how to force half duplex mode?

James Ralston
Wed Jan 26 18:25:22 2000

Hello all,

I apologize if this question has been answered already, but Greg
Siekas's archives (as referenced by the vortex web page) don't seem to

I have a 3Com Boomerang (3c900b) NIC.  It's connected to an Asante
FriendlyNet FS4000P 8-port switch, which performs 10/100Mbps half/full
duplex auto-negotiation.

I am trying to figure out how to tell the 3c59x driver to force the
Boomerang card to half duplex operation.  If it auto-negotiates, it
will (correctly) negotiate to full duplex, but for testing purposes, I
want half duplex operation only.  I know that this is possible in
terms of the hardware of the card, because 3Com's Windows NIC
Diagnostic Utility could do it (back when 3Com was still shipping it
with their drivers).

While passing "full_duplex=1" as a parameter to the driver will force
full duplex, passing "full_duplex=0" does *not* force half duplex.

Is there a way to force half duplex operation?

BTW, if anyone cares, the reason I want to be able to force the card
to half duplex operation is because after I upgraded both of my home
machines to Red Hat 6.1 (RPM kernel-2.2.12-20), TCP/IP throughput
between them dropped into the toilet, to the point where any
high-throughput TCP/IP links between them (e.g., ftp'ing a file from
one machine to the other) would practically stall.  (Connections from
either machine to the outside world (via my DSL line) work fine,

I accidentally wound up creating the state where the NIC on the
machine that I was transferring the file *from* was operating in full
duplex, but the switch had that machine's port in half duplex.  This,
of course, generated a bajillion collisions (due to the duplex
mismatching), but the transfer between the two machines then zipped
along at a much more reasonable rate (700kb/sec or so, which is around
~5.5Mbps).  So what I'm trying to figure out now is whether the
collisions were what eliminated the throughout bottleneck (as strange
as that may sound), or if setting half duplex mode is what did it.


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