The previous chapters covered signaling and forwarding issues tackled with interior (intra-domain) gateway routing protocols (interior gateway protocols, IGPs). These protocols are used for distribution of reachability information within the boundaries of a single administrative realm under a unified administration, also referred to as an autonomous system (AS). In practice, this constitutes a set of routers adhering to a set of (hopefully) coherent policies. Thousands of autonomous systems resemble what we know as today's global Internet, populating the global routing table with approximately 130,000 network announcements (prefixes/aggregates). The Internet is a best-effort nondeterministic architecture of hundreds of thousands of nodes (routers) with a connectionless datagram protocol (IP) at its network layer. Border Gateway Protocol (BGP) provides signaling (not forwarding) intelligence to this network.
This chapter focuses on UNIX aspects of BGP and topics not covered in the standard authoritative guides, such as Sam Halabi's Internet Routing Architectures or John Stewart's BGP4 Inter-Domain Routing in the Internet. My intention is to create a qualitative understanding and awareness of today's Internet operation and challenges and to discuss routing databases, routing registries, and route servers (topics missing from the classical textbooks in the field).
This chapter includes a thorough introduction to interdomain routing and the two BGPv4 flavors: Interior Border Gateway Protocol (IBGP) and Exterior Border Gateway Protocol (EBGP). This chapter also discusses tools and approaches for exchange points, such as route server setups, BGP views, looking glasses, the Internet Routing Registry, and the interface language (Routing Policy Specification Language, RPSL) used to query and update these databases. The load-balancing aspect of BGP scenarios is briefly mentioned as well. BGP filtering and route maps as well as state-of-the-art features of multiprotocol BGP conclude the chapter. BGP is one of the most complex and powerful concepts that networking has to offer. I strongly advise you to make full use of the additional resources listed at the end of this chapter.