Quick Answer
The answer is the CRC errors counter. This is correct because Cyclic Redundancy Check errors confirm that received frames have failed their integrity check, which directly points to physical-layer problems such as collisions, faulty cabling, or signal degradation. When troubleshooting WAN link connectivity, collisions are rare on modern full-duplex links, but if the interface is misconfigured as half-duplex, collisions will corrupt frames and manifest as CRC errors, making this counter the definitive indicator of packet loss from collisions. On the Cisco DevNet Associate 200-901 exam, this concept tests your ability to correlate interface counters with underlying network issues, often appearing in scenario-based questions where a slow WAN link points to a duplex mismatch. A common trap is to look at the “collisions” counter instead, but remember that on a WAN link, collisions are not directly counted; they hide as CRC errors. Memory tip: “CRC = Corrupted Reception Confirms collisions.”
This 200-901 practice question tests your understanding of network fundamentals. 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.
A network engineer is troubleshooting slow connectivity between two sites connected via a WAN link. The engineer suspects packet loss due to collisions. Which interface counter should be examined to confirm this?
Runts
Why wrong: Runts are frames smaller than 64 bytes, not collision-specific.
CRC errors
CRC errors indicate frame checksum failures often caused by collisions.
Output errors
Why wrong: Output errors are a general category, not specific to collisions.
Giants
Why wrong: Giants are frames larger than maximum size, not collision-specific.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
CRC errors
CRC (Cyclic Redundancy Check) errors indicate that frames received on an interface have failed the integrity check, which is often caused by physical-layer issues such as collisions, faulty cabling, or signal degradation. In the context of a WAN link, collisions are not typical (since full-duplex is standard), but if the link is misconfigured as half-duplex, collisions can occur and will manifest as CRC errors. Thus, examining the CRC errors counter is the correct way to confirm packet loss due to collisions.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Runts
Why it's wrong here
Runts are frames smaller than 64 bytes, not collision-specific.
CRC errors
Why this is correct
CRC errors indicate frame checksum failures often caused by collisions.
Related concept
Read the scenario before looking for a memorised answer.
Output errors
Why it's wrong here
Output errors are a general category, not specific to collisions.
Giants
Why it's wrong here
Giants are frames larger than maximum size, not collision-specific.
Common exam traps
Cisco often tests the misconception that collisions are directly indicated by 'runts' or 'output errors', but the correct indicator for collision-induced corruption is the CRC errors counter, especially when the link is suspected of operating in half-duplex mode.
Trap categories for this question
Command / output trap
Output errors are a general category, not specific to collisions.
Detailed technical explanation
CRC errors occur when the frame check sequence (FCS) computed by the receiver does not match the FCS appended by the sender, indicating bit-level corruption during transmission. In half-duplex environments, collisions cause frame fragments that are often counted as runts or CRC errors, but on modern full-duplex WAN links, CRC errors more commonly point to faulty transceivers, signal attenuation, or electromagnetic interference. Real-world troubleshooting often involves checking the 'show interfaces' command output for CRC errors alongside 'input errors' and 'frame errors' to isolate physical-layer problems.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 200-901 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: CRC errors — CRC (Cyclic Redundancy Check) errors indicate that frames received on an interface have failed the integrity check, which is often caused by physical-layer issues such as collisions, faulty cabling, or signal degradation. In the context of a WAN link, collisions are not typical (since full-duplex is standard), but if the link is misconfigured as half-duplex, collisions can occur and will manifest as CRC errors. Thus, examining the CRC errors counter is the correct way to confirm packet loss due to collisions.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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