Border Gateway Protocol(BGP)即边界路由协议，是一种不同自治系统路由器之间进行通信的外部网关协议，作为EGP替代品。BGP系统之间交换网络可达到信息。这些信息包括数据到达这些网络所必须经过的自治系统AS中的所有路径，通过这些信息构造自治系统链接图，然后根据连接图删除选路环，制定选路策略。
参见
RFC 4271 :
The Border Gateway Protocol (BGP) is an inter-Autonomous System routing protocol.
The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability, from which routing loops may be pruned and, at the AS level, some policy decisions may be enforced.
BGP-4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR) [
RFC1518,
RFC1519]. These mechanisms include support for advertising a set of destinations as an IP prefix and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths.
