Hi wb,
1. I've tried manually IRQ assigning as well as automatically.
2. I've installed FreeBSD 10.2-STABLE. Installed the drivers from download.intel.com. Setup a routing, ipfw or pf rules for nat.
The traffic from 172.16.0.0/16 is received in ixl0 interface and forwarded (and NATed) to ixl1 interface. Returning traffic is forwarded vice versa. What details would you like to know?
Did you tried this schema on your machine with FreeBSD with the latest drivers?
Thanks,
Evgeny