8.2 Basic BGP Operations
8.2.1 BGP v4
BGP went through different phases and improvements from its earlier version, BGP1, in 1989 to today's version, BGP4, deployment of which started in 1993. BGP4 is the first version that handles aggregation (classless interdomain routing [CIDR]) and supernetting.

BGP imposes no restrictions on the underlying internetwork topology; it assumes that routing within an AS is done via an intra-AS routing protocol. (For the purposes of this class, intra means routing within an entity, and inter means between entities.) BGP constructs a graph of ASs based on the information exchanged between BGP neighbors. This directed graph is sometimes referred to as a tree. As far as BGP is concerned, the whole internetwork is a graph of ASs, with each AS identified by an AS number. The connections between each pair of ASs forms a path, and the collection of path information forms a route to reach a specific destination. BGP ensures that loop-free interdomain routing is maintained. The Figure illustrates this general path tree concept.