Ethernet Auto-Negotiate fails in 3 macs

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Level 1

34 points

Ethernet Auto-Negotiate fails in 3 macs

Over the past year I have had 2 older iMacs and 1 600MHz iBook brought to me with what appeared to be dead Ethernet ports. For all outward appearances the ports were DOA. No router or switch would show any sign of life on the Ethernet ports when hooked up to these Macs.

BUT in each case I found that if I went into the Ethernet settings and turned off the Auto-Negotiate and manually entered the speed and duplex the ports would come to life. In most cases at any speed/duplex I choose, although one iMac would only connect at 100/half.

So how could a port that had worked flawlessly in AUTO for several years just suddenly decide one day not to negotiate, but still work just fine when forced into a mode?

Has anyone seen this or found a cause or solution?

Thanks.

Posted on Nov 8, 2005 9:10 AM

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Answer 1

Camelot

Level 9

58,527 points

Nov 8, 2005 11:27 AM in response to Zaxcom

In my experience, cables tend to go bad way more often than ports do.

How long is the cable in question?
Has the cable been kinked/pinched/twisted
Have you tried a different cable?

Auto negotiation will attempt to negotiate the fastest reliable connection possible. If it fails, but a manual config works, the signal strength may be below acceptable thresholds meaning you'll be running a high error/retransmission rate even though you do get a link.

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Answer 2

Nov 8, 2005 11:36 AM in response to Zaxcom

Without seeing the switch logs, it's difficult to say what is wrong here. Auto-negotiation only works if both ends of the connection are set to auto-negotiate. You can't auto-negotiate if the switch has a fixed speed and duplex. Otherwise, the device trying to auto-negotiate will default to 10/half. This usually leads to a duplex mismatch which will cause a boatload of collisions. However, while this would cause a severe slowdown in network performance, you should still have some sort of connectivity.

When you say the ports are DOA, do you mean there is no link whatsoever, or just no activity?

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Answer 3

Zaxcom Author

Level 1

34 points

Nov 8, 2005 2:52 PM in response to Zaxcom

I guess I need to be clearer in my explanation. These are not my machines, but those of friends. They not only stopped negotiating on their own routers and modems at their homes, but also on my routers and modems at my home. All different brands and models of equipment. I tried 2 sets of routers and cable modems and several cables that I knew were good. This all points to something in the Macs themeselves that has gone south. No cable was more than a meter long and all switches and routers and modems work just fine with other machines on the networks they were installed in.

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Answer 4

Camelot

Level 9

58,527 points

Nov 8, 2005 3:33 PM in response to Zaxcom

No cable was more than a meter long

As it happens, 1 meter is the minimum cable length for ethernet.

Shorter cables rarely cause problems, but they are outside of spec.

That doesn't solve your issue if you do have faulty ports, but it's an interesting point of fact that most people miss.

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Answer 5

Zaxcom Author

Level 1

34 points

Nov 8, 2005 3:47 PM in response to Camelot

I never knew that. Is there someplace I can read up on that?

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Answer 6

John Strung

Level 6

13,440 points

Nov 8, 2005 4:22 PM in response to Zaxcom

This doesn't answer the specific question, but if you are using home made cables, it is worth reading this:

http://www.bluemax.net/techtips/networking/Wiring_Tips/Wiring100TX/100TXContents .htm

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