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. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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.
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.
SW1#show running-config | section interface
interface GigabitEthernet0/0
description to DHCP Server
ip address 10.0.0.1 255.255.255.252
no switchport
!
interface GigabitEthernet0/1
switchport access vlan 20
spanning-tree portfast
!
interface Vlan20
ip address 192.168.20.1 255.255.255.0
ip helper-address 10.0.0.3
!
SW1#show ip dhcp snooping
Switch DHCP snooping is disabled
DHCP snooping is configured on following VLANs:
none
DHCP snooping is operational on following VLANs:
none
DHCP snooping is configured on the following L3 Interfaces:
Insertion of option 82 is enabled
circuit-id default format: vlan-mod-port
remote-id: 00e0.f711.2233 (MAC)
Option 82 on untrusted port is not allowed
Verification of hwaddr field is enabled
Verification of giaddr field is enabled
Check for address on untrusted interfaces is enabled
Custom option 82 strings are disabled
A
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.
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.
B
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 wrong: 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.
C
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 wrong: This is incorrect because the DHCP server is at 10.0.0.2, not 10.0.0.3. The helper-address must point to the actual server IP; otherwise, DHCP requests will be forwarded to the wrong destination.
D
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 wrong: This is incorrect because trusting both interfaces defeats DHCP snooping security. The access port (G0/1) must remain untrusted to prevent rogue DHCP server attacks from the client side.
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.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Related concept
Access ports place end devices into a single VLAN.
✗
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
This is incorrect because the DHCP server is at 10.0.0.2, not 10.0.0.3. The helper-address must point to the actual server IP; otherwise, DHCP requests will be forwarded to the wrong destination.
✗
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
This is incorrect because trusting both interfaces defeats DHCP snooping security. The access port (G0/1) must remain untrusted to prevent rogue DHCP server attacks from the client side.
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?”
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
Network Services and Security — This question tests Network Services and Security — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 200-301 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 200-301 questions on switching, trunking, and access-port configuration.
What is the key concept behind this question?
Access ports place end devices into a single VLAN.
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