when you FXP one site MUST use PASV while the other MUST use PORT. you tell one site "tell me to which ip/port to connect"(PASV) and the other site "connect to this ip/port"(PORT)
"alternative fxp" let's you deside which site will use PASV/PORT
as for specifics of serv-u, i don't know
i don't use it. either look for some guides or somebody here will help you.