CCNA Network Infrastructure and Connectivity Practice Question
Which TWO interface error counters indicate a Layer 1 issue?
⚠ Common exam trap
Cisco often tests the distinction between Layer 1 errors (CRC, runts, giants, frame errors) and Layer 2/3 congestion indicators (output drops, input drops, ignored counts), so the trap is that candidates mistakenly associate any 'drop' or 'error' counter with the physical layer without understanding the underlying cause.
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
✓
CRC errors
CRC errors occur when the cyclic redundancy check computed at the receiver does not match the value appended by the sender, indicating that the frame was corrupted during transmission. This corruption is typically caused by physical-layer problems such as faulty cabling, bad connectors, or excessive electrical noise. Runts are frames that are smaller than the minimum Ethernet frame size of 64 bytes (excluding preamble), and they often result from collisions or transceiver issues that are Layer 1 phenomena. Both counters directly point to physical-layer impairments rather than logical or congestion-related issues.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
CRC errors
Why this is correct
CRC errors occur when the receiving interface computes the frame check sequence and finds it does not match the value sent by the sender, proving that bits were corrupted during transmission. This is a classic Layer 1 symptom, typically caused by bad cabling, loose connectors, electromagnetic interference, or a failing transceiver. Because the corruption happens on the physical medium, CRC errors directly identify a Layer 1 problem.
- ✗
Output queue drops
Why it's wrong here
Output queue drops occur on egress interfaces when the transmit buffer is full and packets are discarded. This is caused by Layer 3 congestion, traffic bursts, or QoS shaping, not by physical media problems. Since the issue lies in packet scheduling and buffer management rather than signal integrity, it is not a Layer 1 error.
- ✓
Runts
Why this is correct
A runt is an Ethernet frame smaller than the IEEE 802.3 minimum of 64 bytes, often resulting from a collision, a defective NIC, or a faulty cable that truncates the signal. These frames are discarded at the receiving interface because they fail size validation, and their presence typically points to physical layer or hardware issues such as bad connectors or crosstalk. Therefore, runts are correctly classified as a Layer 1 error.
- ✗
Input errors
Why it's wrong here
Input errors is a broad aggregate counter that includes all receive-side errors, such as CRC errors, runts, giants, and framing errors. Because it combines both Layer 1 and Layer 2 issues, it is too general to be a specific Layer 1 error type. A high input error count may coincide with physical layer faults, but it is not a definitive indicator by itself.
- ✗
Ignored packets
Why it's wrong here
Ignored packets are frames discarded because the interface's input buffer or memory is unavailable, often due to high traffic bursts or hardware performance limits. This is a device resource exhaustion problem, not a transmission medium issue, and can happen even with flawless cabling and signal quality. Thus, ignored packets do not indicate a Layer 1 fault.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓CRC errorsCorrect answer▾
Why this is correct
CRC errors occur when the receiving interface computes the frame check sequence and finds it does not match the value sent by the sender, proving that bits were corrupted during transmission. This is a classic Layer 1 symptom, typically caused by bad cabling, loose connectors, electromagnetic interference, or a failing transceiver. Because the corruption happens on the physical medium, CRC errors directly identify a Layer 1 problem.
✗Output queue dropsWrong answer — click to see why▾
Why this is wrong here
Output queue drops occur when the transmit queue is full due to congestion, typically at Layer 3 (IP) or Layer 2 (switching). They are not caused by physical layer issues but by traffic overload or insufficient buffer space.
Why candidates choose this
Students might confuse output drops with input errors, thinking any 'drop' could be physical, but output drops are purely a queuing mechanism issue.
✗Input errorsWrong answer — click to see why▾
Why this is wrong here
Input errors is a catch-all counter that includes CRC, runts, giants, and framing errors. While it can indicate Layer 1 issues, it is not specific to Layer 1 because it also includes errors from higher layers (e.g., alignment errors). The question asks for counters that indicate a Layer 1 issue, and input errors is too broad.
Why candidates choose this
Since input errors often include CRC and runts, students may think it directly indicates Layer 1, but it is a summary counter that can also include non-Layer1 errors.
✗Ignored packetsWrong answer — click to see why▾
Why this is wrong here
Ignored packets are dropped due to buffer overflow, often from high traffic or hardware limitations, not specifically a Layer 1 error. They are typically caused by congestion at Layer 2 or Layer 3, not physical layer faults.
Why candidates choose this
The term 'ignored' might suggest the interface is ignoring bad frames, but it actually refers to packets dropped because the receive buffer is full, which is a resource issue, not a physical layer error.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Go deeper
Related to this question
Learn chapter
Ethernet Standards
Key term
Receiver
A receiver is a hardware device or software component that accepts incoming signals, data, or transmissions from a sender over a communication channel.
Key term
Cyclic Redundancy Check
A Cyclic Redundancy Check (CRC) is an error-detecting code used to verify that data transmitted over a network or stored on a device has not been corrupted.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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