The Border Gateway Protocol (BGP) came to standardized life in 1989 with [RFC 1105]. After more than 20 years of its initial specification, BGP, meant to be a short-term solution awaiting some yet-to-be-determined long-term solution for the inter-domain routing, is still very young and healthy. Production implementations of the protocol have been deployed since 1990. The most current specification of the BGP protocol at the time of this writing is still considered to be version 4 as per [RFC4271], thanks to the embedded extensibility. This chapter considers this version to be the current and most widely deployed one.
Figure 3.1 shows an example of a BGP network composed of interconnected Autonomous Systems (ASs), each with its own address space, that exchange reachability information. The AS number is carried with the advertisements in the form of the AS_PATH attribute, providing loop capability detection.
Integration or division of organizations implies changes in the AS number of the BGP-speaking network devices interconnecting with external peers. AS renumbering is an infrequent yet cumbersome task, with most expertise residing in the technical departments of network integrators or Service Providers that merged smaller ISPs during the consolidation phase after the Internet boom.
Various internal peering topologies are possible. Smaller ASs (or networks with highly scalable equipment) follow a full-mesh arrangement. Following the example in Figure 3.1, the ...