Like Allen already pointed out, the top level forum should contain subforums, but also all postings of all its subforums.
So if you open the postlist for forum1 you will get links to the subforums forum1.1, forum1.2 and already see all posts of forum1, forum1.1, forum1.2.
A click onto subforum forum1.1 then displays just a subset (like a filter) of its parent forum1 posts.