Chapter 5. Auto-Negotiation

Automatic configuration of Ethernet interfaces over twisted-pair links and one fiber optic media type is provided by the Auto-Negotiation protocol. Auto-Negotiation is defined in Clause 28 of the Ethernet standard for twisted-pair links, and Clause 37 for the 1000BASE-X fiber optic link. The Auto-Negotiation system ensures that devices at each end of a link can automatically negotiate their configuration to the highest set of common capabilities.


A separate Auto-Negotiation system was developed for use with backplane Ethernet technology, which is defined in Clause 73 of the standard. However, the system defined in Clause 73 is of interest only to developers of Ethernet switches and other devices that use backplane Ethernet technology, and will not be described here.

The need for an automatic configuration system for Ethernet links becomes obvious once you understand that in order to correctly connect a desktop computer to an Ethernet switch port, for example, you must know the speed at which the Ethernet desktop interface and switch port should be set to operate and whether full-duplex mode is supported on the devices at both ends of the link and should therefore be enabled. However, features like the speed and duplex settings are embedded in the network equipment and are invisible to you.

One RJ45 twisted-pair Ethernet port looks a lot like another, and it is not obvious which network options may be supported by the equipment connected to that port. ...

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