CCNA Network Infrastructure and Connectivity Practice Question
A network administrator is troubleshooting slow file transfers between two servers in different access-layer switches. The administrator runs the 'show interface' command on the uplink connecting the two switches and notices a high number of CRC errors on both ends, but a high number of late collisions on only one interface; the other interface reports no late collisions.
⚠ Common exam trap
Cisco often tests the distinction between CRC errors (which can have multiple causes like cable faults or duplex mismatch) and late collisions (which are a definitive indicator of a duplex mismatch), leading candidates to incorrectly attribute CRC errors alone to a cable issue.
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
✓
Duplex mismatch: the interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex.
A is correct because a duplex mismatch causes one interface to operate in half-duplex (detecting late collisions due to CSMA/CD) while the other operates in full-duplex (no collisions). The half-duplex interface waits for the carrier sense before transmitting, but the full-duplex interface transmits immediately, causing the half-duplex side to detect collisions after the transmission window (late collisions). CRC errors occur on both ends because frames are corrupted when collisions happen. Configuring both ends to auto-negotiate ensures matching duplex and speed, resolving the issue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Duplex mismatch: the interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex.
Why this is correct
Late collisions only occur on half-duplex Ethernet when multiple stations attempt to transmit simultaneously. When one end is full-duplex, the half-duplex end perceives any overlapping transmission as a collision, producing late collisions and CRC errors. Auto-negotiation correctly sets both ends to full-duplex when supported, fixing the problem.
- ✗
Speed mismatch: one interface is set to 100 Mbps, the other to 1 Gbps. This causes frequent link flaps, resulting in CRC errors. Use the 'speed' command to match the rates.
Why it's wrong here
A speed mismatch prevents the Ethernet link from ever coming up—autonegotiation requires both peers to advertise a common speed and duplex via Fast Link Pulses, so a 100 Mbps/1 Gbps mismatch would leave the interface down/down or up/down, not stable and passing traffic. Late collisions, unlike speed errors, are a half-duplex CSMA/CD timing failure that occurs after the 512-bit slot time, typically because the far end is full-duplex and fails to defer. Changing the 'speed' command would not resolve the condition; only matching duplex settings would eliminate the late-collision counter and the resulting CRC errors.
- ✗
Faulty Ethernet cable causing signal degradation, which leads to CRC errors. The late collisions are a result of the switch misdetecting collisions due to the degraded signal. Replace the cable.
Why it's wrong here
A faulty cable can cause CRC errors, but it will not generate late collisions unless the problem causes autonegotiation to fall back to half-duplex — which is a duplex mismatch, not a cable fault per se. Late collisions are a protocol-layer symptom, not physical-layer.
- ✗
A broadcast storm caused by a loop is flooding the uplink with frames, leading to CRC errors and late collisions as the switch discards excess traffic. Enable Spanning Tree Protocol to block the redundant path.
Why it's wrong here
Broadcast storms generate massive amounts of traffic but do not directly cause CRC errors or late collisions. Late collisions are a layer-2 contention mechanism in half-duplex Ethernet, unrelated to loop-induced flooding. CRC errors from a storm would require physical bit errors, which aren’t inherent to an Ethernet frame storm.
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.
✓Duplex mismatch: the interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex.Correct answer▾
Why this is correct
Late collisions only occur on half-duplex Ethernet when multiple stations attempt to transmit simultaneously. When one end is full-duplex, the half-duplex end perceives any overlapping transmission as a collision, producing late collisions and CRC errors. Auto-negotiation correctly sets both ends to full-duplex when supported, fixing the problem.
✗Speed mismatch: one interface is set to 100 Mbps, the other to 1 Gbps. This causes frequent link flaps, resulting in CRC errors. Use the 'speed' command to match the rates.Wrong answer — click to see why▾
Why this is wrong here
Confuses speed mismatch (which prevents link establishment) with duplex mismatch (which allows the link to come up but causes errors and late collisions).
✗Faulty Ethernet cable causing signal degradation, which leads to CRC errors. The late collisions are a result of the switch misdetecting collisions due to the degraded signal. Replace the cable.Wrong answer — click to see why▾
Why this is wrong here
Assumes all interface errors are cable-related and overlooks the characteristic late-collision signature of duplex mismatch.
✗A broadcast storm caused by a loop is flooding the uplink with frames, leading to CRC errors and late collisions as the switch discards excess traffic. Enable Spanning Tree Protocol to block the redundant path.Wrong answer — click to see why▾
Why this is wrong here
Misinterprets high utilization as a source of physical-layer errors and ignores the diagnostic value of asymmetric late collisions.
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?”
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Network Topology Types
Key term
ISP
An ISP is a company that provides individuals and organizations access to the Internet, along with related services like email and web hosting.
Key term
CSMA/CD
Carrier Sense Multiple Access with Collision Detection is a network protocol used in Ethernet to manage data transmission and handle collisions when two devices send data at the same time.
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Written by Johnson Ajibi, MSc IT Security
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