The IGPs OSPF, IS-IS, and RIP maintain the mapping for the topology within a single administrative domain or AS, along with the set of best paths between systems within the domain. Each AS uses one or more common IGPs and common metrics to determine how to route packets within the AS. The administration of an AS appears to other ASs to have a single coherent interior routing scheme and presents a consistent picture of what destinations are reachable through it.
To handle inter-AS routing, IGPs use an EGP. EGPs keep track of how routing domains are connected to each other and the sequence of domains that must be traversed to reach a particular destination. Although a number of EGPs were developed in the late 1980s, the Border Gateway Protocol (BGP) is the only one currently being used on IP networks and the Internet. Version 1 of BGP was introduced in 1989, and the current iteration, Version 4, is defined in RFC 1771 and has been in use since 1995. A number of additional RFCs define extensions to the base BGP protocol (see http://www.bgp4.as/rfc).
BGP is the routing protocol that holds the Internet together, providing the mesh-like connectivity of Internet service provider (ISP) networks that forms what we call the Internet. ISPs use BGP to connect to each other, forming the virtual backbone of the Internet. Large enterprises also sometimes use BGP to connect to their ISPs, as well as to connect portions of their internal corporate network.
BGP uses a ...