Fundamentals of BGP Operation

BGP is structured around the concept that the Internet is divided into a number of Autonomous Systems (ASs). Before you learn how the protocol operates, you should become familiar with ASs.

An Autonomous System (AS) is a network under a single administration, identified by a single two-byte number (1-65536), which is allocated by the InterNIC and is globally unique to the AS. Within an AS, private AS numbers may be used by BGP, but they must be translated to the official AS prior to connectivity with the Internet.

An AS is essentially a network under a single administrative control, and it may be categorized as a stub, multihomed, or transit AS. A stub AS is a network that connects to a single Internet service provider and does not generally provide transit for other ASs. A multihomed AS connects to more than one ISP. A transit AS is the ISP itself. In other words, it provides connectivity between other ASs.

Figure 11-1 shows this arrangement. Stub AS-A reaches other destinations on the Internet through its transit provider, ISP-C. Stub AS-E reaches all Internet destinations through its transit provider, ISP-D.

Figure 11-1. Stub, Multihomed, and Transit ASs

Figure 11-1. Stub, Multihomed, and Transit ASs

Transit providers must either provide connectivity to all other transit providers in the global Internet, or purchase that connectivity through a higher-tier transit provider. Therefore, in the Internet there is a hierarchy of transit providers. The providers at the highest tier of the hierarchy

(typically called Tier 1 ISPs) must provide connectivity to all other Tier 1 ISPs for global connectivity to be complete.

A multihomed AS, such as B shown in Figure 11-1, connects to two or more transit providers. Users in network B may reach Internet destinations through either provider by using basic load sharing of traffic, or through a policy that determines the best route to any particular destination.

The InterNIC allocates AS numbers (ASNs). However, not all networks require an official, globally unique ASN. Unique ASNs are necessary only when an organization must be routable on the Internet as a self-contained entity. Multihomed ASs are sometimes listed in this category, although, through careful use of address translation or load-sharing techniques, you can avoid the use of an official ASN. Networks providing Internet transit to other networks are the most appropriate users of InterNIC-assigned ASNs.

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