Which statement best describes the impact of a duplex mismatch and a valid way to diagnose it?

• A. Increased collisions and late frames on the affected link; diagnose by checking interface stats and duplex settings on both ends.
• B. Increased MTU and packet fragmentation; diagnose by adjusting MTU.
• C. No effect on performance; diagnose by ping tests.
• D. Increased VPN latency; diagnose by rebooting devices.

Answer: (A)

Duplex mismatch disrupts how two devices communicate over the same Ethernet link. If one side is operating in half‑duplex (where devices contend for the wire and collisions can occur) and the other is in full‑duplex (no collisions because send and receive paths are independent), the timing and behavior of the link don’t align. That misalignment leads to more retransmissions, which shows up as increased collisions and late frames on the affected link. You’ll often see these error indicators rise in the interface statistics when a mismatch exists.

The best way to diagnose it is to check the duplex and speed settings on both ends and compare them. Look at the device statistics for the link and note any collisions, late collisions, CRC errors, or other error counters. If the speeds or duplex modes don’t match, or if auto‑negotiation isn’t negotiating consistently, that’s a strong sign of a duplex mismatch. Align both ends—preferably with consistent speed and full‑duplex on both sides, or ensure auto‑negotiation negotiates identically on both ends—and recheck the interface counters.

Other options miss the core issue: MTU changes don’t diagnose duplex problems, ping tests don’t reveal the collision behavior caused by a mismatch, and rebooting won’t fix the underlying misconfiguration.