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I have two Mac Pros and as one is primarily utilized for storage, I thought connecting the two directly under a different subnet would be a worthwhile endeavor that would directly transfer data without having to pass through the switch and therefore augment speed.

Everything seems to work properly in the sense that I can connect via the static IP on the private subnet between the two machines and I likewise I can connect just fine outside of this subnet to the LAN and Internet by the 2nd ports that are connected to the network infrastructure.

Unfortunately, something strange is happening where transfer rates are exceptionally slow. With the switch I normally get between 70-90 MB/s with AFP, whereas with the a crossover cable connecting them, I'm only seeing < 100 KB/s!

Any ideas as to what may be going on here?

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My first guess would be the crossover cable you're using to connect them. Gigabit ethernet requires a different crossover wiring than 10- or 100-megabit ethernet, and if you have the wrong kind, the computers will either figure out the problem and switch to a slower connection, or just get massive amounts of errors. I don't have 2 Mac Pros handy to check myself, but you can check this in the Network Utility's Info pane: Select the relevant interface from the pop-up menu, then check the Link Speed listed on the left, and the error counts on the right.

If this is the problem, I'm pretty sure the fix is easy too: just use a straight-through CAT-5 (or 5e or 6) cable instead. Pretty much all modern network interfaces do auto-MDIX meaning that the interface can do the crossover internally. Again, I don't have Mac Pros handy to test, but I'm pretty sure their ethernet interfaces support this.

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No, I had already attempted a straight through with the same results. Similarly 0 errors (send & receive) and link speed is 1 Gbit. You are right, Mac Pros do auto negotiate cross-overs, but i wanted to try it both ways to be certain there were no cable problems. –  ylluminate Nov 17 '12 at 19:27
    
Hmm, just to be sure, try the ifconfig command, and look at the "media" line under the relevant interface to make sure it matches between the two computers. It shows the connection in a little more detail than Network Utility, including e.g. whether flow control is enabled on the port. Also, compare the MTU on the interconnect vs. switched interface; I wouldn't expect it to make that much difference, but check just to be sure. –  Gordon Davisson Nov 17 '12 at 22:16
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