A speed mismatch occurs when two connected Ethernet devices are configured to operate at different speeds. In scenario A, one device is manually set to 1 Gbps, while the other is set to 100 Mbps. Since both ends are hard-coded to different speeds, they cannot successfully negotiate a common speed, leading to a mismatch and resulting in a failed or unstable link.
In contrast, scenario B, where both ends are manually set to the same speed, ensures compatibility and stable communication. Scenario C can lead to a duplex mismatch rather than a speed mismatch; the auto-negotiating end may default to half-duplex if it cannot determine the duplex setting of the manually configured end. Scenario D is less common; if auto-negotiation fails, devices may revert to their lowest common speed, but this typically results in reduced performance rather than a complete mismatch.