The problem is, that this site is some kind of searcher, but not only the searcher. On the left left frame there are various options, on the right frame there are searching results, textbox, button etc. After searching, if you click on some listed record, right frame is being reloaded, and on this frame the content of picked document is being showed. Additionally you can pick some options on the left frame, and in order that, right frame is reloaded and lets say some form is being showed, or other pages.
It is like you said. I've got contentplaceholders, and in this holders I put iframes, into which are loaded specific aspx pages. I just tried to avoid reload all page (options part, and found record part). I guess, that using frames is the only solution. Maybe using MasterPage with Iframe is weird, but it is not a problem to get rid of MasterPage. I dont know what is the best way to achieve this, so application could be scalable. Improtant issue is that browser's backbutton has to work, so AJAX, is not an option.
Thanks