Downgrade 40Gb/s connectivity to 10Gb/s without changing network card and fiber cable.

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nephri

Active Member
Sep 23, 2015
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Paris, France
Hi,

I would done a temporary transition from 40Gb/s to 10Gb/s for one link on my home network.

It's because, i have plan to change 2 switches in my home lab but not in the same time (for budget cost distribution).
For a while, i will not have anymore 40 Gb/s connectivity.

I have 4 links to 40 Gb/s but:
  • 3 of theses can be off until i have again 40Gb/s connectivity.
  • 1 of theses must remain active.

Current installation :
  • My current switch use QSFP port at 40 Gb/s
  • My PC is using a Mellanox Connect-X3 at 40Gb/s
  • The cable between the switch and the PC is a MTP-12 (Female) to MTP-12 (Femal) OM4 Multimode, 12 Fibers, Type B

Is it possible to downgrade my link to 10Gb/s like this ?
  • Keeping the MTP12 cable since it's wired from the office to the basement (where switches are located)
  • Keeping the Connect-X3 as is on the PC
  • Using a break-out cable but that must break-out an existing MTP12 cable (and not be a full cable)
  • Connect only one break-out endpoint to the switch on an SFP+

So, questions are:
  • The connect-x 3 will accept to work only on a single fiber and establish a 10Gb/s connection ?
  • Is exising breakout cable that can breakout an existing MTP-12 cable ?

Thanks in advance,
Best regards,
Sébastien.
 

ano

Active Member
Nov 7, 2022
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1) no really, but also yes see below
is no by default, but you can use an XVR QSFP+ to SFP+ and then nr 2 kicks in, you can use SFP+ on a pair, or even single lambda, but then its also possible to use a SR4 bidi module on the breakout cable.


2) yes
but it might be easier to just use a cable box in each end like this https://www.fs.com/products/57017.html instead of breakout cable, you need A on one end, and AF on other
so one side https://www.fs.com/products/57017.html
other side https://www.fs.com/products/57038.html?attribute=32069&id=1090620

this gives you a pair of LC MM, and you can grab a QSFP+ bidi module off ebay, probably cheaper than XVR QSFP+ to SFP+ and SFP+ module




Guess part of what I do at work... :D
 

klui

Well-Known Member
Feb 3, 2019
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63
@nephri, if you will be keeping your 40G switch, you should try flashing your CX3 with MCX354A-QCBT firmware and see if your existing link downgrades to 10G. I recently purchased one with SAS passthrough to play with and before I flashed with FCBT firmware, my 100G-capable DAC only linked at 10G. I didn't get a chance to try CWDM/40G transceivers.

Obviously if you use an SFP+ switch you'll need breakout cable/module. Modules offer better manageability because otherwise you will have unused pairs dangling.

If you have multiple MPO drops then I would follow @ano's advice; otherwise it's too troublesome.
 

klui

Well-Known Member
Feb 3, 2019
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430
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@ano I'm having difficulty understanding why you recommend a flipped module on one end since OP has a method B cable. Is this a Fiberstore-specific solution?

My modules are "universal" and I don't need to have a different one on either end of my method B trunks. I use the same LC cords on both ends. The trunk does the swapping. If I have method A MPO trunk I would use a method B patch cable between the trunk and one [added for clarity] module just so I can have every module the same.
 
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nephri

Active Member
Sep 23, 2015
541
106
43
45
Paris, France
If i understand, i could use (to the switch side)


But before, i would check if i can negociate my Connect-X 3 to only 10Gb/s using a single pair of fibers
The breakout cable is not so cheap (~90€ without shipping). For handling a transition, it's a bit costly.

Thanks,
Sébastien.
 

klui

Well-Known Member
Feb 3, 2019
798
430
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The difficulty you will have is your "trunk" is unpinned. You will need a pinned MPO breakout. Almost all good cables (Amphenol, Commscope, Corning, Molex) you can get from eBay are unpinned MPO. Look at your transceivers' datasheets for OM3 compatibility. Some vendors' modules are pinned so price compare with breakout cables.

Because your current MPO is method B, you can use method A or B as long as both ends are the same.