DHCP Snooping for Rogue Server Prevention
A security analyst has enabled DHCP snooping on all VLANs of the company's switches to mitigate the risk of rogue DHCP servers. After implementation, the analyst discovers that clients are still receiving IP addresses from an unauthorized DHCP server. The unauthorized server is connected to a switch port that is currently configured as a trusted port. What should the analyst do to stop the rogue DHCP server from offering addresses?
Quick Answer
The fix here comes down to understanding what trusted actually means in DHCP snooping: it isn't a security label, it's a permission to send server-side DHCP traffic. By default, only uplink ports toward legitimate DHCP infrastructure should be marked trusted, while every access port facing end users or unknown devices should be untrusted so the switch drops any DHCP OFFER or ACK it sees there. In this scenario the rogue server ended up on a trusted port, which told the switch to treat its offers as legitimate rather than filtering them, so the feature was enabled but misconfigured rather than broken. Reconfiguring that specific port to untrusted restores the intended behavior: the switch will now silently discard any DHCP server message arriving from it, regardless of how convincing the response looks. This distinction matters because DHCP snooping doesn't inspect content to judge whether a server is legitimate - it only cares which port the message arrived on, which is why port configuration, not detection logic, is almost always the actual point of failure in these questions. Whenever a scenario says a security feature is enabled but the attack is still succeeding, look for a misapplied trust setting on the specific port involved rather than assuming the feature itself needs to be replaced.
⚠ Common exam trap
CompTIA often tests the misconception that DHCP snooping alone blocks all rogue servers, but the trap is that it only works if the rogue server's port is correctly classified as untrusted; candidates may forget that a trusted port bypasses all DHCP snooping filtering.
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
✓
Change the port connecting the unauthorized server to an untrusted port.
DHCP snooping operates by designating switch ports as either trusted or untrusted. Trusted ports are allowed to send DHCP server messages (OFFER, ACK), while untrusted ports are blocked from sending such messages. Since the rogue server is connected to a trusted port, it can still offer IP addresses. Changing the port to untrusted will cause the switch to drop all DHCP server messages from that port, stopping the rogue server.
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 Dynamic ARP Inspection on the VLAN.
Why it's wrong here
Dynamic ARP Inspection validates ARP packets, not DHCP messages. It does not prevent a rogue DHCP server from offering addresses on a trusted port.
When this WOULD be correct
Dynamic ARP Inspection would be correct in a scenario where a rogue device is performing ARP spoofing or man-in-the-middle attacks by sending fake ARP replies, and the goal is to validate ARP packets against the DHCP snooping binding table.
- ✓
Change the port connecting the unauthorized server to an untrusted port.
Why this is correct
DHCP snooping treats trusted ports as authorized sources of DHCP offers. By making the port untrusted, the switch will drop any DHCP server messages received on that port.
- ✗
Configure port security on the unauthorized server's port to limit MAC addresses.
Why it's wrong here
Port security restricts the number of MAC addresses on a port but does not filter DHCP messages. The rogue server could still issue DHCP offers.
When this WOULD be correct
Port security would be correct in a scenario where an attacker is performing MAC flooding or a CAM table overflow attack to intercept traffic, and the goal is to restrict the number of MAC addresses per port.
- ✗
Increase the rate limit on the unauthorized server's port.
Why it's wrong here
Rate limiting controls the amount of traffic, not the type of traffic. It would not prevent the rogue server from sending DHCP offers.
When this WOULD be correct
This option would be correct in a scenario where a legitimate DHCP server is being overwhelmed by excessive DHCP requests (e.g., due to a DoS attack), and the goal is to protect the server by limiting the rate of incoming DHCP packets.
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 N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓Change the port connecting the unauthorized server to an untrusted port.Correct answer▾
Why this is correct
DHCP snooping treats trusted ports as authorized sources of DHCP offers. By making the port untrusted, the switch will drop any DHCP server messages received on that port.
✗Enable Dynamic ARP Inspection on the VLAN.Wrong answer — click to see why▾
Why this is wrong here
DHCP snooping already filters unauthorized DHCP servers by trusting only specific ports; Dynamic ARP Inspection (DAI) validates ARP packets, not DHCP offers, so it wouldn't stop the rogue DHCP server from assigning IP addresses.
★ When this WOULD be the correct answer
Dynamic ARP Inspection would be correct in a scenario where a rogue device is performing ARP spoofing or man-in-the-middle attacks by sending fake ARP replies, and the goal is to validate ARP packets against the DHCP snooping binding table.
Why candidates choose this
Candidates may confuse DHCP snooping with Dynamic ARP Inspection, thinking both are needed to fully secure DHCP, or they might believe DAI can block rogue DHCP servers since it relies on DHCP snooping bindings.
✗Configure port security on the unauthorized server's port to limit MAC addresses.Wrong answer — click to see why▾
Why this is wrong here
Port security limits the number of MAC addresses on a port but does not prevent a rogue DHCP server from offering addresses. The issue is DHCP snooping trust, not MAC flooding.
★ When this WOULD be the correct answer
Port security would be correct in a scenario where an attacker is performing MAC flooding or a CAM table overflow attack to intercept traffic, and the goal is to restrict the number of MAC addresses per port.
Why candidates choose this
Candidates may confuse port security with DHCP snooping, thinking that limiting MAC addresses can block unauthorized servers, but port security does not filter DHCP server messages.
✗Increase the rate limit on the unauthorized server's port.Wrong answer — click to see why▾
Why this is wrong here
Increasing the rate limit on the unauthorized server's port would allow more DHCP traffic, not block it. The issue is that the port is trusted, so DHCP snooping does not filter DHCP server messages from that port.
★ When this WOULD be the correct answer
This option would be correct in a scenario where a legitimate DHCP server is being overwhelmed by excessive DHCP requests (e.g., due to a DoS attack), and the goal is to protect the server by limiting the rate of incoming DHCP packets.
Why candidates choose this
Candidates may confuse rate limiting with security controls, thinking that limiting traffic can stop rogue servers, or they may misapply rate limiting as a general mitigation for unauthorized devices.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Related to this question
Learn chapter
Wireless Security Protocols
Key term
CAN
A CAN (Controller Area Network) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other without a host computer.
Key term
DHCP
Dynamic Host Configuration Protocol (DHCP) automatically assigns IP addresses and network settings to devices on a network, so they can communicate without manual configuration.
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Same concept, more angles
1 more way this is tested on N10-009
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A security analyst notices that a network switch is receiving DHCP discover messages from a rogue device offering IP addresses. The rogue device is causing clients to obtain invalid IP addresses and lose network connectivity. Which security feature should be implemented on the switch to prevent this type of attack?
hard- A.Dynamic ARP inspection (DAI)
- ✓ B.DHCP snooping
- C.Port security
- D.802.1X authentication
Why B: B is correct because DHCP snooping is a security feature that filters untrusted DHCP messages on a switch. It distinguishes between trusted ports (connected to legitimate DHCP servers) and untrusted ports (connected to clients or rogue devices). When a rogue device sends DHCP discover messages offering IP addresses, DHCP snooping on untrusted ports drops those messages, preventing the rogue server from assigning invalid IP addresses.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This N10-009 practice question is part of Courseiva's free CompTIA 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 N10-009 exam.