How to optimize routing performance

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I've performed a test on the routing capacity of a Linux 2.4.2 box versus a
FreeBSD 4.2 box. I used two Pentium Pro 200Mhz computers with 64Mb memory,
and two DEC 100Mbit ethernet cards. I used a Smartbits test-tool to measure
the packet throughput and the packet size was set to 64 bytes. Linux dropped
no packets up to about 27000 packets/s, but then it started to drop packets
at higher rates. Worse yet, the output rate actually decreased, so at the
input rate of 40000 packets/s almost no packets got through. The behaviour
of FreeBSD was different, it showed a steadily increased output rate up to
about 70000 packets/s before the output rate decreased. (Then the output
rate was apprx. 40000 packets/s).
I have not made any special optimizations, aside from not having any
background processes running.

So, my question is: are these figures true, or is it possible to optimize
the kernel somehow? The only changes I have made to the kernel config was to
disable advanced routing.

Thanks,

Mårten