What is a Load Balancer?
Load Balancers ensure that all incoming queries are distributed to several devices within a network.
Why does FTP balancing not work with NAT in a passive mode?
Open all ports on your SLB, not only 20 & 21. Furthermore you have to add a passive-sender-address to your FTP-configuration, as the address of your FTP-server will be send to your client, but will not be rewritten in NAT. Use “session” as balancing method. Please note to set the default Gateway of your SLB to your virtual address.