Have an testbed configuration where we're running 24 servers with Intel XL710DA1 40G NIC into a switch with 40/100G ports. Trying to use DACs to hold the cost down, but parsing the specs for QSFP+ vs. QSFP28 has been frustrating.

It looks like 40GBase-CR4 requires QSFP+ DACs, but the QSFP28 spec seems to imply that it will step-down (auto-negotiate) from 100G to 40G. If that is true, is there any performance penalty in doing so vs. using QSFP+ DACs?

I've been trying to get some real world recommendations in the 40G space, but so far most folks either go 4x10G to 40G or just ramp up to 100G where QSFP28 is required.

Any insights would be greatly appreciated.

Thank you for your time,

Ed Kiefer

2 Answers 2


Autonegotiation is up to the port, not the DAC. As long as the port accepts the QSFP28 PHY and links with the far side the port will be fine.

However, it's quite likely that the QSFP+ port will not accept a QSFP28 PHY - check the specs: unless QSFP28 compatibility is explicitly listed it most probably won't work.

The XL710-QDA1's feature support matrix doesn't list compatible QSFP28 DACs.

  • Sorry for the imprecise problem statement WRT ports vs. cables doing the adapting. Going to go with the QFSP+ cables and spend some more $ if/when the environment is upgraded to 100G. Thanks.
    – Ed Kiefer
    Commented Sep 17, 2018 at 13:55
  • Don't forget to vote and/or accept the answer if it is useful and satisfies you.
    – Zac67
    Commented Sep 17, 2018 at 14:11

So far, 100G NIC is very expensive. If the server is installed with Intel XL710DA1 40G NIC into a switch with 40/100G ports, only 40G QSFP+ DAC can be used.

Tip: One end of 40G QSFP+ DAC plugged into the Intel XL710DA1 40G NIC must be compatible with Intel. If the switch is Cisco, 40G QSFP+ DAC port plugged into the the switch must be compatible with Cisco. enter image description here

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