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Question

Can I connect a 25Gbps QSFP28 port to a 10Gbps SFP+ port using either fiber or copper?

Background

I'm building out a network with some high speed interconnects coupled to a slower gigabit network. On the high speed side, I plan to have several servers that will be connected to a Mellanox MSN2700 (or similar) which is QSFP28. These will be running at 25Gbps over fiber. On the low speed side, we'll have a couple 1Gbps switches with a few dozen clients, with 10Gbps ports bridging them to the Mellanox switch. Most 1/10Gbps switches I've seen use SFP+ connectors for the 10Gb lines.

I've looked and I've found adapters for QSFP28-SFP28, QSFP-SFP, QSFP+-SFP+, and others. I assume that some of these are physically identical (possibly QSFP, QSFP+, and QSFP28, and SFP variants perhaps?) but I'm not sure which ones are protocol compatible, i.e. if I have a QSFP28 fiber transciever on one end and a SFP+ fiber transciever on the other, will that work?

Any help would be appreciated.

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QSFP and QSFP28 are the same physical form factor - the former supports 40G (or 4x10G) while the latter supports 100G (..or 4x25G / 2x50G). For the most part QSFP's will work in QSFP28 ports but the inverse is not true.

SFP, SFP+ and SFP28 also share a common form factor (although different than QSFP, obviously). SFP was the original 1G version while SFP+ is 10G. The modules supporting 25G are sometimes just generically referred to as "25G SFP" but are actually SFP28.

So - there is compatibility...for breakouts. As an example, four independent 10G SFP+ SR's can connect to a single 40G-SR QSFP (obviously only if the associated 40G Ethernet port supports breakouts) or a QSFP28 100G can connect to 4 SFP28 25G (assuming both breakout and FEC are supported).

Outside of this, however, you generally can't connect, say, a 1G SR optic to a 10G SFP+ SR or a 25G SFP28 to a 10G SFP+. In some limited cases within certain vendors some optics allowed for multiple speeds (ex: in the Cisco world some of the workgroup-class switches can support 100- or 1000- on a TX SFP) but this is highly uncommon in modern optics.

To your question - if your Mellanox has a QSFP28 port and you wanted to connect some 10GE devices then you'd need a QSFP 40GBase-SR4 adapter and an appropriate set breakout cables with an MPO connector running to 4 LC duplex connectors (this could be via mountable cassettes, structured cable or a pre-made patch cable). The LC would connect to 10GBase-SR SFP+'s on the downstream switches. You'd need to configure the Mellanox's port to run in breakout mode and then would configure the resulting 4 10GE interfaces independently.

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  • I think that answers some of my questions. From the sound of it, a "40Gbps QSFP28" port is really a QSFP+ port, which can be broken out into 4 10Gbps SFP+ (assuming the switch supports breakout mode)... right? – Whimsical Seaplane Feb 8 '17 at 21:42
  • So there's another terminology issue - the literal QSFP uses 4G lanes and was only found in some FC/IB contexts. In the Ethernet world QSFP actually means QSFP+ - which is the same QSFP form factor but with 10G lanes. QSFP28 is equivalent to QSFP+28 which uses 28G (25G effective) lanes. So - you want a QSFP+ port (..which may be referred to simply as QSFP if it's Ethernet) with an optic that breaks out to 4x10G. Keep in mind that you have both single mode and multimode (SR / LR) and twinax/AOC options that may be available. – rnxrx Feb 9 '17 at 0:52
  • Alright, a couple more questions just to check my assumptions. I think, from all of this information, that a QSFP28 can break out into either 4x25G or 4x10G lanes, depending on the transciever used, similar to SFP28 which will accept SFP+ transcievers and run at the lower 10G speed. (All of this, of course, assuming there's no weird manufacturer compatibility issues.) And, if I have this correct, a QSFP28 broken out into 4x10G can interface with an SFP+ 10G transciever, assuming the fiber connectors on both ends are the same. At least I think that's right. :) – Whimsical Seaplane Feb 9 '17 at 20:32
  • OK - so a QSFP28 port (i.e. 100G) can generally take either a QSFP+ or QSFP28 optic. The QSFP28 optic supports 25G lanes - so can do 4x25G breakout, 2x50G breakout or 1x100G (no breakout). The QSFP+ optic supports 10G lanes and so can run 4x10GE or 1x40GE. A QSFP28 optic cannot break out 10GE links, but a QSFP+ optic in a QSFP28 port can. Put another way QSFP+ can connect to itself or 4xSFP+, QSFP28 can go to itself or 4xSFP28 (..or 2xSFP56 - which isn't totally a thing yet). – rnxrx Feb 9 '17 at 21:46
  • Yep, that's what I meant - it basically comes down to the optic (both ends have to match) and the port (form factor needs to match plus port speed needs to be equal or greater than the optic used). Thanks for the help! – Whimsical Seaplane Feb 10 '17 at 0:23

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