BGP multihoming

BGP multihoming involves a variety of multihoming techniques, all of which require that there be an exchange of information from the autonomous systems being multihomed, to one or more other AS. In this discussion, the AS that is obtaining multiple paths will be called the "user AS". When multihoming without payment, between AS that provide mutual backup or other mutually beneficial AS, those AS will be called "peer 1 (P1), peer 2 (P1), etc." When the multihoming is to an "upstream" provider or providers that the user AS pays for transit to the Internet, those AS will be called "Transit1 (T1), transit 2 (T2), etc."

The cases include:

• Multiple BGP sessions to different points of presence of the same AS (i.e., T1) . Any type of physical connectivity can be used at the POP, including a switch operated by T1
• BGP sessions to the POPs of two transit providers; the principles can be extended to any number of AS.
• BGP session with peer P1, who provides the preferred path to P1's customers and mutual backup to the Internet.
• BGP sessions with peers P1 and P2, who do not provide mutual backup