally and on VLAN 20, configure both interfaces G0/0 and G0/1 as trusted, and change the ip helper-address on the SVI for VLAN 20 from 10.0.0.3 to 10.0.0.2.
Why it's wrong here
Configuring both G0/0 and G0/1 as trusted completely undermines DHCP snooping's security model. The access port G0/1 is the attachment point for end users, and a rogue DHCP server there would be allowed to send replies if trusted. Only the uplink G0/0 to the legitimate DHCP server should be trusted; all other ports should remain untrusted to filter unauthorized DHCP traffic.
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.
✓Enable DHCP snooping globally and on VLAN 20, configure interface G0/0 as trusted, and change the ip helper-address on the SVI for VLAN 20 from 10.0.0.3 to 10.0.0.2.Correct answer▾
Why this is correct
This is correct because DHCP snooping must be enabled globally and per VLAN to filter untrusted DHCP messages. The uplink to the DHCP server (G0/0) must be trusted to allow server replies, while access ports remain untrusted. The ip helper-address must point to the actual DHCP server IP (10.0.0.2) to relay broadcasts correctly.
✗Enable DHCP snooping globally and on VLAN 20, configure interface G0/1 as trusted, and change the ip helper-address on the SVI for VLAN 20 from 10.0.0.3 to 10.0.0.2.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the access port (G0/1) should be untrusted, not trusted. Only the uplink port to the legitimate DHCP server should be trusted.
Why candidates choose this
Candidates might think that because the access port connects to clients, it needs to be trusted to receive DHCP requests, but DHCP snooping trusts only ports that receive DHCP server messages, not client messages.
✗Enable DHCP snooping globally and on VLAN 20, configure interface G0/0 as trusted, and keep the ip helper-address as 10.0.0.3 because that is the correct server address.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the ip helper-address must be set to the actual DHCP server IP (10.0.0.2), not 10.0.0.3.
Why candidates choose this
Candidates might misread the question or assume the existing helper-address is correct, especially if they overlook the server IP given in the stem.
✗Enable DHCP snooping globally and on VLAN 20, configure both interfaces G0/0 and G0/1 as trusted, and change the ip helper-address on the SVI for VLAN 20 from 10.0.0.3 to 10.0.0.2.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that only the uplink port (G0/0) should be trusted; trusting the access port (G0/1) allows any device connected to that port to act as a DHCP server.
Why candidates choose this
Candidates might think that trusting all ports simplifies configuration or that both ports need to be trusted for DHCP to work, but this ignores the security purpose of DHCP snooping.
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 server
A DHCP server is a network device or service that automatically assigns IP addresses and other network configuration parameters to devices on a network, eliminating the need for manual configuration.
Key term
DHCP snooping
DHCP snooping is a network security feature that filters untrusted DHCP messages to prevent rogue DHCP servers from giving out false IP addresses.
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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.