Full vs. 1/2 duplex oddity

B. James Phillippe bryan@terran.org
Wed Sep 1 15:34:05 1999


We have a lab environment at work where we are doing IP forwarding
performance testing with several versions of the 0.91 tulip driver (under
2.0.3x).  We have noticed that when the media type autonegotiates to
100BaseTX-FD, the performance is worse than with 1/2 duplex, by as much as
30Mbit/sec.  There are very few errors or collisions on FD, and performance
there is about 12Mb/sec, so I do not believe the problem is caused by a
duplex mismatch.  Both the tulip-diag and the ethernet switch report FD and
the ethernet driver emits no errors.  The switches we've tried are Compaq
SW3322 and SW3323 with latest firmware.  Again, there are no apparent
errors being reported, performance is just lower.  The clients are 533Mhz
21164's at 1/2 duplex.  The only FD connection is between our system and
the switch.  For now we have simply masked out 100BaseTX-FD capability from
the NWay advertisement to guarantee 100Mbit FD is not negotiation.  This
improves performance.  Anyone know what might cause this phenomenon?

# bryan at terran dot org
# http://www.terran.org/~bryan