Which symptom best indicates a duplex mismatch on a copper link?

• A. CRC errors only.
• B. High CPU utilization on the switch.
• C. Excessive retransmissions with no collisions.
• D. Collisions and late collisions with poor throughput.

Answer: (D)

Duplex mismatch on a copper link shows up because one end is operating in half-duplex while the other is in full-duplex. The half-duplex side must contend for the medium using CSMA/CD, which leads to collisions. Since the timing between the two ends is off, some of these collisions are detected late, giving you late collisions. All of this forces multiple retransmissions and results in noticeably poor throughput. CRC errors can result from physical issues like bad cabling, and high CPU on the switch isn’t a specific telltale sign of a duplex problem. Excessive retransmissions without collisions don’t fit the typical mismatch pattern, where collisions (and late collisions) are the telltale symptom. So the combination of collisions and late collisions with reduced throughput best indicates a duplex mismatch.