Split QSFP+ without splitter cale

Bjorn Smith

Active Member
Sep 3, 2019
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Hi,

I know I can buy a QSFP+ splitter cable so I get 4xSFP+ ports in the other end - but does anyone know if its possible to split the port without a cable?

I know on my mellanox switch I can just do a:

Code:
interface ethernet 1/5 module-type qsfp-split-4 force
Which then seems to appear as 4 ports in their interface.

But it does not make 4 intefaces available on the network card side, so I assume I need to tell the card that it should split the port into 4 - but I don't know if its possble.

I theory it should be possible, since the QSFP+ cable is just 4 bonded cables as far as I understand?
 

Bjorn Smith

Active Member
Sep 3, 2019
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QSFP+ is not just 4 bonded cables.
Ok - that might be true - but you can get a splitter cable and when you plug in a splitter cable - suddenly one port turns into 4 ports - at least on a switch.

I don't know what happens if you plug in a splitter cable into a network card - but I would assume the same happens on the operating system - that it turns into 4 ports instead of one.

On my switch this is what happens:
1590154093480.png

I have plugges in a splitter cable into my #1 port and can control each "port" individually as if it were a real port - the magic might be inside the QSFP+ connector, but it would be nice if it was built into the nic/switch port itself.
 

tsteine

Member
May 15, 2019
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You are drastically oversimplifying this problem, it's not a matter of "the connector fits, so this should work both ways"


The magic *does* happen in the switch, but the thing is that a lot of good engineering, design, and forethought has gone into making your switch a device that is compatible with a large variety of connections, your switch is built for this.

Your Network Interface Card on the other hand is most likely not built for this. Network Cards for the most part, with the exception of the card I linked, aren't made to split a qsfp+ port into multiple ports. This has to be implemented in the circuitry on the NIC, not the QSFP+ port itself, and must be supported in the network processing silicon on your card, as well as in the drivers.
This would be a major cost driver for NIC manufacturers, for a feature that's not likely to be used most of the time.
 

necr

Member
Dec 27, 2017
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Has been discussed here many times already - you can split a port on the switch, but not on the NIC. On the switch, a 40g interface is destroyed, and 4 new interfaces are created, 10G lanes are re-mapped to the new interfaces. You can do it switch-to-switch this way.

On the NIC, there’s just 1 interface per port. You can’t re-map lanes and create more devices than there already is. So, you’re limited by the number if interfaces on the NIC.
 

Freebsd1976

Active Member
Feb 23, 2018
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Has been discussed here many times already - you can split a port on the switch, but not on the NIC. On the switch, a 40g interface is destroyed, and 4 new interfaces are created, 10G lanes are re-mapped to the new interfaces. You can do it switch-to-switch this way.

On the NIC, there’s just 1 interface per port. You can’t re-map lanes and create more devices than there already is. So, you’re limited by the number if interfaces on the NIC.
not exactly , now some nic indeed support split,


and

https://www.servethehome.com/hpe-620qsfp28-4x-25gbe-single-qsfp28-port-ethernet-adapter-review/
 

tsteine

Member
May 15, 2019
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@Freebsd1976 This is a bit misunderstood, what necr is saying is that you cannot dynamically create and remap interfaces on the NIC side, which is completely correct, and is still correct even for the HPE-620 card you have linked.

The linked card is a 4 port adapter that uses a QSFP port to save space and is built with a QSFP splitter cable in mind, it does not support a single 100gbe link, and can only be used as a 4x 25gbe port adapter.

Edit: I can confirm that an XL710-QDA2 supports this though, but that comes back to my earlier point which is that this must be built into the network card and explicitly supported, and is a feature that is not present on many cards.
 
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Freebsd1976

Active Member
Feb 23, 2018
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@Freebsd1976 This is a bit misunderstood, what necr is saying is that you cannot dynamically create and remap interfaces on the NIC side, which is completely correct, and is still correct even for the HPE-620 card you have linked.

The linked card is a 4 port adapter that uses a QSFP port to save space and is built with a QSFP splitter cable in mind, it does not support a single 100gbe link, and can only be used as a 4x 25gbe port adapter.

Edit: I can confirm that an XL710-QDA2 supports this though, but that comes back to my earlier point which is that this must be built into the network card and explicitly supported, and is a feature that is not present on many cards.
you right about 620qsfp28(my fault),and only few nic support split. some ql45xxx nic also support split, for example QL45412HLCU support 40g and can split 4x10g
 
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Freebsd1976

Active Member
Feb 23, 2018
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but i still do not know the performance when use this kind of nic to get rid switch ( connect 4 or 5 servers )
 

necr

Member
Dec 27, 2017
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Yes, I should’ve written “absolute majority” of the cards don’t support splits. For the notorious XL710-QDA2, not only second port won’t work, but you’ll be trying your luck with the offloads. I’m also interested in the performance of these cards - of course, single stream will be limited to 10G, not all applications support something like SMB Multichannel or have scalability problems, finally there could be a uneven distribution/fat RX flow on one of those 10G interfaces. TX side looks better potentially, with more HW queues to manage.