●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 __(71)__ 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 __(72)__ are partitioned into well-known and optional.The provision for optional attributes allows experimentation that may involve a group of BGP __(73)__ without affecting the rest of the Internet.New optional attributes can be added to the protocol in much the same fashion as new options are added to the Telnet protocol，for instance.
（71）A．routers
B．systems
C．computers
D．sources
（72）A．routing
B．switching
C．transmitting
D．receiving
（73）A．connection
B．policy
C．source
D．consideration
（74）A．states
B．searches
C．attributes
D．researches
（75）A．routers
B．states
C．meters
D．costs