[Freeswitch-users] switch_r_sdp or switch_l_sdp modify the media ip or port?

Hi , Who can tell me how to use switch_r_sdp or switch_l_sdp modify the media ip or port? Now I am want to redirect media to a transcoding card. But I don't know how to rewrite the rtp ip and port point to the transcoding card. Please can you give me the way to do it. Thanks very much!