| 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.
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