Chapter 3. Auto-Negotiation

When I get called to a client’s site to diagnose a network slowdown or a “slow” device, the first things I look at are the error statistics and the auto-negotiation settings on the switches and the devices connected to them. If I had to list the most common problems I’ve seen during my years in the field, auto-negotiation issues would be in the top five, if not number one.

Why is auto-negotiation such a widespread problem? The truth is, too many people don’t really understand what it does and how it works, so they make assumptions that lead to trouble.

What Is Auto-Negotiation?

Auto-negotiation is the feature that allows a port on a switch, router, server, or other device to communicate with the device on the other end of the link to determine the optimal duplex mode and speed for the connection. The driver then dynamically configures the interface to the values determined for the link. Let’s examine these parameters:

Speed

Speed is the rate of the interface, usually listed in megabits per second (Mbps). Common Ethernet speeds include 10 Mbps, 100 Mbps, and 1,000 Mbps. 1,000 Mbps Ethernet is also referred to as Gigabit Ethernet.

Duplex

Duplex refers to how data flows on the interface. On a half-duplex interface, data can only be transmitted or received at any given time. A conversation on a two-way radio is usually half-duplex—each person must push a button to talk, and, while talking, that person cannot listen. A full-duplex interface, on the other hand, can ...

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