Unless and until you _KNOW_ that the NIC and switch cannot get along via autosense/autoneg etc, my personal belief is one should leave things at both ends to AUTO. Even for a 100BT NIC rather than a GbE NIC.
Once one starts hardcoding one side or the the other, the following comes into play:
How Autoneg is supposed to work:
When both sides of the link are set to autoneg, they will "negotiate"
the duplex setting and select full duplex if both sides can do
full-duplex.
If one side is hardcoded and not using autoneg, the autoneg process
will "fail" and the side trying to autoneg is required by spec to use
half-duplex mode.
If one side is using half-duplex, and the other is using full-duplex,
sorrow and woe is the usual result.
So, the following table shows what will happen given various settings
on each side:
Auto Half Full
Auto Happiness Lucky Sorrow
Half Lucky Happiness Sorrow
Full Sorrow Sorrow Happiness
Happiness means that there is a good shot of everything going well.
Lucky means that things will likely go well, but not because you did
anything correctly :) Sorrow means that there _will_ be a duplex
mis-match.
When there is a duplex mismatch, on the side running half-duplex you
will see various errors and probably a number of late collisions. On
the side running full-duplex you will see things like FCS errors.
Note that those errors are not necessarily conclusive, they are simply
indicators.
Further, it is important to keep in mind that a "clean" ping (or the
like - eg "linkloop") test result is inconclusive here - a duplex
mismatch causes lost traffic _only_ when both sides of the link try to
speak at the same time. A typical ping test, being synchronous, one at
a time request/response, never tries to have both sides talking at the
same time.
there is no rest for the wicked yet the virtuous have no pillows
