The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIB-In. The output of the Decision
Process is the set of routes that will be advertised to all peers;
the selected routes will be stored in the local speaker's Adj-RIB-
Out.
The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns a non-
negative integer denoting the degree of preference for the route.
The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the
existence of other routes, the non-existence of other routes, or the
path attributes of other routes. Route selection then consists of
individual application of the degree of preference function to each
feasible route, followed by the choice of the one with the highest
degree of preference.
The Decision Process operates on routes contained in each Adj-RIB-In,
and is responsible for:
selection of routes to be advertised to BGP speakers located in
the local speaker's autonomous system
selection of routes to be advertised to BGP speakers located in
neighboring autonomous systems
route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each
triggered by a different event:
Phase 1 is responsible for calculating the degree of preference
for each route received from a BGP speaker located in a
neighboring autonomous system, and for advertising to the other
BGP speakers in the local autonomous system the routes that have
the highest degree of preference for each distinct destination.
Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into
the appropriate Loc-RIB.
Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each peer
located in a neighboring autonomous system, according to the
policies contained in the PIB. Route aggregation and information
reduction can optionally be performed within this phase.