Markus Echterhoff

If you have a multisite setup, you normally cannot have the same bbPress forums across all sites of the network. Now you can.
Install this plugin and activate it on the main site and for each subsite that should use the main site’s forums, or network-activate to enable on all sub sites.
If you want both main site and sub site forums displayed on the sub site, set a different root slug and topics slug in the forum settings. After that, both slugs will be known on the sub site and display their respective forums.
BUT! There is a reason why you cannot normally do this and that is because the code base is not really supportive of it. I have monkey patched my way to the goal and I’m doing some evil black magic like hooking ‘query’ and running regular expressions on the query. All of this is only ever done to your forum pages, so be cool, give it a try. If it works for you: great. If it doesn’t: deactivate, relax, look for a different solution (or patch mine).
Profile Note: Profiles are not merged! Favorites, topics started etc show main/sub site only. If you want your users to see all their subscriptions in one place, you’ll have to write some custom code and likely patch my code too.
BuddyPress Note: It does work with network enabled BuddyPress, but they don’t play together nicely because of the BuddyPress/bbPress integration stuff. I have monkey patched this to work. If you do not have BuddyPress installed in /members/ you’ll have to monkey patch my monkey patch, please see the code. And as for the profiles: Same thing as with bbPress profiles.

Multisite bbPress Slave

Make sub sites use the main site's bbPress forums

useStrict

bbPress Notify (No-Spam) is the ultimate notification plugin for bbPress and BuddyBoss forums. 
It replaces the limited default subscription system with a flexible, no-spam solution that gives your users personalized, reliable email updates about new topics and replies.
Stop flooding inboxes. With bbpnns you control exactly who gets notified, when, and how. Perfect for community managers, membership sites, and anyone who wants professional-grade forum notifications.
New! Join our Facebook Group to discuss all things bbpnns: <a href="https://www.facebook.com/groups/bbpressnotifynospam" rel="nofollow ugc">Better Forum Notifications with bbPress Notify (No-Spam)</a>.
<h4>Key Features</h4>
<ul>
<li>📧 Send professional emails — Choose HTML, plain text, or multipart with image support. </li>
<li>🎯 Target the right audience — Notify by user roles (admins, moderators, members, etc.). </li>
<li>🚀 Faster performance — Background notifications prevent post-submission timeouts. </li>
<li>🧪 Preview before sending — Dry-run mode shows exactly who will receive notifications. </li>
<li>🔌 Developer-friendly — Dozens of filters and actions for easy customization. </li>
<li>✅ BuddyBoss compatible — Works out of the box with BuddyBoss forums. </li>
</ul>
<a href="https://usestrict.net/bbpress-notify-no-spam-documentation/" rel="nofollow ugc">📖 View the full documentation</a>
<h3>Premium Add-Ons</h3>
Take bbpnns to the next level with powerful extensions:
<ul>
<li>Reply by Email – Post new topics or replies directly from your inbox. </li>
<li>Bulk Mailer – Send thousands of notifications via Mailgun or SendGrid without server timeouts. </li>
<li>Digests – Give users daily, weekly, or monthly summaries to reduce inbox noise. </li>
<li>Opt-Out – One-click unsubscribe options for CAN-SPAM and CASL compliance. </li>
<li>Membership & LMS Bridges – Integrate with BuddyPress, BuddyBoss, MemberPress, Ultimate Member, LearnDash, AccessAlly, Private Groups, and more. </li>
</ul>
👉 Browse all premium add-ons here: <a href="https://usestrict.net/product-category/premium-wordpress-plugins/?utm_source=bbpnns-readme&utm_medium=wporg&utm_campaign=bbpnns" rel="nofollow ugc">bbpnns Premium Add-Ons</a>

bbPress Notify (No-Spam)

Great plugin

Powerful, customizable email notifications for bbPress and BuddyBoss forums — without the spam.

Multisite bbPress Slave vs bbPress Notify (No-Spam)