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I am trying to figure out if a QSFP-DD transceiver can break out 8x 10G lanes if I make/use a 1:8 fiber optic breakout cable. It seems like most transceivers are rated for one speed only, so if I get a QSFP-DD transceiver(such as a 400G-SR8), and break it out to 8x 10G SFP+ transceivers, will that work? All the QSFP-DD Transceivers seem to be rated for breakouts of 50G or 25G.

I tried reaching out to local vendors but they were unsure as well. I know QSFP and SFP are supposed to be backwards compatible, but that seems to be only for the ports and not the transceivers (like the ports can support older iterations), so I can use a QSFP+ to 4x SFP+ cable but then I'm losing 4 lanes of data? I may be also misunderstanding that as well.

Has anyone worked or applied this specific use case before?

Any guidance would be greatly appreciated.

Thanks!

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    – Ron Maupin ♦
    Dec 23, 2021 at 19:19

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Common splits are 100G->4x25G using a QSFP28-to-4xSFP28 breakout or 40G->4x10G using a QSFP+-to-4xSFP+. You cannot split from optical transceivers.

I have yet to see a QSFP-DD eight-way split. Check your device's manual first if that is possible. If it is, there should be a DAC to be found somewhere.

PS: QSFP-DD to 8xSFP56 breakout cables (400G->8x50G) actually do exist from some vendors - but 10G/SFP+ ends aren't available, I'm afraid. You might need an intermediate switch to terminate the DACs and launch MMF.

Also, I'd ask the vendor which transceivers they recommend.

For a breakout cable, you need to start with an SFP standard that is based on the required number of lanes: QSFP+ = 4x 10G, QSFP28 = 4x 25G, QSFP56 = 4x 50G.

(Q)SFP-DD doubles the number of lanes within the same module size and QSFP-DD is only used for 200G and 400G => 25G or 50G with breakout. Whether these split lanes also link down to 10G depends completely on the device in question and which transceiver specs it requires (whether the transceiver needs to be marked for 10G compatibility (10GBASE-SR) which is very unlikely to exist).

If you had two QSFP(-DD) ports, you could use two standard QSFP+-to-SFP+ breakouts for a total of eight 10G ports (or using a common 40GBASE-SR4 & 4x 10GBASE-SR transceivers and an MPO-to-4xduplex-LC fanout cable).

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  • So I have to use fiber (multimode and such). My device is able to support 8x 10G/25G. Unfortunately, that manual just says to check with vendors to find a cable to support it (I reached dead end upon dead end which is why I tried reaching out here. It seemed trivial, but apparently not). So if fiber only able to support essentially one data rate (QSFP+ = 40G, QSFP28=100G, QSFP56=200G, etc) and not anything in between?
    – bchang32
    Apr 26, 2021 at 12:27
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    @Zac67 The only possible breakout is via direct-attach cable... You cannot split from optical transceivers. We're running dozens of breakouts from bread-and-butter type QSFP-40G-SR Modules, using an MTP-to-4xLC-Duplex breakout cable with each a plain vanilla SFP-10-SR on the breakout end (example: To connect Cisco N2K-2248TP-E-1GE with 4x10G to a single port of a N9K-9332PQ). No DACs involved whatsowever... or am I misreading your statement?
    – Marc 'netztier' Luethi
    Apr 26, 2021 at 19:34
  • @Marc'netztier'Luethi Yeah, thanks for pointing that out - actually hadn't seen those yet. Live and learn. ;-)
    – Zac67 ♦
    Apr 26, 2021 at 21:02
  • There will eventually be support for 8x breakouts, but said optics will use MPO-24 connectors.
    – rnxrx
    Apr 27, 2021 at 0:52
  • @rnxrx wouldn't it be MPO-16? I'm new to all this but doesn't 16 lanes = 8 channels for full duplex? Also do you know if there is an industry timeframe for when that may be?
    – bchang32
    Apr 27, 2021 at 12:20
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What do you mean by "make a fiber optic breakout"? A PON spliter can't work because it's not wavelength specific. MTP/MPO isn't technically a spliter as each lane is put on it's own fiber -- that would be a fan-out cable (and there are plenty of sources for those.)

As for what can be supported, that will depend on how each lane is signaled. A 40G interface can be broken into 4x10G because that fundamentally what it is. Likewise 100G breaks down to 4x25G, because it's four 25G lanes.

Quoting the rev 5.1 QSFP-DD specs:

QSFP-DD supports up to 400 Gb/s in aggregate over an 8 x 50 Gb/s electrical interface

So, that's the best you can hope to get... eight 50G interfaces. (Table 9, p.33 may interest you)

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  • I guess I meant a fan-out cable. My device has QSFP-DD and is able to do 80G (as well as the 200/400G). If I plug in a QSFP-DD TRX that fans out to 8 SFP TRX, does the transceiver automatically balance it to 8x 10G? Thats my big concern and unknown and what I am trying to solve (I probably did a bad job explaining initially)
    – bchang32
    Apr 27, 2021 at 12:25
  • @bchang32 You need to make sure that your device in combination with the selected transceiver supports the desired link speed. A split 400GBASE-SR8 port might be compatible with 8x10G but it might just as well only be compatible with 8x50G. Chances are rather slim since there's no eight-lane 80G PHY.
    – Zac67 ♦
    Apr 27, 2021 at 15:49
  • @Zac67 my device is able to do 8x10G so that part is covered (it nicely explicitly says it in its datasheet) which is why im trying to find a compatible transceiver. The transceiver realm is more of the gray space where some vendors are saying its not possible while others are saying the spec of "max data rate" is a limit and anything slower is possible. There seems to be a lot of uncertainty.
    – bchang32
    Apr 27, 2021 at 17:29
  • @bchang32 Very likely, the switch restricts the link speed to the ones the transceiver advertises (which isn't technically required really) - it's not a question of "is this possible (at all)?" but "is this possible with these parts?". I'd select a transceiver and then ask the vendor if it should work.
    – Zac67 ♦
    Apr 27, 2021 at 17:36
  • @Zac67 so I have a question about that. What is meant by the switch? I see it mentioned multiple times but I'm not sure whats it referring to. Is it the port? I don't think my device has a network switch in it (if thats whats meant by switch)? ok, I've been asking vendors about that, but it seems like a blackhole in itself and was hoping someones had experience with this application.
    – bchang32
    Apr 27, 2021 at 23:56
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You'd actually most likely be looking for QSFP-DD SR8 in a 200G speed. These optics (...which would also often be compatible with 400G ports) use MPO-16 or MPO-24 connectors (as mentioned in the comments) instead of the MPO-12 that current SR4 (40/100) optics employ for breakout. These 200G optics break out to 2 x 100G-SR4 connections, or 8 25G links. It's up to the specific switch vendor whether they'll support clocking the individual link speeds down to 10G, but that's the basic principle of how you'd potentially get 8 10G out of a single connector.

I can tell you that 200G-SR8 QSFP-DD modules with 8x10G fixed cables exist and that a few of the big vendors have some support as of the time of this writing, but that widespread use / sane pricing and density (...including availability of 8-way breakout fiber adapters) are still somewhat mid-term roadmap items - with 200G and 400G QSFP-DD ports starting to replace 100G on blades and switches (ex: 200G ports at 1.5 - 2X density of 400G ports, 200G switches with 400G uplinks, etc).

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  • By switch, do you mean my device? My device is capable of doing 80G, I just need to find the fiber optic transceivers to convert the data and break it out to 8 links. Do you know of any vendors that provide that? I've reached out to many to no definitive answer.
    – bchang32
    Apr 28, 2021 at 2:16
  • Yes - it's up to your device, and it's not a function of supporting "80G" or any particular aggregate bitrate. 200G SR8 implies an interface that has 8 SerDes (serial / de-serialization) channels running at 28 gigabits (25 w useful). Not all 40G interfaces support all optics types and, indeed, some are incapable of supporting optics that break out into four separate component interfaces. In the case of 100G-SR4 the breakout is at 25G and (to my knowledge) nobody currently supports down-clocking the sub-channels to 10G. I'd ask about roadmaps from specific vendors.
    – rnxrx
    Apr 28, 2021 at 2:44

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