Ok, this isn't really hard. You just need javascript. Of course navigation with JavaScript is almost never a good idea, but here is how.
I'm assuming you have a page called index.asp. index.asp has an iframe called iframe_index.asp. We will link to a page called target.asp with an iframe called target_iframe.asp.
So first your javascript. In the head of your index.asp put:
function nav(target){<blockquote>window.location='target.asp?param='+target;</blockquote>}
Ok, so this javascript function navigates to target.asp with a dynamic param querystring.
On our iframe_index.asp we have the following links:
<a href="javascript:window.top.nav(1)">link1</a>
<a href="javascript:window.top.nav(2)">link2</a>
<a href="javascript:window.top.nav(3)">link3</a>
This code calls the nav function on our index.asp page and redirects us to target.asp
Target.asp uses the following iframe:
<iframe src="target_iframe.asp?param=<%=request.querystring("param")%>"></iframe>
Now, in your target_iframe.asp code, you can do another request.querystring("param") to get the value passed from the iframe_index.asp page and do whatever with it.
DISCLAIMER:
Your iframes must reside on the same domain and server in order for this to work, and you may have some cross-browser issues. I don't recommend using this, but this is how you can do it. You could use sessions and ajax and a few other methods as well.
Cheers.