The Internal update process is concerned with the distribution of
routing information to BGP speakers located in the local speaker's
autonomous system.
When a BGP speaker receives an UPDATE message from another BGP
speaker located in its own autonomous system, the receiving BGP
speaker shall not re-distribute the routing information contained in
that UPDATE message to other BGP speakers located in its own
autonomous system.
When a BGP speaker receives a new route from a BGP speaker in a
neighboring autonomous system, it shall advertise that route to all
other BGP speakers in its autonomous system by means of an UPDATE
message if any of the following conditions occur:
the degree of preference assigned to the newly received route
by the local BGP speaker is higher than the degree of preference
that the local speaker has assigned to other routes that have been
received from BGP speakers in neighboring autonomous systems, or
there are no other routes that have been received from BGP
speakers in neighboring autonomous systems, or
the newly received route is selected as a result of breaking a
tie between several routes which have the highest degree of
preference, and the same destination (the tie-breaking procedure
is specified in 9.2.1.1).
When a BGP speaker receives an UPDATE message with a non-empty
WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all
routes whose destinations was carried in this field (as IP prefixes).
The speaker shall take the following additional steps:
if the corresponding feasible route had not been previously
advertised, then no further action is necessary
if the corresponding feasible route had been previously
advertised, then:
if a new route is selected for advertisement that has the
same Network Layer Reachability Information as the unfeasible
routes, then the local BGP speaker shall advertise the
replacement route
if a replacement route is not available for advertisement,
then the BGP speaker shall include the destinations of the
unfeasible route (in form of IP prefixes) in the WITHDRAWN
ROUTES field of an UPDATE message, and shall send this message
to each peer to whom it had previously advertised the
corresponding feasible route.
All feasible routes which are advertised shall be placed in the
appropriate Adj-RIBs-Out, and all unfeasible routes which are
advertised shall be removed from the Adj-RIBs-Out.