A.P. Lawrence

This article is from a FAQ concerning SCO operating systems. While some of the information may be applicable to any OS, or any Unix or Linux OS, it may be specific to SCO Xenix, Open Desktop or Openserver.

There is lots of Linux, Mac OS X and general Unix info elsewhere on this site: Search this site is the best way to find anything.

TCP/IP and NFS FAQ

I don't understand all this half/full duplex negotiation stuff

You may be confused because hardware changes have made what used to be good advice become not so good advice.

Here's the basic problem. 10/100 capable network devices have 5 possible modes:

• 10 Mbs half duplex
• 10 Mbs full duplex
• 100 Mbs half duplex
• 100 Mbs full duplex
• AUTO, which will negotiate one of the above 4 modes

Jeff Liebermann points out that I simplified a bit: See

• /Bofcusm/2095.html
• /Bofcusm/2096.html

Typically you have a card connected to a 10/100 switch. If both devices are set to auto, and everything works as it should, then you end up with both devices set to 100 Mbs full duplex, which is the fastest possible connection (duplex means that the devices can "talk" in both directions at the same time).

At one time, most tech folk would say NOT to use auto negotiation because it was more likely to screw up and leave you with a substandard connection. That's not usually the case nowadays but you should understand the underlying possibilities just in case.

Symptoms

If the speeds don't match, you get no communication at all. You have no link, and no network.

If the duplex setting doesn't match, you MAY not get communication at all, but more commonly you get problems: lousy performance, intermittent dropouts, etc.

This Cisco link goes into more detail: http://www.cisco.com/warp/public/473/46.html

How to check the settings

I can't tell you how to check the settings on your switch or hub, but you should be able to find that on the web.

For SCO OpenServer, "ndstat -l" will show your current setting. To change it, see How do I force the speed or duplex settings of my NIC.

For Unixware, I think it's under Advanced Settings of Scoadmin->Network

For Linux, it's "mii-tool" that can query and set your NIC.

Obviously if auto negotiation is not working, you want to manually set BOTH devices to the best possible speed and to full duplex. Note that "best possible" might be 10 Mbs if you are a long distance away: technically both 10 and 100 Mbs are supposedly limited to 100 meters, but in actual practice 10 Mbs can often go further; 100 Mbs cannot. Don't forget to take patch cables into account if you are getting close to these limits, and quality can make the difference here.

Psst - Wanna work for yourself?

Comments


Add your comments

Why no "Digg this!" etc.?

Need eyes on the ground at your customer's site?
Installation and light training Boston and New England
Reliable and experienced, punctual and professional.

Enter your email address for automatic notification of new posts here
(be sure to whitelist 'feedburner.com' if you use spam filtering)

/SCOFAQ/FAQ_scotec4autonegotiate.html copyright 1997-2003 (various) All Rights Reserved

Have you tried Searching this site?

Unix/Linux/Mac OS X support by phone, email or on-site: Support Rates

This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more. We appreciate comments and article submissions.

Publishing your articles here