25G SFP28 capable of rate-adaption down to 1G?

Bill Woodcock woody at pch.net
Mon Jan 31 21:12:27 UTC 2022

> On Jan 31, 2022, at 8:02 PM, Randy Carpenter <rcarpen at network1.net> wrote:
> Are you talking about an SFP28 module that can link at 25Gb, but also 1Gb?
> We just put 1Gb SFPs in the SFP28 ports and they work fine. I have not seen a single module that does both, but admittedly, I have not looked too hard, as the 1Gb modules are so cheap.
> Or, are you talking about a module that presents as 25Gb to the switch, but 1Gb to the client device?

The latter.

I remember there were two kinds of copper SFPs: cheap ones, which would talk 1G on both sides, and expensive ones which would talk 1G on the switch/router side, and 10M/100M/1G on the client side.

There seem to be similar kids of copper SFP+, though I haven’t actually tested them:


Those seem like they might talk 10G on the switch/router side, and 10M/100M/1G/2.5G/5G/10G on the client side.  Or it might be that they establish whatever link speed they can on the client side, and then try to signal the switch/router side to adapt to that rate; which seems possible but improbable.  Or it might be that they accept whatever speed the swich/router side tells them it’s running at, and then only provide link at that rate on the client side; again, possible but seems improbable.  I haven’t actually taken any of these into the lab to test them, anyway.  And those are 10G on the switch/router side, and I’m curious whether anybody knows of one that goes 25G/10G/1G on the switch/router side, and 1G (specifically) on the client side.

I don’t actually want the SFP28, I just need to find a chip that does that in the size/power budget of an SFP, and it seemed like the easiest way to do that would be to find an SFP28 that did what I needed and bust it open to see what chip they were using.

I’m sure you can guess why, given recent threads.  :-)


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