In that case, maybe you could have the auth page call the frameset page and pass the name of the page to load in the "main" frame.
It would then reload each frame with whatver page you specified.
Something like this ...
<a href="index.php?main_frame=auth">click here</a>
Change your index file to a PHP page, so it can parse the variables and it might look something like this ...
<?
if (main_frame == "auth") {
$main_frame = "authentication.php";
} else {
$main_frame = "whatver.htm"
}
?>
<frameset cols="100,*">
<frame name="main" src="<?=$main_frame; ?>">
<frame name="whatever" src="whatever.php">
</frameset>
Hope this makes sense.