Hello Ramesh,
The Abap is using an RFc destination to reach the trex. behing the RFC destination, in fact it is TREX that connects to the rfc gateway when it starts.
In fact the RFC server is takin in charge all teh rfc communications between trex and teh abap server. The abap server pushes al the communication through the rfc destination then it is the couple rfc server -index server that are in charge of the high availability....
regards
julien.