Question Is QSFP28 (or QSFP56) Backwards Compatible With QSFP+

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Feb 12, 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/netw...06a-hdat-spconnectx-6vpiadaptercardhdr200gbe/

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

When I asked whether this was possible before: https://forums.tomshardware.com/thr...compatibility-question.3749824/#post-22610189

...I was told that QSFP56 (which is what the Mellanox uses) is not backwards compatible with QSFP+ (or at least that's how I understood the responses).

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
 

kanewolf

Titan
Moderator
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/netw...06a-hdat-spconnectx-6vpiadaptercardhdr200gbe/

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

When I asked whether this was possible before: https://forums.tomshardware.com/thr...compatibility-question.3749824/#post-22610189

...I was told that QSFP56 (which is what the Mellanox uses) is not backwards compatible with QSFP+ (or at least that's how I understood the responses).

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
What I said was you have to be careful, and I recommended you contact the manufacturer. The Mellanox data sheet shows multi-speed compatibility. What I said was it might be a software controlled function with one set of optics or it might be different optics (they have a PROM in them) that controls the link speed. I have no hands on with that card,.
This Wikipedia page says that QSFP56 is not backward compatible -- https://en.wikipedia.org/wiki/Small_form-factor_pluggable_transceiver#SFP_types

I think this is why you need to contact Mellanox. I worked for a large defense contractor, so companies like Mellanox would answer my e-mail. If you can use company e-mail, rather than a personal e-mail address, you might get a better response.
 
Feb 12, 2022
38
0
30
What I said was you have to be careful, and I recommended you contact the manufacturer. The Mellanox data sheet shows multi-speed compatibility. What I said was it might be a software controlled function with one set of optics or it might be different optics (they have a PROM in them) that controls the link speed. I have no hands on with that card,.
This Wikipedia page says that QSFP56 is not backward compatible -- https://en.wikipedia.org/wiki/Small_form-factor_pluggable_transceiver#SFP_types

I think this is why you need to contact Mellanox. I worked for a large defense contractor, so companies like Mellanox would answer my e-mail. If you can use company e-mail, rather than a personal e-mail address, you might get a better response.

Hello Kane Wolf,

I did contact Mellanox (a few days ago), and they said the following:

"Hi Nelson,

I was able to confirm that dual-port version of this card was tested, so this model should work fine.

Please let me know if any other questions."

...
but I found the answer a bit vague (as they didn't mention what they tested the dual port card with - I'm doubtful it was with my Arista switch), and to be honest, thought that they had just misunderstood my question (which is why I turned to the online community).

I just sent them another email, asking if their QSFP56 ports are compatible with QSFP+ ports (for my switch). Hopefully they get back to me, but if not, hopefully someone in the community will be able to provide some clarification (as I'm seeing contradictory information online).

Thank you,
Nelson
 
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