I understand that most ports that support SFP+ transceivers can step down to allow an SFP to be plugged into it, but my question is around the transceivers/optics themselves. Can an SFP+ transceiver run at 1GbE, for example a 10GBASE-SR SFP+, or would the only solution be to get an SFP specifically?
My understanding is that, SFP+ transceivers should be used for 10GbE and SFP for 1GbE and this lack of backward compatibility is one of the few reasons 10GBASE-T is appealing (due to auto-negotiation).
UPDATE – It appears that there are some dual-rate optics out there. Link to Finisar's offering: https://www.finisar.com/sites/default/files/downloads/finisar-ftlx8574d3bcv-1g-10g-850nm-multimode-datacom-sfp-transceiver-product-specification-revb.pdf
Best Answer
Short answer: in theory yes, in practice no.
Long answer: There are SFP+ optics which can also work at 1 GbE, I've talked (sorry, no other source than that) to people who were experimenting with it and were able to get it running. Unfortunately it is highly dependable on the device you plug the optics in, i.e. not every device in the test was able to tell the transceiver to step down to 1 GbE.