The answer is ACL logging with the 'log' keyword, along with broadcast storms and STP topology changes, as these are three common causes of high CPU utilization on a Cisco Catalyst switch. ACL logging forces every matched packet to be processed by the switch CPU rather than being forwarded in hardware, which rapidly consumes CPU cycles under heavy traffic. Broadcast storms overwhelm the CPU because each broadcast frame must be examined by the CPU to determine forwarding decisions when the storm exceeds the switch’s hardware forwarding capacity. On the ENCOR 350-401 exam, this topic tests your understanding of control plane policing and the distinction between hardware-switched and process-switched traffic. A common trap is assuming high CPU is always due to a virus or bandwidth saturation, but the exam focuses on features that explicitly punt traffic to the CPU. Memory tip: think “LOG = CPU hog” — any feature that logs or processes traffic in software will spike CPU usage.
CCNP Network Assurance Practice Question
This 350-401 practice question tests your understanding of network assurance. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which THREE are common causes of high CPU utilization on a Cisco Catalyst switch? (Choose three.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Broadcast storms
A broadcast storm occurs when excessive broadcast traffic overwhelms the switch CPU, as each broadcast frame must be processed by the CPU to determine forwarding decisions. This consumes CPU cycles, especially when the storm exceeds the switch's hardware forwarding capacity, leading to high CPU utilization.
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.
✓
Broadcast storms
Why this is correct
Option B is correct because broadcast storms flood the CPU with interrupts.
Related concept
Read the scenario before looking for a memorised answer.
✗
Excessive hardware switching of packets
Why it's wrong here
Option A is wrong because hardware switching is done in ASICs, not CPU.
✗
Low memory conditions
Why it's wrong here
Option E is wrong because low memory does not directly cause high CPU.
✓
Frequent STP topology changes
Why this is correct
Option C is correct because STP processing consumes CPU cycles.
Related concept
Read the scenario before looking for a memorised answer.
✓
ACL logging with 'log' keyword
Why this is correct
Option D is correct because logging each match is CPU-intensive.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between control plane (CPU-processed) and data plane (ASIC-switched) traffic; the trap here is assuming hardware switching tasks consume CPU cycles, when in fact they are offloaded to dedicated hardware.
Detailed technical explanation
How to think about this question
Under the hood, the switch CPU handles control plane processes such as STPBPDU processing, ACL logging, and broadcast frame punting. In a broadcast storm, the CPU is forced to process each broadcast frame, leading to high CPU utilization. Real-world scenarios include a misconfigured loop or a faulty NIC generating excessive broadcasts, which can be mitigated using storm-control commands to limit broadcast traffic.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Assurance — This question tests Network Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Broadcast storms — A broadcast storm occurs when excessive broadcast traffic overwhelms the switch CPU, as each broadcast frame must be processed by the CPU to determine forwarding decisions. This consumes CPU cycles, especially when the storm exceeds the switch's hardware forwarding capacity, leading to high CPU utilization.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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