is a QSFP+ card a replacement for a quad SFP+ card?

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rootgremlin

Member
Jun 9, 2016
42
14
8
I have build a NAS in an 1HE server.
Now i want to connect up to 4 Different machines to this NAS. Because of reduction in power requirement, i wanted to use a direct connection, so i do not have to run a separate 10G switch.

So my question is:
Can i accomplish that with a QSFP+ Card, in any mode (IB, ETH, whatnot)

I have searched the forum:
https://forums.servethehome.com/ind...a-qsfp-to-4xsfp-to-connectx2.4903/#post-41471

And the answer: "No, the 4->1 cables are designed to connect between switches or link the card to 4 ports on the same switch to get 40Gbps. What you don't get are 4 Ethernet devices out of the cable."

So for switches in a stack (one logical switch across multiple chassis) this would be possible?

If so, from my understanding, it is a "Software / Firmware" limitation of an qsfp+ card not to be able to serve 4 x 10 Gig SFP+ connections.

I do not understand the usecase of a 1 -> 4 splittercable, because if the qsfp+ card can link with a 4x 10G switch, the switch must have a lagg for those 4 ports to uplink to the qsfp+ device.

But then, if the switch does simple link aggregation, so must the qsfp+ card do. And if it does, it must also be possible to link to 4 different sfp+ devices.

Can somebody please clarify this for me!
 

Mithril

Active Member
Sep 13, 2019
354
106
43
I have build a NAS in an 1HE server.
Now i want to connect up to 4 Different machines to this NAS. Because of reduction in power requirement, i wanted to use a direct connection, so i do not have to run a separate 10G switch.

So my question is:
Can i accomplish that with a QSFP+ Card, in any mode (IB, ETH, whatnot)

I have searched the forum:
https://forums.servethehome.com/ind...a-qsfp-to-4xsfp-to-connectx2.4903/#post-41471

And the answer: "No, the 4->1 cables are designed to connect between switches or link the card to 4 ports on the same switch to get 40Gbps. What you don't get are 4 Ethernet devices out of the cable."

So for switches in a stack (one logical switch across multiple chassis) this would be possible?

If so, from my understanding, it is a "Software / Firmware" limitation of an qsfp+ card not to be able to serve 4 x 10 Gig SFP+ connections.

I do not understand the usecase of a 1 -> 4 splittercable, because if the qsfp+ card can link with a 4x 10G switch, the switch must have a lagg for those 4 ports to uplink to the qsfp+ device.

But then, if the switch does simple link aggregation, so must the qsfp+ card do. And if it does, it must also be possible to link to 4 different sfp+ devices.

Can somebody please clarify this for me!

AFAIK, this is entirely down to the card. QSFP to 4xSFP+ DAC breakout cables are simply direct electrical connections without much in the way of "smarts". I know I've read that some switches don't work (or only some ports work) with that kind of cable (on the QSFP end), on the SFP+ end, as far as that device is concerned it is just another SFP+, so long as there are no brand/type locking issues it will work, so long as the QSFP end does.

If you need to go longer than a DAC will get you, then you need to make sure the QSFP optical transceiver ALSO supports working in 4x10g mode, and get the correct optical breakout cable, connectors, etc.