Greetings Mate,

If you want the simplest of solutions, configuring GRE between the Main branch and remote branches and configuring default route to pass all the traffic to main site.

The above scenario considers that your branches connect to Internet through your Main branch.

If your branches connect to Internet themselves, you might have to do the necessary changes in routes and perform NAT at branches.

For configuration examples, you can refer to the above post by sprem

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