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New Member

100BaseFX auto negotiation

Does 802.3u auto negotiation include fibre 100BaseFX? It is available on Cisco 100BaseFX ports but other manufacturers don't include it.

New Member

Re: 100BaseFX auto negotiation

from the IEEE 802.3u standard: Speed handling

If the PHY has the capability of detecting speeds other than 100 Mb/s, then the repeater set shall have the capability of blocking the flow of non-100 Mb/s signals. The incorporation of 100 Mb/s and 10 Mb/s

repeater functionality within a single repeater set is beyond the scope of this standard.

So, in answer to your question, speed handling (which is 50% of the autonegotiation task), is not a part of the base standard. Keep in mind that all of part 28 of the specification deals with autonegotiation, this is only for 10/100/1000baseT (twisted pair) media. The citation in the above paragraph covers all PHY independent media, including 100baseFX.

If other manufacturers do not cover it, it is not required by the IEEE 802.3u specification. Regardless, always hard code the speed and duplex if/when possible, except for variable use ports.


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