The correct answer involves enabling DHCP snooping globally and on VLAN 50, trusting the uplink port to R1, correcting the excluded range on R1 to 192.168.50.1-192.168.50.20, and changing the helper-address on SW1’s G0/1 to 192.168.50.1. This resolves the rogue DHCP server issue because DHCP snooping filters untrusted messages from ports like Fa0/5, while the trusted port designation allows only the legitimate server’s replies through; fixing the excluded range ensures hosts can lease addresses beyond the first 20, and correcting the helper-address directs DHCP requests to the correct server interface. On the CCNA 200-301 v2 exam, this scenario tests your ability to combine DHCP snooping configuration with troubleshooting rogue DHCP servers, a common trap being that candidates forget to adjust both the excluded range and the helper-address after moving the server to an SVI. A key memory tip is "Trust the uplink, trim the exclusion, and point the helper to the SVI" — remember that a helper-address should target the server’s VLAN interface, not a point-to-point link.
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Exhibit
R1# show running-config | section dhcp
ip dhcp excluded-address 192.168.50.1 192.168.50.100
ip dhcp pool VLAN50
network 192.168.50.0 255.255.255.0
default-router 192.168.50.1
dns-server 8.8.8.8
!
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.252
no shutdown
!
SW1# show running-config | section interface
interface GigabitEthernet0/1
description uplink to R1
ip address 10.0.0.2 255.255.255.252
ip helper-address 10.0.0.1
!
interface VLAN50
ip address 192.168.50.1 255.255.255.0
!
interface FastEthernet0/5
description rogue server
switchport mode access
switchport access vlan 50
!
SW1# show ip dhcp snooping
Switch DHCP snooping is disabled
DHCP snooping is configured on the following VLANs:
none
DHCP snooping trust/untrusted ports:
Trusted ports:
Untrusted ports:
You are connected to R1. Configure DHCP server on R1 to assign addresses from 192.168.50.0/24 to hosts on VLAN 50, excluding 192.168.50.1-192.168.50.20, with default-router 192.168.50.1 and DNS server 8.8.8.8. On switch SW1, configure DHCP snooping globally and on VLAN 50, and enable trusted ports on the uplink to R1. Then, a host on VLAN 50 reports it received an incorrect IP address; troubleshoot and fix the issue: the wrong helper-address is configured on SW1, the excluded range is too large, and a rogue DHCP server is present on port Fa0/5.
R1# show running-config | section dhcp
ip dhcp excluded-address 192.168.50.1 192.168.50.100
ip dhcp pool VLAN50
network 192.168.50.0 255.255.255.0
default-router 192.168.50.1
dns-server 8.8.8.8
!
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.252
no shutdown
!
SW1# show running-config | section interface
interface GigabitEthernet0/1
description uplink to R1
ip address 10.0.0.2 255.255.255.252
ip helper-address 10.0.0.1
!
interface VLAN50
ip address 192.168.50.1 255.255.255.0
!
interface FastEthernet0/5
description rogue server
switchport mode access
switchport access vlan 50
!
SW1# show ip dhcp snooping
Switch DHCP snooping is disabled
DHCP snooping is configured on the following VLANs:
none
DHCP snooping trust/untrusted ports:
Trusted ports:
Untrusted ports:
A
Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, correct the excluded range on R1 to 192.168.50.1-192.168.50.20, and change the helper-address on SW1's G0/1 to 192.168.50.1.
This option correctly addresses all issues: DHCP snooping blocks rogue DHCP servers, trusting the uplink allows legitimate DHCP offers, fixing the excluded range ensures hosts get addresses, and correcting the helper-address ensures DHCP requests reach the correct server.
B
Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and change the helper-address on SW1's G0/1 to 10.0.0.1.
Why wrong: This is incorrect because the helper-address should point to the DHCP server's IP (192.168.50.1), not the point-to-point link address (10.0.0.1). Also, it does not fix the excluded range issue.
C
Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and correct the excluded range on R1 to 192.168.50.1-192.168.50.20.
Why wrong: This is incorrect because it does not address the wrong helper-address on SW1. Without correcting the helper-address, DHCP requests from hosts will not reach the correct server.
D
Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and change the helper-address on SW1's G0/1 to 192.168.50.1.
Why wrong: This is incorrect because it does not fix the excluded range on R1. The excluded range is too large (excluding all addresses), so hosts cannot obtain any IP address from the legitimate server.
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 50, trust the uplink port to R1, correct the excluded range on R1 to 192.168.50.1-192.168.50.20, and change the helper-address on SW1's G0/1 to 192.168.50.1.
The host received a wrong IP because a rogue DHCP server on Fa0/5 was responding. First, enable DHCP snooping globally with 'ip dhcp snooping' and on VLAN 50 with 'ip dhcp snooping vlan 50'. Then, trust the uplink port to R1 (G0/1) with 'ip dhcp snooping trust'. Next, fix the excluded range on R1: change it to exclude only the first 20 addresses (192.168.50.1-192.168.50.20) so that hosts can obtain other addresses. Finally, correct the helper-address on SW1's G0/1: change 'ip helper-address 10.0.0.1' to 'ip helper-address 192.168.50.1' because the DHCP server is on the VLAN 50 SVI, not on the point-to-point link.
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 50, trust the uplink port to R1, correct the excluded range on R1 to 192.168.50.1-192.168.50.20, and change the helper-address on SW1's G0/1 to 192.168.50.1.
Why this is correct
This option correctly addresses all issues: DHCP snooping blocks rogue DHCP servers, trusting the uplink allows legitimate DHCP offers, fixing the excluded range ensures hosts get addresses, and correcting the helper-address ensures DHCP requests reach the correct server.
Related concept
Access ports place end devices into a single VLAN.
✗
Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and change the helper-address on SW1's G0/1 to 10.0.0.1.
Why it's wrong here
This is incorrect because the helper-address should point to the DHCP server's IP (192.168.50.1), not the point-to-point link address (10.0.0.1). Also, it does not fix the excluded range issue.
✗
Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and correct the excluded range on R1 to 192.168.50.1-192.168.50.20.
Why it's wrong here
This is incorrect because it does not address the wrong helper-address on SW1. Without correcting the helper-address, DHCP requests from hosts will not reach the correct server.
✗
Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and change the helper-address on SW1's G0/1 to 192.168.50.1.
Why it's wrong here
This is incorrect because it does not fix the excluded range on R1. The excluded range is too large (excluding all addresses), so hosts cannot obtain any IP address from the legitimate server.
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 50, trust the uplink port to R1, correct the excluded range on R1 to 192.168.50.1-192.168.50.20, and change the helper-address on SW1's G0/1 to 192.168.50.1.Correct answer▾
Why this is correct
This option correctly addresses all issues: DHCP snooping blocks rogue DHCP servers, trusting the uplink allows legitimate DHCP offers, fixing the excluded range ensures hosts get addresses, and correcting the helper-address ensures DHCP requests reach the correct server.
✗Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and change the helper-address on SW1's G0/1 to 10.0.0.1.Wrong answer — click to see why▾
Why this is wrong here
The helper-address must be the IP of the DHCP server, which is the SVI address 192.168.50.1, not the link address 10.0.0.1.
Why candidates choose this
Candidates might think the helper-address should be the next-hop IP or the IP of the router interface facing the switch.
✗Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and correct the excluded range on R1 to 192.168.50.1-192.168.50.20.Wrong answer — click to see why▾
Why this is wrong here
The helper-address misconfiguration prevents DHCP requests from being forwarded to the correct server, so fixing only the excluded range is insufficient.
Why candidates choose this
Candidates might focus only on the excluded range issue and overlook the helper-address, especially if they assume the helper-address is correct.
✗Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and change the helper-address on SW1's G0/1 to 192.168.50.1.Wrong answer — click to see why▾
Why this is wrong here
The excluded range must be corrected to allow hosts to receive addresses from the pool; otherwise, the DHCP server will not assign addresses.
Why candidates choose this
Candidates might think that DHCP snooping alone will solve the issue, or they may forget to adjust the excluded range after troubleshooting.
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 50, trust the uplink port to R1, correct the excluded range on R1 to 192.168.50.1-192.168.50.20, and change the helper-address on SW1's G0/1 to 192.168.50.1. — The host received a wrong IP because a rogue DHCP server on Fa0/5 was responding. First, enable DHCP snooping globally with 'ip dhcp snooping' and on VLAN 50 with 'ip dhcp snooping vlan 50'. Then, trust the uplink port to R1 (G0/1) with 'ip dhcp snooping trust'. Next, fix the excluded range on R1: change it to exclude only the first 20 addresses (192.168.50.1-192.168.50.20) so that hosts can obtain other addresses. Finally, correct the helper-address on SW1's G0/1: change 'ip helper-address 10.0.0.1' to 'ip helper-address 192.168.50.1' because the DHCP server is on the VLAN 50 SVI, not on the point-to-point link.
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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