bbPress.org

@Ashfame – You keep insisting that no plugin is needed – however, I could not make the integration work until I installed the plugin –

https://wordpress.org/extend/plugins/bbpress-integration/

I have WP installed in a subdirectory, and BBPress installed in a sub-domain. The plugin helped me to discover how to a) make the cookies register in the same root domain, and b) define the cookie hash that was missing.

In short, although it may not be needed – it is useful. In my humble opinion.