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.

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.