auto-negotiation
termThe exchange in which two twisted-pair Ethernet ports advertise their speeds and duplex modes and settle on the best they share.
Defined in Clause 28 of IEEE 802.3 and carried in bursts of link pulses. Optional at 10 and 100 Mb/s, mandatory from 1000BASE-T on.
Cisco describes it as 'an optional function of the IEEE 802.3u Fast Ethernet standard that enables devices to automatically exchange information over a link about speed and duplex abilities'. The information travels in Fast Link Pulse bursts, which the University of New Hampshire's lab describes as 'simply a sequence of 10BASE-T Normal Link Pulses', sent every 16 ms, give or take 8.
From Gigabit on copper it stopped being optional: an IEEE interpretation confirms that 'selecting 1000BASE-T mode of operation still requires that Auto-Negotiation be used', because negotiation is also where the two ends settle which one is the clock master. Fibre at a fixed speed usually does not negotiate speed at all.
Its weak point is a neighbour that does not negotiate. The negotiating side then falls back on parallel detection, which can sense speed from the signal but never full duplex, so it settles on half: the origin of most duplex mismatches. Leave both ends on auto, or fix both.
Also known as: autonegotiation, autoneg, nway, clause 28, parallel detection, flp burst