RFC 3221 (rfc3221) - Page 3 of 25
Commentary on Inter-Domain Routing in the Internet
Alternative Format: Original Text Document
RFC 3221 Commentary on Inter-Domain Routing December 2001 This network-wide connectivity is described in the routing table used by the BGP4 protocol (referred to as the Routing Information Base, or RIB). Each entry in the table refers to a distinct route. The attributes of the route, together with local policy constraints, are used to determine the best path from the local AS to the AS that is originating the route. Determining the 'best path' in this case is determining which routing advertisement and associated next hop address is the most preferred by the local AS. Within each local BGP-speaking router this preferred route is then loaded into the local RIB (Loc-RIB). This information is coupled with information obtained from the local instance of the interior routing protocol to form a Forwarding Information Base (or FIB), for use by the local router's forwarding engine. The BGP routing system is not aware of finer level of topology of the network on a link-by-link basis within the local AS or within any remote AS. From this perspective BGP can be seen as an inter-AS connectivity maintenance protocol, as distinct from a link-level topology management protocol, and the BGP routing table can be viewed as a description of the current connectivity of the Internet using an AS as the basic element of connectivity computation. There is an associated dimension of policy determination within the routing table. If an AS advertises a route to a neighboring AS, the local AS is offering to accept traffic from the neighboring AS which is ultimately destined to addresses described by the advertised routing entry. If the local AS does not originate the route, then the inference is that the local AS is willing to undertake the role of transit provider for this traffic on behalf of some third party. Similarly, an AS may or may not choose to accept a route from a neighbor. Accepting a route implies that under some circumstances, as determined by the local route selection parameters, the local AS will use the neighboring AS to reach addresses spanned by the route. The BGP routing domain is intended to maintain a coherent view of the connectivity of the inter-AS domain, where connectivity is expressed as a preference for 'shortest paths' to reach any destination address as modulated by the connectivity policies expressed by each AS, and coherence is expressed as a global constraint that none of the paths contains loops or dead ends. The elements of the BGP routing domain are routing entries, expressed as a span of addresses. All addresses advertised within each routing entry share a common origin AS and a common connectivity policy. The total size of the BGP table is therefore a metric of the number of distinct routes within the Internet, where each route describes a contiguous set of addresses that share a common origin AS and a common reachability policy. Huston Informational
