RFC 3221 (rfc3221) - Page 3 of 25

Commentary on Inter-Domain Routing in the Internet

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