BGP: the Border Gateway Protocol
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 Border Gateway Protocol

The Border Gateway Protocol (BGP) is the routing protocol used to exchange routing information across the Internet. It makes it possible for ISPs to connect to each other and for end-users to connect to more than one ISP. BGP is the only protocol that is designed to deal with a network of the Internet's size, and the only protocol that can deal well with having multiple connections to unrelated routing domains.

BGP first became an Internet standard in 1989 and was originally defined in RFC 1105. The current version, BGP4 was adopted in 1995 and is defined in RFC 4271. An overview of all BGP RFCs can be found in the BGP RFC section on this website.

BGP has proven to be scalable, stable and provides the mechanisms needed to support complex routing policies. When people talk about "BGP" today, they implicitly mean BGP4. There is no need to specify the -4 version number because no one uses earlier versions, and very few vendors even still support them.

The Border Gateway Protocol 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 (AS) that reachability information traverses. This information is sufficient to construct a graph of AS connectivity from which routing loops may be pruned and some policy decisions at the AS level may be enforced.


BGP4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR) defined in RFC 4632. These mechanisms include support for advertising a set of destinations as an IP prefix and eliminating the concept of network "class" within BGP. BGP version 4 also introduces mechanisms which allow aggregation of routes, including aggregation of AS paths.

Routing information exchanged via BGP supports only the destination-based forwarding paradigm, which assumes that a router forwards a packet based solely on the destination address carried in the IP header of the packet. This, in turn, reflects the set of policy decisions that can (and can not) be enforced using BGP. BGP can support only the policies conforming to the destination-based forwarding paradigm.

A unique AS number (ASN) is allocated to each AS for use in BGP routing. The numbers are assigned by IANA and the Regional Internet Registries (RIR), the same authorities that allocate IP addresses. There are public numbers, which may be used on the Internet and range from 1 to 64511, and private numbers from 64512 to 65535, which can be used within an organization.

Most AS numbers are currently 16-bit integers, but deployment of 32-bit AS Numbers (4-Byte ASN) is starting. Why? Because of expected ASN shortage in the 16-bit range around the year 2010. In RFC 4893 (May 2007), some new extensions to BGP are described to carry the Autonomous System number as a four-octet entity. In RFC 5668 (Oct 2009), a new type of a BGP extended community which carries the 4-octet Autonomous System (AS) number, is defined.

Whether you are completely new to the Border Gateway Protocol or are an experienced routing professional, by reading the BGP Articles & Whitepapers and BGP Presentations on this website you will certainly learn a lot about the workings of this important technology. New is a dedicated section on BGP Security.

Related Reading
Report from the IAB Workshop on Routing and Addressing


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