1. Basic: Here the ISP will offer only the default route to the AS. This kind of connection is least processor intensive and recommended for simple networks with only one ISP connection.
2. Medium: This uses default routes and BGP. Internal AS can select the best ISP to use depending on the preferences.
3. Full: Full multi-homing uses only BGP. Here the routes are learned using the AS_PATH attribute information to make routing
Posted by vishwesh