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To preface, this is my first experience with 10GbE networking and I have encountered an issue which my research does not seem to locate a solution for...

I have two servers (older DL580G5 and DL380G5), each with a HP NC522SFP, 10GbE dual sfp+ port adapter. I have purchased copper "passive" direct connect adapter cables (that look like twinax), which seem to work well when I connect them to the sfp+ ports on my Dell 5524 switch. However, if I directly connect the two servers with the same cable, the link doesn't come up. I am running Window Server 2012 Standard on each server. My intention is to use one of these servers as a homebrew SAN and I would like to enable multiple 10GbE paths for iSCSI traffic.

My question(s):

Can I connect the two adapters to each other, such as I would with previous, less speedy generations of Ethernet?

If I can, do I require a crossover cable, or is there some type of other sfp+ cable solution to do this? My 10GbE sfp+ switch ports are premium, but server-to-server connections are doable in small numbers for me, and I would really like the multiple paths this would give me.

Is there a simple solution to this issue?

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4 Answers 4

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The "crossover" is already built in to these cables. When working with cables like these you don't have a concept of a DTE side and DCE side as you do with ethernet cables. On a typical ethernet cable the DTE(terminal) side will have transmit on pins 1 and 2, with receive on pins 3 and 6. The DCE(switch) side or the other end of that cable will be flipped with transmit on 3 and 6 and receive on 1 and 2. These direct attach cables are built with the transmit of one side built already connected to the receive of the other.

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Automatic MDI/MDI-X is required part of Ethernet standard since 1Gbps era.

I'd guess that it is just disabled (either BIOS or driver level) in your case.

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    According to Wikipedia: "Auto MDI-X was specified as an optional feature in the 1000BASE-T standard, in practice it is implemented widely on most interfaces."
    – Zhro
    Commented Dec 21, 2018 at 13:42
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There are two types of DAC cables, Active and Passive. You might have the wrong cable for your SPF+ ports.

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For the record, the short answer (as has been pointed out several times) is: you do not need a crossover cable for SFP+ ports, because the SFP+ terminals sort the RX and TX part out for you.

If you have SFP+ GBICs connected with fibre or a direct attached copper cable (either active or passive) that are/is compatible with your card, then it is possible to make the direct connection work. In my case, I have two Broadcom Corporation NetXtreme II BCM57810 10 Gigabit Ethernet devices talking happily over a passive directa attached copper SFP+ cable. No fiddlying in the BIOS was required.

It may happen that in the initial setup everything gets connected, but no packet moves over the link between two 10Gb cards and you get Destination Host Unreachable errors, although all network configuration appears to be fine. (That happened to me and has also happened to other people). In my case it was possible to get over the problem by rebooting the servers once the direct attached copper cable connection was in place.

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