BGP is an inter-autonomous system routing protocol; it is designed to be used between multiple autonomous ( ) . BGP assumes that routing within an autonomous system is done by an intra-autonomous system routing protocol.BGP does not make any assumptions about intra-autonomous system （ ） protocols employed by the various autonomous systems. Specifically， BGP does not require all autonomous systems to run the same intra-autonomous system routing protocol. BGP is a real inter-autonomous system routing protocol. It imposes no constraints on the underlying Internet topology. The information exchanged via BGP is sufficient to construct a graph of autonomous systems connectivity from which routing loops may be pruned and some routing ( ) decisions at the autonomous system level may be enforced. The key feature of the protocol is the notion of Path Attributes. This feature provides BGP with flexibility and expandability. Path （ ） are partitioned into well-known and optional. The provision for