Q: network port hangs

I am using a cisco slm-2048 gigabit ethernet switch, category 6e cables, apple airport extreme, freenas raids. all settings are either automatic or standard (gigabit full duplex).

The reason why I had said that I ruled out all of these is because the fault ONLY happens on the Mac Pro's running 10.8/9/10. I can't replicate the fault on the Mac Pro's when I boot them into snow leopard, also I can't replicate the fault on any other machine using the same cables, switch, router, drives, OS and settings.

regular frames with no flow control

same settings on mac pro, freeness drive and cisco switch.

I've never really learned much about these settings, would I get better results if I enable jumbo frames and flow control on the macs, drives and switch?

thanks, I will give this a try

I've had some progress with setting everything to jumbo frames although I can't figure out how to activate flow control on the mac while using LACP. I haven't had any errors as you can see below although one of the ports still hangs. before i switched to jumbo frames, 1 port would hang then a few minutes later the other would go down however now, the remaining port seems to be more stable and keeps the connection up for longer.

(I don't think I mentioned that I use LACP earlier, but I have tried extensively with a single connection as well as with LACP and it doesn't seem to affect things)

looks like I spoke too soon, the second port has just gone down with 0 errors...

after a quick reboot...

I Can appreciate that bonding does create other complexities, however, when I wrote the original post, the ports were not bonded, I was just using a single port and then switching to a different port when it went down. After your advice to switch to jumbo frames I got excited about an increase in speed (we edit 4K raw video footage) and decided to do a test with it bonded. I have since tried jumbo frames with no bonding but still get issues. It's fair to say that it is far more reliable now but I still get the hangs although less frequent. I'm wondering if it's just cheap hardware built in to the Mac and if I should get a top of the range pcie network card to test

I have managed to do some more testing, flow control when used on the mac, switch, raids etc all has no effect on the problem. The ports still hang when (and only when) a large file transfer is initiated through finder or carbon copy cloner. It's not a case of extensive network activity as we often have final cut performing complex renders over the network and when it does, it stretches the networks read and write to capacity without causing the ports to hang. 

So after some more testing, I have found that the problem only occurs with heavy traffic for both send and receive, i.e copying data from one network drive to another network drive. the problem does not happen when say data is copied from network drive to a local disk or from a local disk to a network drive.

So to recap:

Symptoms:

Network ports hang/freeze when copying data from network drive to network drive using either finder or carbon copy cloner. it only happens on 2006 & 2008 mac pro, it does not happen on macbook pro 2010 using identical settings, OS, network connections/cables, routers, switches, network drives etc etc. It does not happen on OS X 10.6 or earlier, i'm not sure if it happens on 10.7, it definitely happens on 10.8, 10.9, 10.10. (I have 2 drives in my mac pro, one with 10.10 installed and one with 10.6 installed. boot into 10.10 and the problem happens, reboot into 10.6 without changing any network cables/connections/settings and the problem goes away.)

Things tried but don't fix the problem:

installing pcie network card

single connection or LACP

jumbo frames

flow control

fresh OS install

PRAM/SMC reset

permissions/disk repair

deleting and re-installing network card kexts/drivers

Work arounds:

copy files from network drive to local drive and then from local drive back to network drive

boot into 10.6

don't use mac pro, instead use macbook pro

use wifi