A technician investigates a workstation that copies files very slowly across a 100 Mbps copper link. The switch port, configured for full duplex, reports rising CRC and runt frame errors, while the workstation NIC, which has fallen back to half duplex, logs rising late collisions. What is the most likely cause of these symptoms?
- AThe run exceeds the 100 metre limit for twisted-pair copper, so attenuation is corrupting frames.
- BExcessive crosstalk between adjacent wire pairs in the patch cable is corrupting the signal.
- CA duplex mismatch, with the switch forced to full duplex while the NIC operates in half duplex. Correct
- DA speed mismatch between a 100 Mbps interface and a 10 Mbps interface on the same link.
Why A is wrong: An over-length run does raise errors, which makes this tempting, but attenuation does not force one end into half duplex or generate late collisions, so it does not fit the counters described.
Why B is wrong: Crosstalk can cause CRC errors, so it is a plausible guess, but it does not drive one interface into half duplex nor produce late collisions, which are the distinguishing symptoms here.
Why C is correct: With the switch forced to full duplex it transmits without carrier sense, so the half-duplex NIC detects those transmissions as late collisions while both ends log CRC and runt errors from the truncated frames, and throughput collapses.
Why D is wrong: A true speed mismatch usually prevents the link from coming up at all rather than passing traffic slowly, and it would not produce the late-collision counters seen here.