appendix A. RFC 1771: Border Gateway Protocol 4

The objective of this appendix is to present the concepts and terminology contained in RFC 1771 in order to help you understand the operation of BGP. Many of the details have been omitted, such as packet format and contents. These topics are of interest to developers of the code that runs on a BGP-capable router. Border Gateway Protocol (BGP) is an interauto-nomous system routing protocol. An autonomous system (AS) is a network or group of networks that are under a common administration and that have common routing policies. BGP is used to exchange routing information for the Internet and is the protocol used between Internet service providers (ISPs). Customer networks, such as universities and corporations, usually employ an Interior Gateway Protocol (IGP) such as Routing Information Protocol (RIP) or Open Shortest Path First (OSPF) for the exchange of routing information within their networks. Customers connect to ISPs, and ISPs use BGP to exchange customer and ISP routes. When BGP is used between autonomous systems, the protocol is called External BGP (EBGP). If a service provider uses BGP to exchange routes within its AS, the protocol is called Interior BGP (IBGP). Figure A-1 illustrates this distinction.

Figure A-1. External and Interior BGP

BGP is a very robust and scalable routing protocol, as evidenced by the fact that BGP is the routing protocol employed on the Internet. Currently, the Internet BGP routing tables have more than 90,000 routes. In order to achieve scalability at this level, BGP uses many route parameters, called attributes, to define routing policies and maintain a stable routing environment.

In addition to BGP attributes, BGP uses Classless Interdomain Routing (CIDR) to reduce the size of the Internet routing tables. For example, assume that an ISP owns the IP address block 195.10.x.x from the traditional Class C address space. This block consists of 256 Class C address blocks, 195.10.0.x to 195.10.255.x. Assume that the ISP assigns a Class C block to each of its customers. Without CIDR, the ISP would advertise 256 Class C address blocks to its BGP peers. With CIDR, BGP can supernet the address space and advertise one block, 195.10.x.x. This block is the same size as a traditional Class B address block. The class distinctions are rendered obsolete by CIDR, allowing a significant reduction in the BGP routing tables.

BGP neighbors exchange full routing information when the TCP connection between neighbors is first established. When changes to the routing table are detected , the BGP routers send to their neighbors only those routes that have changed. BGP routers do not send periodic routing updates, and BGP routing updates advertise only the optimal path to a destination network.