Flexoptix' Universal SFP28 - 10G and 25G with Manual or Automatic CDR Bypass

You're probably familiar with the problem: you've finally got your hands on the new 25G network devices, but not all clients can be connected via 25G, because some are still running on 10G. So now you would need to buy both 10G and 25G transceivers. But what if a client is supposed to be upgraded from 10G to 25G later on? Wouldn’t it be more practical if the optics could be flexibly used for both 10G and 25G?

Then we have the solution for you – though there are a few specific requirements that need to be considered.

For 10G to work on SFP28 transceivers, the CDR must be bypassed, because the CDR is designed to lock onto 25G. If you try to run 10G without bypassing the CDR, it will try to regenerate the signal and leave large jitters.

Our SFP28 transceivers come with two different features to bypass the CDR. One version uses "Rate Select" and the other uses "Auto-Bypass".

You can think of it like a car that needs to slow down from 100 km/h to 50 km/h. To avoid the engine stalling, the gearbox must downshift. You either have to shift down manually (Rate Select), or your car has an automatic transmission (Auto-Bypass).

The CDR in the Auto-Bypass version can detect different data rates and automatically switches into bypass mode. These can be programmed with a 10G/25G configuration and operated in SFP28 interfaces, as well as in devices with SFP+ ports, if the transceiver is programmed with a 10G configuration.

For the Rate Select version, the host must actively set the CDR to bypass mode via the hardware pins RS0/RS1 or through the EEPROM. For this to work, the host must recognize that it’s dealing with a 10G-capable SFP28 transceiver.
This becomes a problem when trying to operate an SFP28 transceiver programmed for 10G in a switch with SFP+ interfaces. Since the host detects a 10G SFP+ transceiver, it won’t bypass the CDR through RS0/RS1 pins or via EEPROM. As a result, the ports either won’t come up or will start flapping.

So, if you’re planning to operate our SFP28 transceivers at 10G, please make sure to specify in your order which of the two hardware versions you need (CDR Rate Select or CDR Auto-Bypass)