The correct answer is ARP poisoning, also known as ARP spoofing. This attack exploits the lack of authentication in the Address Resolution Protocol by flooding the network with forged ARP replies that map the attacker’s MAC address to the IP of the default gateway, causing all inter-subnet traffic to be intercepted. On the CompTIA Network+ N10-009 exam, this scenario tests your ability to recognize the symptoms of a man-in-the-middle attack, where high CPU utilization on a switch often results from processing the flood of spoofed packets and redirected traffic. A common trap is confusing this with a DHCP starvation attack, but ARP poisoning specifically targets the gateway’s IP-to-MAC mapping. For detection and prevention, remember that Dynamic ARP Inspection (DAI) validates ARP packets against a trusted DHCP snooping database, dropping invalid replies. Memory tip: “Poisoned ARP, traffic gets scarred—DAI checks the binding card.”
N10-009 Network Security Practice Question
This N10-009 practice question tests your understanding of network security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
A network security analyst notices high CPU utilization on the core switch and detects a large volume of ARP replies from a single IP address that claims to be the default gateway for all local subnets. Which type of attack is MOST likely occurring?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Sending forged ARP replies to redirect traffic is the classic definition of ARP poisoning. The high volume of replies and claiming to be the gateway are strong indicators.
B
DHCP starvation
Why wrong: DHCP starvation floods the network with DHCP requests to exhaust the address pool. It does not involve ARP replies or claiming to be the gateway.
C
MAC flooding
Why wrong: MAC flooding overwhelms a switch's MAC address table with fake MAC addresses to force it into hub mode, but it does not involve ARP replies pretending to be the gateway.
D
DNS amplification
Why wrong: DNS amplification is a DDoS attack that floods a target with large DNS response traffic. It does not operate at the ARP level.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ARP poisoning
The attack described is ARP poisoning (also known as ARP spoofing), where an attacker sends forged ARP replies to associate their MAC address with the IP address of the default gateway. This causes all traffic destined for other subnets to be redirected to the attacker's machine, leading to high CPU utilization on the switch as it processes the flood of ARP packets and forwards the intercepted traffic.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
ARP poisoning
Why this is correct
Sending forged ARP replies to redirect traffic is the classic definition of ARP poisoning. The high volume of replies and claiming to be the gateway are strong indicators.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
DHCP starvation
Why it's wrong here
DHCP starvation floods the network with DHCP requests to exhaust the address pool. It does not involve ARP replies or claiming to be the gateway.
✗
MAC flooding
Why it's wrong here
MAC flooding overwhelms a switch's MAC address table with fake MAC addresses to force it into hub mode, but it does not involve ARP replies pretending to be the gateway.
✗
DNS amplification
Why it's wrong here
DNS amplification is a DDoS attack that floods a target with large DNS response traffic. It does not operate at the ARP level.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between ARP poisoning and MAC flooding by describing symptoms like 'high CPU utilization' and 'large volume of ARP replies,' which can mislead candidates into thinking MAC flooding is the answer because it also causes high CPU, but the key clue is the specific use of ARP replies targeting the default gateway IP.
Detailed technical explanation
How to think about this question
ARP poisoning exploits the stateless nature of ARP; hosts accept unsolicited ARP replies (gratuitous ARPs) without verification, allowing an attacker to overwrite the ARP cache of other devices. In a real-world scenario, an attacker could use tools like arpspoof or Ettercap to perform a man-in-the-middle attack, intercepting and potentially modifying traffic between clients and the gateway. The high CPU utilization on the core switch often results from the switch having to process and forward the large volume of ARP traffic, as well as the increased load from redirecting all subnet traffic through the attacker's MAC address.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Security — This question tests Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ARP poisoning — The attack described is ARP poisoning (also known as ARP spoofing), where an attacker sends forged ARP replies to associate their MAC address with the IP address of the default gateway. This causes all traffic destined for other subnets to be redirected to the attacker's machine, leading to high CPU utilization on the switch as it processes the flood of ARP packets and forwards the intercepted traffic.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
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 discovers that an unauthorized device is sending forged ARP replies, causing other devices to map the default gateway IP address to the attacker's MAC address. Which security feature should be implemented on the switches to prevent this attack?
medium
A.Port security
B.DHCP snooping
✓ C.Dynamic ARP Inspection
D.BPDU guard
Why C: Dynamic ARP Inspection (DAI) is the correct choice because it validates ARP packets on a per-port basis, ensuring that only legitimate ARP replies with correct IP-to-MAC bindings are forwarded. DAI uses a DHCP snooping binding table (or static ARP ACLs) to intercept and verify ARP packets, dropping forged replies that attempt to poison the ARP cache of other devices.
Variation 2. A security analyst discovers that an unauthorized device is sending forged ARP replies to poison the ARP caches of other devices on the network. Which security feature should be implemented on the switches to prevent this?
medium
A.Port security
B.DHCP snooping
✓ C.Dynamic ARP Inspection
D.STP BPDU guard
Why C: Dynamic ARP Inspection (DAI) validates ARP packets on a per-interface basis by intercepting all ARP requests and replies and verifying that they match entries in the DHCP snooping binding table. If an ARP reply contains a forged IP-to-MAC mapping, DAI drops the packet, preventing ARP cache poisoning. This directly stops the described attack where an unauthorized device sends forged ARP replies.
Last reviewed: Jun 11, 2026
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