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