If the NIC dropped all the way to 10 Mbit half-duplex, it suggests that the other side of the link was incapable of doing 100 Mbit and may be incapable of doing full duplex.
Forcing one side to 100FD does not automagically make the otherside accept it, so it may not actually fix anything.
Some boilerplate I use:
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
