(Editor's note: This article was written by Michael J. Bennett of the LBLnet Services Group.)

Most folks who have worked with Fast Ethernet (IEEE 803.3u) have either loved or hated a feature of the IEEE standard known as Auto Negotiation (AN). AN is used when two network elements, such as a computer and a switch, need to try to figure out the speed and duplex capability of each end of the link. Since AN was standardized in 1995, there have been a number of complaints from the user community regarding failures to properly negotiate speed and duplex between link partners. Many users actually disable the feature as a work-around to the perceived failures.

AN is based on a protocol created by National Semiconductors Inc. National Semiconductors offered their protocol to the IEEE 802.3u task force in 1994 as a possible solution to the problem of maintaining backward compatibility with 10 Megabit Ethernet (10BASE-T). National Semiconductors' trademark name for this protocol is Nway. Another protocol called Auto-selection was developed as an alternative; however, this protocol could only negotiate speed. Finally, the Nway protocol was adopted into the standard.

AN is based on a point-to-point network architecture. AN uses a combination of Normal Link Pulses (NLP) which, are defined in the 10BASE-T standard, and Fast Link Pulses (FLP) to communicate the capabilities of each link partner using arbitration. Basically, each link partner sends a burst of pulses that define the link partner type, speed and duplex capabilities. If neither link partner can agree on a common speed and duplex ability, then the link fails to be established.

An exception to this is Parallel Detection. Parallel Detection takes place when one link partner is AN enabled and the other link partner is either AN disabled or incapable of AN. The AN-enabled link partner listens for either NLPs or FLPs and if detected, will enable the link at the appropriate speed but will always set the duplex capability to half duplex.

Some users mistakenly believe that if they force the configuration of one end of a link to be 100 Megabits per second, full duplex, the end with AN enabled will discover and configure itself to be the same. The problem is that Parallel Detection will cause the AN end of the link to configure as half duplex which leads to a problem known as a duplex mismatch.

Duplex mismatch problems exhibit symptoms in the form of poor performance or "the network is slow." This is because one end of the link can "talk and listen" simultaneously while the other end can "talk or listen, but not both." This causes frame collisions, which cause the interface detecting those collisions to wait before attempting to send again. This waiting is what causes the degraded performance, which is certainly confounding.

While the latter case would be considered a misconfigured system, there are some cases in which the hardware implementation is faulty. Such is the case with, of all vendors, National Semiconductors Inc, which is deployed in many network equipment vendor's products. The good news is that this generation of equipment is nearly obsolete and the newer generations of equipment that include 1000BASE-T (gigabit Ethernet over twisted pair copper) do not appear to have the problem.

The rule of thumb in connecting and configuring this equipment is that each end of the link should have the same settings. If you think you need to disable the AN feature on your computer, you need to be sure the network switching equipment is also configured with AN disabled. This means contacting the LBLnet Services Group at X4559 to discuss the issue before changing anything to be sure disabling auto negotiation is really the correct action.