On Tue, Jun 17, 2003 at 08:06:00AM +0100, David Laight wrote:
> On Tue, Jun 17, 2003 at 12:48:12AM -0400, Chuck Yerkes wrote:
> > Chant the matra with me:
> >   Autoselect is bad, I will always force my interfaces to what
> >   I want.
> 
> More dangerous, that is very likely to end up with the ends of a
> link disagreeing about FDX - which seems to work until you load the
> network...
> 
> IMHO the only safe way to do things is to use auto-negotiation.
> The 2 transceivers then mutually agree the correct procedure.
> Unfortunately the ethernet MAC needs to know whether the link is
> FDX or HDX (but typically not whether it is 10M or 100M).
> This can be tricky to determine reliably - the actual operating mode
> of a transceiver isn't actually available in one of the standard MII
> registers :-(

Unfortunately, for those cards/chipsets/systems where autonegotiation is
*broken*, you have no choice.  One of the more popular router interfaces
of a few years ago (cisco FE on 7xxx series routers) has autonegotiation
which just *does* *not* work, and when you rely on it, you invariably
wind up with duplex mismatch.

Most exchange points require their participants to hardcode speed/duplex
for this reason.  It also makes sure everyone knows what the settings
are.

+j
-- 
Jeff Rizzo                                         http://boogers.sf.ca.us/~riz