Port, Speed, and Duplex Mismatches

Some links fail not because hardware is broken but because settings disagree. Speed and duplex mismatches, along with port misconfigurations, cause poor or no connectivity even on perfect cabling. These are logical Layer 1/2 faults: the physical link forms, but the two ends cannot communicate cleanly because they are configured differently.

Duplex describes how a link sends and receives. Half-duplex allows transmitting or receiving but not both at once, like a walkie-talkie. Full-duplex allows both simultaneously, like a phone call. Modern wired Ethernet uses full-duplex. When two ends disagree on duplex, performance collapses even though the link appears up.