Can I Use SFP Transceiver in SFP+ slot?

A common confusion about SFP and SFP + compatibility is that I can use the SFP module in the SFP + slot. Can SFP + negotiate on 1G when I connect the SFP transceiver to the SFP + transceiver? Or is it possible to use the SFP + module in the SFP port of my switch? Can an SFP + copper twinax cable negotiate up to 1G to support SFP? Can 1G and 10G exist in the same compound? All of these questions are usually a huge headache for many engineers. After looking through a lot of relevant documentation, I'll now summarize the answers in this article.

SFP can be connected to SFP + ports in most situations

I'm not sure about the specific switch model, but as a general rule of thumb, SFPs will work in SFP + slots, but SFP + optics will not work in SFP slots. It's just a matter of power availability. When you plug the SFP module into the SFP + port, the speed of that port is 1G, not 10G. And sometimes that port will lock the speed down at 1G until you reload the switch or run some fancy commands. Also, the SFP + port cannot normally support speeds below 1G. That means we cannot plug the 100BASE SFP into the SFP + port. In fact, this question can depend a lot on the switch models - sometimes SFPs are supported in SFP + ports and sometimes not. For example, almost all SFP + ports on the Cisco switch can support SFPs, and many SFP + ports on the Brocade switch support only SFP +. While it can often be done, it is safer to ask your switch provider for some information.

SFP + cannot automatically negotiate down to 1G to support the SFP module

Unlike copper SFPs, which are available in 10/100/1000 auto-negotiation, optics like SFP and SFP + don't support auto-negotiation at all to my knowledge. In fact, most (95+%) SFPs and SFP + s only run at nominal speed, no more, no less. Also, there is no SFP + doing 1G on one side (towards the fiber optic) and then 10G on the other (towards the device). While in many cases we can use SFP in SFP + ports, that does not mean that an SFP + connected to the SFP + port can support 1G. In a fiber optic link, if we plug an SFP into the SFP + port on one side (1G) and then plug an SFP + into the SFP + port on the other (10G) it may not work! You just can't have 10 GbE on one end and 1 GbE on the other. If you are using SFP + copper twinax cable on this issue, it can't negotiate on 1G either.

If you are using SFP and SFP + modules on your network, make sure that the speed of both ends of the fiber optic link is the same. SFP modules can be used in SFP + ports, but an SFP can never be connected to an SFP + module. For their different speeds, transmission distances and wavelengths. 10 SFP + can only be used for the 10G SFP + port and can never auto-negotiate on 1G.

This entry was filed on September 21, 2016 by Cheer Chen under Fiber Optic Transceiver, SFP Transceiver, SFP + Transceiver and with 10G, 1G, SFP and SFP + compatibility, SFP module in SFP + slot, SFP port, SFP + ,Tagged copper twinax cables.