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100Base-FX no autoNeg

For 100Base-FX (100mbps with optical fibers) it's standardized to not use autoNegotiation. This is probably because speed and duplexmode are fixed. The disadvantage is that when only 1 fiber is connected (so other fiber in reverse direction is disconnected) one side has ethernet link up and the other side ethernet link down. Although this case can be caught by the Far End Failure Indication (FEFI) feature I guess. Even if a sloppy signal is received by a PHY in 100Base-FX mode sometimes an ethernet link is detected.

For 1000Base-SX (for example), autoNegotiation is enabled and in the above example both sides would have ethernet link down.

Also, 100Base-FX linkPartners cannot negotiate about flow control settings.

Can anybody comment on this, why is there no autoNeg done for

100Base-FX ?
Reply to
bolnvhuis
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100FX and 10FL (is FL correct?) operate at different optical wavelengths and so the same physical diodes cannot be used.

The other issue you mention, the detection of unidirectional links, is not included in the pre-gigabit physical layer standards or practical implementations. Cisco, and possibly others, implement a Layer 2 method called Unidirectional Link Detection (UDLD). This is available for

10 and 100 M ethernet interfaces but it is off by deault (IIRC).
Reply to
Bod43

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