Yes Karl, you've the reason.
I've searched in the archives of the present list and I've found a lot
of useful info about the initial question.
At present moment I consider the next tips:
* sk(4)-based NICs have the best performance
* one dedicated data-bus per NIC (I think a PCI-E x8/x16 will be
enough in my environment)
* OpenBSD 4.2 PF's (it seems the performance has been improved
until 50%!!! in relation to previous releases)
So, now I've one additional question
¿Can the hard-disk performance to mean a bootle-neck in the PF-based FW
box? I understand the major part of PF operations are made in RAM
memory, but maybe I'm not right.
