Chapter 1 on Ethernet mentioned about autonegotation using Fast Link Pulses so I wanted to find a little bit more about how this works. Cisco devices detect link speed and duplex settings using Fast Link Pulses (FLP) which is adapted from Normal Link Pulses (NLP) defined in the 10BASET specifications. This negotiation will only occur over 8 wire twisted pair copper and as such wont happen on other media types such as Fiber.

Upon an interface coming up a FLP burst will send 17 pulses of 100ns each representing a Link Code Word (LCW) which informs the other end of the link about the media speed and type and standards to use. The LCW only sends enough information to setup link speed of 100Mbps, so for Gigabit speeds multiple LCWs are sent which contain extra information such as master/slave settings, and multiport capabilities.

Although other media types such as fiber don’t use FLP they do use similar auto negotiation protocols.

Before FLP was defined devices would send an idle signal down the wire and the other end would train to it this is called parallel detection.

Its really important to note that the 802.3u standard does not specify what FLP should do if the link has statically defined speed/duplex settings, so it reverts to parallel detection and correctly sets the speed but assumes the link it half duplex. This results in a duplex mismatch if one end of link is coded for 100Mbps Full and the other for autonegotiate.