You can just connect a normal fiber between a 10G SFP and a 40G QSFP and let the 40G end auto negotiate to 10G. Fanout cables are better for density though.

[–] MxM111@kbin.social 2 points 1 year ago (1 children)

Does 40G QSFP transponder be able to even communicate with 10G SFP? Do they even have the same encoding? OOK? QPSK?

[–] lud@lemm.ee 3 points 1 year ago* (last edited 1 year ago)

Seems to work mostly fine: https://community.fs.com/blog/40g-qsfp-to-10g-sfp-configuration-guide.html

Considerations for QSFP+ to SFP+ Split

Usually, this split configuration is supported on most switches provided by different vendors regardless of standalone and stacked ones. Certainly, you still need to check this function in the instruction of your switch or consult your vendor, as it’s the most basic condition of splitting QSFP+ to SFP+ configuration. Sometimes, most ports on a switch support split, but some don’t. In other cases, when a switch is deployed as a leaf switch, some ports do not support the split. Or when the port is already used as a stack port, this may also limit split configuration. Thus the conditions can be quite different based on your applications.

[–] admin@sh.itjust.works 3 points 1 year ago (1 children)

Yeah they would need a breakout cable. And there’s no way in hell they can have a link of 40G via SFP+ to a 10G appliance.

[–] tjhart85@kbin.social 5 points 1 year ago (1 children)

Agreed that they can't get a 40G link when attaching to a 10G device, but the 40G QSFP can be split into 4 10G SFP+ connections instead

[–] admin@sh.itjust.works 2 points 1 year ago

Also for whoever is curious, there’s 100G QSFP28 which has breakdown cables to 4x 25G SFP28, I’m not a networks guy but I think at that point it’s not Ethernet anymore but InfiniBand.