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Is 1000 Base SX fiber multimode (850nm) interface backwards compatible with 100 Base Sx fiber multimode (850nm) interface?

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This question is too vague to answer as asked. It's not even clear what you mean by "interface". –  David Schwartz Jun 19 '12 at 22:17
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No - 1000BaseSX has no negotiation capability. A 100M fiber interface is incompatible.

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Either way, it's a question of whether you can connect a 1000BaseSX interface to a 100M fiber interface. –  David Schwartz Jun 20 '12 at 0:17
    
The question is vague, but the term 'interface' tends to connote a connection on a host of some sort. I'll add to this the fact that I have yet to see a fiber cable labeled either 100- or 1000- baseSX, much less mentioning the wavelength of light, but I have seen plenty of host interfaces that have both of these specified. As to the negotiation - that would be implicit in connecting two interfaces of different speeds. I had assumed the question was whether the fiber interface could negotiate down to 100M much as a 100/1000 copper interface might. –  rnxrx Jun 20 '12 at 1:09
    
Re-read your own link. That interface has both an SFP and an RJ-45. You choose which one you want to use with the media-type command. The speed command only applies to the RJ45 interface (10/100/1000). There are SFP optics that can support 100SX and there are SFP optics that support 1000SX. Connect the two and you do not get a link. –  rnxrx Jun 20 '12 at 2:01
    
I'm not missing any point. The product in the link you just supplied takes -two- optics to achieve conversion between 100SX and 1000SX - one 100, one 1000. -It's a conversion box-. You need conversion boxes because these two link types are not compatible...which was the original question. –  rnxrx Jun 20 '12 at 2:57
    
Thanks a lot for all your feedbacks! –  Tomaso Jun 20 '12 at 15:01
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