Saying Y here enables direct NIC-to-NIC (NIC = Network Interface Card) data transfers on the local network, which is fast.

IMPORTANT NOTE: This option is NOT COMPATIBLE with "Network packet filtering" (CONFIG_NETFILTER). Say N here if you say Y there.

However, it will work with all options in the "Advanced router" section (except for "Use TOS value as routing key" and "Use FWMARK value as routing key").

At the moment, few devices support fast switching (tulip is one of them, a modified 8390 driver can be found at <ftp://ftp.inr.ac.ru/ip-routing/fastroute/fastroute-8390.tar.gz>).