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Network Warrior, 2nd Edition by Gary A. Donahue

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Chapter 3. Autonegotiation

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 autonegotiation settings on the switches as well as the devices connected to them. If I had to list the most common problems I’ve seen during my years in the field, autonegotiation issues would be in the top five, if not number one.

Why is autonegotiation 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 Autonegotiation?

Autonegotiation 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 the configuration 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. Many switches now support 10 Gbps Ethernet, and 100 Gbps is on the horizon as well.

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 ...

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