Hack the Route Start free Open app Ethernet and ARP
Duplex and speed
Whether a link's two ends may send at the same time, and how fast: agreed by auto-negotiation, or set by hand.
Full duplex sends and receives at once; half duplex (the old shared hub) waits for silence and backs off on a collision. Two ends auto-negotiate speed and duplex. If one end is set by hand, the other can still sense the speed but not the duplex, and falls back to half: a duplex mismatch. The half side then counts late collisions, the full side CRC errors and runts, and the link works slowly instead of failing. A speed mismatch brings no link at all.