4G8 allows you to capture traffic from a third party in a switched environment at the expense of a slight increase in latency to that third party host. Utilizing ARP cache poisoning, packet capture and packet reconstruction techniques, 4G8 works with nearly all TCP, ICMP and UDP IPv4 traffic flows.

4g8 -i eth0 -g 192.168.1.1 -G 00:18:4D:9B:65:FF -s 192.168.1.2 -S 00:18:4D:7C:76:6E

Here:
-g IP address of gateway to impersonate
-G MAC address of gateway to impersonate

-s IP address of victim host
-S MAC address of victim host

[ http://www.intrusense.com/software/forgate/ ]