I've encountered the same problem with my Mac at work. Caused some problems with the installation as well, as apparently once the installed forced the connection back to half-duplex, with the resulting connectivity loss, software update left a number of directories (like my home directory) with the wrong file ownership.
Also, in my case, the ethernet port that I was using (Mac Pro tower) appears to be dead. After the install, and cleaning up the file ownership issues, I had to switch to the other port, and allow half duplex to regain connectivity to our corporate network.
So, I back up and running... kinda...
Actually, no -- the desk is bolted to the wall in such a way that the jack is inaccessible. But I doubt the cable is to blame since OS 10.4 - 10.5.8 all use full duplex just fine.
I also tried a clean install with the same result -- half duplex accepted. Full duplex immediately returns "cable unplugged" error.
The switch is set to manual 100/Full duplex. Will ask the admin to set the switch to auto-negotiate and see if that helps...
+Will ask the admin to set the switch to auto-negotiate and see if that helps...+
No luck -- got bounced down to 10Mb/Full duplex. Network admin says, "I think it is some issue with the NIC card of your machine."
FWIW, here are the ethernet card specs:
Type: Ethernet Controller
Vendor ID: 0x8086
Device ID: 0x1096
Subsystem Vendor ID: 0x8086
Subsystem ID: 0x3499
Revision ID: 0x0001
Link Width: x4
You probably already know this but in case you don't. The info that you have posted about the networks suggests its an Intel Pro 1000 EB see http://www.pcidatabase.com/vendor_details.php?id=1302.
I have had a look around the net but cannot find anything about the card in relation to Apple.
The software in my pack states a 90 day support by telephone, if you have that available it may be worth giving Apple a call. You pretty well have got the problem down to the driver/ethernet card. The only other thing I can suggest is a replacement card of different make.
Each subsequent upgrade of OSX seems to make the Mac's network hardware "pickier" about cabling.
As long as you are certain that all the wiring from your Mac to the switch is quality Cat 5E minimum than no worries. However, I have beat my head against many walls only to find that it was a simple cable issue.
Thanks for the tips
It is looking more and more like a driver issue with the Intel Pro 1000 card that came with the Mac Pro. I've found people with similar loss of full duplex with this card (sometimes unrelated to Snow Leopard).
Intel no longer supports the card or driver apparently, having sold that operation to www.small-tree.com. SmallTree's Web site states that a problem exists with the driver and Snow Leopard and offers a possible work-around at http://www.small-tree.com/gigabit-ethernet-driver-download-a/198.htm.
(The driver must be installed with the current ethernet card removed. Unfortunately, every how-to guide I've seen assumes you're replacing the easy-to-access PCI cards, but the preinstalled Mac Pro card appears to be hidden back behind the main fan and I don't see an easy way to get to it.
Am I missing something obvious?)
Don't think you're missing anything, as I'm experiencing the identical problem on our corporate LAN. Everything worked great with 10.5.8, with full-duplex being manually set, but stopped working after (actually during) the 10.6 upgrade. Sounds like the vendor (SmallTree) is working with Apple based on the link you posted, so I expect we'll see a fix in 10.6.2+
I was thinking of performing a clean install, but others have said that doesn't fix the problem, so I guess I just wait. Sigh...
As an aside, when I upgraded my Mac Pro at home, I didn't have this problem, since at home I'm currently connected via WiFi as I install structured wiring in the house.
Not much help, but confirms the problems you're seeing.
My experience keeps getting murkier, partly because I'm getting conflicting info from our tech people about the true extent of the Cat-5 cabling in the building -- and a ton of other stuff that muddies the picture further.
But I'll skip top the punchline:
Earlier today I used Cocktail to force a setting that I'd never heard of -- and that does not appear in the Network control panel: 100Mb at Full Duplex (hw-loopback) (click "force speed and duplex settings").
Problem solved instantly. Measured throughput is now 92Mb. "About This Mac" shows connection as 100Mb/Full Duplex.