SFP+ / SFP28 / SFP backwards compatibility I have the opportunity to upgrade from VDSL to any of 1/10/25 GbE fiber. I have only an SFP router presently (Edgerouter 4), but would like to have the service provider upgrade to optimally the 25 service, and replace the router on my side later. Can you run SFP28 or SFP+ modules in an SFP cage at the lower speeds? Alternatively, can you find SFP modules that sync against a faster physical interface?

Generally, slots may be downward compatible but are not upward compatible. Of course, there are exceptions.

Since a module faster than the slot doesn't really provide any benefit, you should use a module that matches the best mutual mode supported by the far side - your ISP should be able to tell you which these are (possibly single-strand 1000BASE-BX or duplex 1000BASE-LX).

In no way can you link a 1G SFP port with a 10G-only counterpart, regardless of what module you try to fit.