When you specify route-lookup, the ASA looks up the routing table to determine which interface to use for the NAT'd traffic. This is useful when you have multiple interfaces and want the ASA to dynamically choose the best path based on the routing table. no route-lookup is when you want to use the interface configured in nat statement to be used. Proxy-arp is really used for static NAT where you want the ASA to respond to a arp to the mapped ip.. Hope that help.
**Please rate helpful posts**
