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Posted by on January 18, 2007, 1:23 pm
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bolnvhuis@wanadoo.nl wrote:
> 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 ?
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).
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