CCNA Network Infrastructure and Connectivity Practice Question
Which TWO statements correctly describe the causes or implications of CRC errors, runts, giants, or output errors as seen in the output of 'show interface' or 'show interface status'?
⚠ Common exam trap
Cisco often tests the misconception that CRC errors always indicate a bad port (trap A) and that giants are always discarded (trap C), when in reality both can have multiple causes and switches can be configured to forward larger frames.
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
✓
A high number of runts on an interface typically indicates excessive collisions or a faulty NIC.
Runts—frames smaller than 64 bytes—often result from collisions truncating frames on half-duplex links or a faulty NIC. Option D is correct because duplex mismatch can cause late collisions, which appear as output errors in 'show interface'; a device on one side full-duplex and the other half-duplex leads to collisions and framing errors. Option A is wrong because CRC errors can stem from faulty cabling, interference, or a mismatched NIC, not exclusively a bad switch port. Option C is wrong because giants (frames over maximum MTU) may be forwarded if the interface is configured with jumbo frames or the switch is set to accept oversize frames. Option E is wrong because 'show controllers' displays frame-size errors like runts and giants, including details beyond CRC errors.
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 are always caused by a faulty switch port and require port replacement.
Why it's wrong here
CRC errors indicate frames received with corrupted bits after the FCS calculation. While a faulty port can cause them, far more common causes are damaged or low-quality cabling, electromagnetic interference, marginal transceivers, or a duplex mismatch. Replacing the port is a last resort, not a default action; first reseat cables, replace patch cords, and verify interface statistics.
- ✓
A high number of runts on an interface typically indicates excessive collisions or a faulty NIC.
Why this is correct
A runt is an Ethernet frame shorter than the 64-byte minimum, often generated when a collision aborts transmission or when a faulty NIC emits undersized frames. On a modern full-duplex link, collisions should not occur, so a persistent runt count usually points to a defective NIC, bad cabling, or a misconfigured interface. High runt counts can also indicate excessive network congestion in legacy half-duplex environments.
- ✗
Giants are frames that exceed the maximum transmission unit (MTU) and are always discarded by the switch.
Why it's wrong here
Giants are frames that exceed the standard maximum Ethernet frame size (1518 bytes). However, modern switches can be configured to accept jumbo frames, which are intentionally larger, so not every oversized frame is discarded. True giants often stem from malfunctioning NICs, software bugs, or misconfigured devices, and the switch's forwarding behavior depends on its jumbo frame support.
- ✓
Output errors, including late collisions, can be caused by a duplex mismatch between the switch and the connected device.
Why this is correct
A duplex mismatch occurs when one device operates in full duplex while the other runs half duplex, disrupting the collision detection mechanism. The full-duplex side transmits without checking the medium, while the half-duplex side sees a busy line and asserts its carrier, causing collisions after the slot time — known as late collisions. These collisions manifest as output errors and late-collision counters on the switch interface, and can severely degrade throughput.
- ✗
The 'show controllers' command provides a detailed view of CRC errors but does not show runts or giants.
Why it's wrong here
'show controllers' is a platform-specific command that exposes hardware-level interface statistics, including CRC errors, and on many Cisco platforms it also reports runts, giants, and other frame-level errors. The exact output varies by IOS version and hardware, so it is not accurate to say it omits runt or giant counts. For general troubleshooting, 'show interface' is normally the first stop for error counters, with 'show controllers' used for deeper physical-layer diagnostics.
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.
✓A high number of runts on an interface typically indicates excessive collisions or a faulty NIC.Correct answer▾
Why this is correct
A runt is an Ethernet frame shorter than the 64-byte minimum, often generated when a collision aborts transmission or when a faulty NIC emits undersized frames. On a modern full-duplex link, collisions should not occur, so a persistent runt count usually points to a defective NIC, bad cabling, or a misconfigured interface. High runt counts can also indicate excessive network congestion in legacy half-duplex environments.
✗CRC errors are always caused by a faulty switch port and require port replacement.Wrong answer — click to see why▾
Why this is wrong here
This statement is too absolute; CRC errors often stem from Layer 1 issues like bad cables or noise, not always a defective port.
✗Giants are frames that exceed the maximum transmission unit (MTU) and are always discarded by the switch.Wrong answer — click to see why▾
Why this is wrong here
The statement is too definitive; giants can be forwarded if jumbo frame support is enabled.
✗The 'show controllers' command provides a detailed view of CRC errors but does not show runts or giants.Wrong answer — click to see why▾
Why this is wrong here
This statement is incorrect because 'show controllers' often includes runt and giant counters on many Cisco platforms.
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
Half-Duplex vs Full-Duplex
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
Frame
A frame is a unit of data at the Data Link layer of networking, containing header, payload, and trailer information for local network delivery.
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