Re: e1000 speed/duplex error

[Rick Jones <rick.jones2@hp.com>]

> I thought the common behavior is that if one side force any particular
> parameter, other side should "sense" that and go to that mode too.

Nope.  That is a common misconception and perhaps the source of many 
duplex mismatch problems today.  Here is some boilerplate I bring-out 
from time to time that may be of help:

$ cat usenet_replies/duplex
How 100Base-T 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
("normal" collisions don't count here).  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" or default netperf TCP_RR) 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.

Finally, when/if you migrate to 1000Base-T, everything has to be set
to auto-neg anyway.

rick jones