In today’s world of computer networks auto-negotiation is an important plug-and-play technology. Auto-negotiation as an algorithm was defined by Section 28 of the IEEE 802.3 standard and first introduced in 1997 as part of the IEEE 802.3u standard on Fast Ethernet. Auto-negotiation was designed to be backward compatible with original Ethernet networking standards as well. Auto-negotiation was further enhanced in 1999 by the IEEE standard 802.3ab with the introduction of Gigabit Ethernet. Auto-negotiation is best defined as the mutual agreement by two network devices sharing a wire on the speed, duplex, and controls to govern the use of that wire. As a protocol auto-negotiation exists strictly at the PHY (physical) layer of the OSI and is implemented by software, hardware, or a mixture of both.
For a link to function properly the devices on either side of the wire must be configured in the same manner; either both set to auto-negotiation or both set to the same hard-coded speed and duplex settings. In an environment where one device is set to auto-negotiate and the other device is set to a hard-coded speed and duplex the auto-negotiate algorithm can detect speed and set that appropriately. The duplex setting of the remote device is indeterminable by the auto-negotiating device. Following the IEEE standard, the auto-negotiating device falls back to half-duplex. This presents an issue if the remote device is set to full-duplex. It should be noted that, according to the IEEE specification the use of Gigabit Ethernet requires the use of auto-negotiation therefore 1000Mb/s is not a valid hard-coded option in a true IEEE compliant networking device.
Most common duplex mismatch error：
如果接口不使能auto-negotiation，IEEE的标准定义接口的协商能力信息不被传递，因此对端的auto-negotiation接口会自动设置成half-duplex；The symptoms of this situation will most often show up as a sluggish network link or applications with excessive timeouts. On a correctly configured connection CRC errors should be negligible so an excessive CRC count is often considered symptomatic of a duplex mismatch.
A link will become saturated when the connection between two devices has more data to transmit than bandwidth to transfer that data. This is an easy scenario to create on a link between two switches if the uplink port is the same speed as the user ports. This introduces the need for flow control, a process that allows one device to ask the other to pause in order to let it catch up. This pause procedure could include a timer until a restart, require a unpause notification, or simply be a stall tactic with dummy data to delay communications.
Usually built-in Gigabit Ethernet ports are capable of negotiation, but in cases like modular SFP or GBIC types, they do not negotiate.
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