CCNA Network Infrastructure and Connectivity Practice Question
Which TWO symptoms are most likely to appear in the output of 'show interfaces' when a duplex mismatch exists between a switch port and a connected host?
⚠ Common exam trap
Cisco often tests the distinction that 'runts' and 'frame errors' are symptoms of duplex mismatch on the full-duplex side, while 'excessive collisions' and 'CRC errors' appear on the half-duplex side, and candidates may incorrectly assume both symptoms appear on the same interface.
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
✓
Excessive collisions and CRC errors on the interface
A duplex mismatch causes collisions on the half-duplex side, resulting in excessive collisions and CRC errors (option A). On the full-duplex side, the host receives truncated frames from the half-duplex side's collisions, leading to runts and frame errors (option B). Option C is incorrect because 'Auto-negotiation failed' would appear only if negotiation itself failed, not from a mismatch after successful negotiation. Option D is wrong because a duplex mismatch typically reduces throughput and causes errors, not a high input rate. Option E is incorrect because the line protocol remains up; duplex mismatch does not bring the line protocol down.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Excessive collisions and CRC errors on the interface
Why this is correct
In a duplex mismatch, the side forced to half-duplex continues to use CSMA/CD and expects to transmit only when the medium is clear. Because the full-duplex peer transmits without first performing carrier sense, the half-duplex side experiences collisions on nearly every frame. These collisions are reported as excessive collisions in the interface output, and the frames that are truncated or otherwise corrupted during the collisions fail the FCS, resulting in CRC errors. Thus, excessive collisions and CRC errors are the hallmark symptoms on the half-duplex side of a mismatch.
- ✓
Runts and frame errors on the interface
Why this is correct
On the full-duplex side of a duplex mismatch, the NIC does not listen for collisions because CSMA/CD is disabled. When the half-duplex peer detects a collision, it aborts transmission and sends a jam signal, which leaves a truncated, non-aligned frame on the wire. The full-duplex interface receives this fragment—shorter than the 64-byte minimum—and counts it as a runt, while the lack of a valid byte boundary produces alignment or frame errors. This is why runts and frame errors are the characteristic symptoms on the full-duplex side.
- ✗
Auto-negotiation failed message in the interface output
Why it's wrong here
The 'show interfaces' output does not display 'auto-negotiation failed' as a direct symptom. It shows the negotiated duplex and speed, but a mismatch can occur even if auto-negotiation succeeds (e.g., when one side is manually configured).
- ✗
High input rate on the interface
Why it's wrong here
A duplex mismatch reduces effective throughput but it does not cause the input rate counter to spike. Errors dominate the interface statistics: damaged frames are discarded, and the upper-layer protocols must retransmit, which consumes bandwidth but does not show as an abnormally high input rate. If the input rate is high, it is more likely caused by genuine traffic such as a broadcast storm, not by a duplex mismatch. Therefore, high input rate is not a valid indicator of this problem.
- ✗
Line protocol is down
Why it's wrong here
A duplex mismatch does not bring the line protocol down. The interface status remains up/up (or up/down if there is a physical issue), but errors accumulate. Line protocol down indicates a more fundamental problem like no keepalives or a Layer 2 issue.
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.
✓Excessive collisions and CRC errors on the interfaceCorrect answer▾
Why this is correct
In a duplex mismatch, the side forced to half-duplex continues to use CSMA/CD and expects to transmit only when the medium is clear. Because the full-duplex peer transmits without first performing carrier sense, the half-duplex side experiences collisions on nearly every frame. These collisions are reported as excessive collisions in the interface output, and the frames that are truncated or otherwise corrupted during the collisions fail the FCS, resulting in CRC errors. Thus, excessive collisions and CRC errors are the hallmark symptoms on the half-duplex side of a mismatch.
✗Auto-negotiation failed message in the interface outputWrong answer — click to see why▾
Why this is wrong here
The 'show interfaces' command does not display an 'auto-negotiation failed' message. It shows the negotiated duplex and speed, but a mismatch can occur even when auto-negotiation succeeds if one side is manually configured. The absence of such a message makes this option incorrect.
Why candidates choose this
Students may think that a duplex mismatch always results from auto-negotiation failure, but mismatches can occur even with successful auto-negotiation when manual configuration overrides it. The 'show interfaces' output does not explicitly indicate negotiation failure.
✗High input rate on the interfaceWrong answer — click to see why▾
Why this is wrong here
Duplex mismatch typically causes high error rates and retransmissions, which reduce actual throughput. The input rate shown in 'show interfaces' is usually not elevated; instead, it may be low due to errors. High input rate is not a typical symptom of duplex mismatch.
Why candidates choose this
Some might assume that errors lead to retransmissions, increasing the input rate. However, in practice, the interface counters show errors, not high throughput, and the input rate is often normal or low.
✗Line protocol is downWrong answer — click to see why▾
Why this is wrong here
A duplex mismatch does not cause the line protocol to go down. The interface remains up/up (or up/down if there is a physical issue), but errors accumulate. Line protocol down indicates a more fundamental Layer 2 problem, such as no keepalives or a misconfigured encapsulation.
Why candidates choose this
Students may confuse duplex mismatch with other issues that cause the line protocol to go down, such as a cable fault or misconfiguration. However, duplex mismatch only affects data integrity, not the line protocol state.
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
ARP — Address Resolution Protocol
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
Key term
Throughput
Throughput is the rate at which data is successfully transferred from one point to another over a network, typically measured in bits per second.
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