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 for100Base-FX ?