I recently came across an issue while setting up a stack to replace an older M switch setup. I stacked 3 switches - 2 Cisco 2960's and 1 C2960 PoE using SFP cables to their backplane.
My plan was to use the SFP modules on the front end of the switches to uplink straight to the core rather than to a bottom M and a Top M that were uplinked to Core1 and Core2.
We purchased non-cisco SFP's (Cisco's are much more expensive) and installed them. What I noticed via Termmon, was that each SFP was going down as soon as it was plugged in.
000016: *Feb 28 22:35:32.507 EST: %PHY-4-SFP_NOT_SUPPORTED: The SFP in Gi2/0/50 is not supported (B2-FL1-Stack-2) 000017: *Feb 28 22:35:32.507 EST: %PM-4-ERR_DISABLE: gbic-invalid error detected on Gi2/0/50, putting Gi2/0/50 in err-disable state (B2-FL1-Stack-2)
It's outputting that they are not compatible which is confusing considering the SFP is an international standard.