Solved! Is QSFP28 (or QSFP56) Backwards Compatible With QSFP+

Jan 13, 2022
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Hello,

I had a question regarding QSFP compatibility.

My company has an old Arista switch (7050SX-72Q) - https://www.arista.com/assets/data/pdf/Datasheets/7050SX-128_64_Datasheet.pdf

...which they wish to use for a few more years.

The Arista has 6x 40 Gbps (QSFP+) ports.

I'm currently building out a system for my company, and one of the requirements is for a 40 Gbps+ connectivity.

I wanted to purchase the following Mellanox NIC MCX653106A-HDAT-SP - https://store.nvidia.com/en-us/networking/store/product/MCX653106A-HDAT-SP/nvidiamcx653106a-hdat-spconnectx-6vpiadaptercardhdr200gbe/

...for the system build (and then upgrade the switch at a later time).

When I asked whether this was possible before (on Toms Hardware forum), I was told that QSFP56 (which is what the Mellanox uses) is not backwards compatible with QSFP+.

But after doing some more research, I found out that might not be the case.

So I pose the question to all of you; can I plug the Mellanox into my company's Arista switch (for the time being - connectivity would be capped at 40 Gbps), and then upgrade my switch at a later time, or must I purchase an older NIC with QSFP+ ports (to connect to the Arista switch)?

If it is possible to QSFP56 ports (from the Melalnox) to QSFP+ ports (on the Arista), can I assume I'd need some sort of breakout optical cable to go from QSFP56 to QSFP+?

If so, could someone recommend such a cable?

Thank you,
Nelson
 

Fallen Kell

Diamond Member
Oct 9, 1999
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307
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I am pretty sure that for most equipment the answer is no. This is one of the reasons why used/refurb QSFP+ network switches and cards have been so cheap the last 2 years. All the corporations are ripping them out and replacing with the new switches across entire business campuses as they upgrade to the newer standards. Otherwise we would still see a huge use case for these older switches, especially for servicing standard desktop/laptop drops in the buildings as those typically do not need better than 1000MBit service, but due to them wanting to upgrade their core switches to support QSFP56 for the big fileservers and virtual machine infrastructure, they need to upgrade everything in that building that needs to connect back to that network core.
 
Jan 13, 2022
52
1
11
It seems that the question is: Can one plug a QSFP+ tranceiver into QSFP56 port?
Hello MV2DevNull,

That, I don't believe is possible (as a QSFP56 is much larger than a QSFP+).

However, there are active breakout cables that go from QSFP56 to (usually 4) QSFP+.

That said, I'm not 100% certain that will work, as there is still an auto-negotiation component (I think) that has to be done between the NIC and switch.

Thanks,
Nelson
 

Fallen Kell

Diamond Member
Oct 9, 1999
5,784
307
126
I am pretty sure that for most equipment the answer is no. This is one of the reasons why used/refurb QSFP+ network switches and cards have been so cheap the last 2 years. All the corporations are ripping them out and replacing with the new switches across entire business campuses as they upgrade to the newer standards. Otherwise we would still see a huge use case for these older switches, especially for servicing standard desktop/laptop drops in the buildings as those typically do not need better than 1000MBit service, but due to them wanting to upgrade their core switches to support QSFP56 for the big fileservers and virtual machine infrastructure, they need to upgrade everything in that building that needs to connect back to that network core.
 
Jan 13, 2022
52
1
11
I am pretty sure that for most equipment the answer is no. This is one of the reasons why used/refurb QSFP+ network switches and cards have been so cheap the last 2 years. All the corporations are ripping them out and replacing with the new switches across entire business campuses as they upgrade to the newer standards. Otherwise we would still see a huge use case for these older switches, especially for servicing standard desktop/laptop drops in the buildings as those typically do not need better than 1000MBit service, but due to them wanting to upgrade their core switches to support QSFP56 for the big fileservers and virtual machine infrastructure, they need to upgrade everything in that building that needs to connect back to that network core.
Hello Fallen Kell,

Thank you for the information.

Can I assume that just like QSFP56, QSFP28 suffers from the same problem (i.e. is not backwards compatible with QSFP+)?

Thank you,
Nelson
 

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