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They seem to transmit data fine, but don't provide any feedback to the switch in terms of capabilities, symbol error rate, cable length, et cetera. They don't even report as 1000SX/LX or operating speed properly, just that they're connected. Returning mine and going with another brand which I know works and doesn't have these problems. Given it's only slightly more expensive to get a much better product, skip these and go with a better SFP.