CISSP Communication and Network Security Practice Question
This CISSP practice question tests your understanding of communication and network 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.
Which TWO of the following are common causes of network performance degradation that can be detected by network monitoring tools?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Duplex mismatch on a link
Duplex mismatch occurs when one end of an Ethernet link is configured for full-duplex and the other for half-duplex, causing frame collisions, CRC errors, and severe performance degradation. Network monitoring tools detect this through interface error counters (e.g., runts, FCS errors, late collisions) and can alert on excessive errors. This is a common physical-layer issue that directly impacts throughput and latency.
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.
✗
Incorrect default gateway
Why it's wrong here
Incorrect default gateway breaks connectivity to remote networks but does not degrade performance locally.
✗
High CPU utilization on routers
Why it's wrong here
High CPU is a symptom of other issues, not a direct cause of network performance degradation.
✓
Duplex mismatch on a link
Why this is correct
Duplex mismatch causes collisions and retransmissions, significantly degrading performance.
Related concept
Read the scenario before looking for a memorised answer.
✓
Excessive broadcast traffic
Why this is correct
Broadcasts consume bandwidth and can cause performance degradation on the network.
Related concept
Read the scenario before looking for a memorised answer.
✗
DNS misconfiguration
Why it's wrong here
DNS issues cause name resolution failures, not network throughput degradation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
ISC2 often tests duplex mismatch as a classic performance issue, and the trap here is that candidates confuse 'high CPU utilization on routers' (a symptom) with a direct cause of degradation, or they mistakenly think DNS misconfiguration affects network throughput rather than just name resolution.
Detailed technical explanation
How to think about this question
Duplex mismatch triggers the half-duplex side to detect collisions, causing it to invoke the backoff algorithm (CSMA/CD), which introduces random delays and retransmissions. The full-duplex side does not defer, leading to frame drops and excessive late collisions (collisions beyond the slot time of 512 bit-times for 10/100 Mbps Ethernet). Real-world scenario: a switch port set to auto-negotiation fails to negotiate with a manually configured full-duplex server, resulting in the switch defaulting to half-duplex while the server remains full-duplex, causing massive packet loss and retransmissions.
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 analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CISSP question in full detail.
Communication and Network Security — This question tests Communication and Network Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Duplex mismatch on a link — Duplex mismatch occurs when one end of an Ethernet link is configured for full-duplex and the other for half-duplex, causing frame collisions, CRC errors, and severe performance degradation. Network monitoring tools detect this through interface error counters (e.g., runts, FCS errors, late collisions) and can alert on excessive errors. This is a common physical-layer issue that directly impacts throughput and latency.
What should I do if I get this CISSP 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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