[Users] Load-Balancing

Hi, I'm trying to set a load balancing solution with SER and twos Asterisk. The Asterisks will be use as REGISTRAR, REDIRECT, GATEWAY .... The ser will just load balance the traffic on one Asterisk or an other. I use the dispatcher module for the SIP message and I use 2 public ip addresses for the asterisk and so, I'm able to directly send the RTP traffic to them (I didn't use mediaproxy or rtpproxy). On each asterisk, I set the SER as default GW to be sure that all message go through this computer. (Client are behind a FW/NAT and so, I would like to be sure that all message come from the same IP address:port else, the FW/NAT will DROP these messages). For the moment, with this solution, I have a problem. The message from client will go to SER which will resend it to the asterisk and then, the asterisk will try to answer responding on the same IP -> Loop detected !! On some device I saw that we can forward the request for the phone to the asterisk without changing the source ip address .... Do you know how can I do this ? Thanks a lot for your help, Thomas D