Help with understanding 40Gbe/56GbIB cabling

iphands

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Jan 11, 2020
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Hi all, n00by here... but this place has a wonderful amount of knowledge.

I asked a similar question in the Mellanox forums here:
https://community.mellanox.com/s/question/0D51T00007CGDzUSAX/which-cables-for-a-pair-of-mcx353afcbt

I have two MCX353A-FCBT cards on the way and I want to connect them over DAC or AOC.
I would like to play around with NFS over RDMA, 40GbE and 56Gb IB. I figured one cable between two NICs would let me do that... after reading the Mellanox cable descriptions I am not so confident.

1) When I see a QSFP cable for sale... will that also work for 56Gb IB?
2) Or will the 56Gb IB rated cables work for 40GbE?
3) Furthermore.... can I get a much higher rated cable like the 100GbE QSFP28 and use it for lowly 40GbE/56GbIB links?

Sorry if some of this is obvious... I am so new to this stuff.
 
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iphands

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Jan 11, 2020
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iphands

New Member
Jan 11, 2020
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I guess the real question is.... do the 56G cables work with 40GbE. I would assume yes, but because I see this in the Mellanox PDF:
MC2210126-005 Mellanox® passive copper cable, ETH 40GbE, 40Gb/s, QSFP, 5m
MC2207126-004 Mellanox® passive copper cable, VPI, up to 56 Gb/s, QSFP, 4m
MC2206125-007 Mellanox® passive copper cable, IB QDR, 40 Gb/s, QSFP, 7m

It kinda seems like the MC2206125 is IB specific, the MC2207126 is VPI specific, and the MC2210126 is Ethernet specific.
I would guess the 56G cable is backwards compatible though.