Why must switch port speed and duplex match the connected device, and what does autonegotiation do?

Speed and duplex must match on both ends or the link malfunctions; autonegotiation lets the two devices automatically agree on the best speed and duplex. If you turn it off, you must hard-set both sides identically.

A link is a two-party agreement: each end must run at the same speed and the same duplex or the connection either fails outright or, worse, "works" but performs terribly. Autonegotiation exists to make that agreement happen automatically, and the only real rule is to keep both ends consistent — both negotiating, or both manually pinned to identical settings.

Matching requirement:

• Speed (bandwidth) and duplex are configured per switch port
• Both ends of a link must agree — a mismatch causes performance problems (the classic duplex-mismatch case)

Autonegotiation:

• An optional feature on most Ethernet switches and NICs
• Lets the two link partners automatically negotiate the best common speed and duplex
• Both sides should have it on, or both off — never one of each, or autonegotiation can fail and produce a mismatch

Gotcha: Gigabit Ethernet ports only operate in full-duplex, so half-duplex is not an option at gigabit speeds.

Go deeper:

• Autonegotiation (Wikipedia) — how two link partners agree on the best common speed and duplex, and why a one-sided configuration causes a duplex mismatch.

• Duplex — telecommunications (Wikipedia) — the full- vs half-duplex distinction the two ends must match for the link to work cleanly.