An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity as specified in Section 6.3.
If an optional non-transitive attribute is unrecognized, it is
quietly ignored. If an optional transitive attribute is
unrecognized, the Partial bit (the third high-order bit) in the
attribute flags octet is set to 1, and the attribute is retained for
propagation to other BGP speakers.
If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible
propagation to other BGP speakers.
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as IP
prefixes) contained in this field shall be removed from the Adj-RIB-
In. This BGP speaker shall run its Decision Process since the
previously advertised route is not longer available for use.
If the UPDATE message contains a feasible route, it shall be placed
in the appropriate Adj-RIB-In, and the following additional actions
shall be taken:
If its Network Layer Reachability Information (NLRI) is identical
to the one of a route currently stored in the Adj-RIB-In, then the
new route shall replace the older route in the Adj-RIB-In, thus
implicitly withdrawing the older route from service. The BGP speaker
shall run its Decision Process since the older route is no longer
available for use.
If the new route is an overlapping route that is included (see
9.1.4) in an earlier route contained in the Adj-RIB-In, the BGP
speaker shall run its Decision Process since the more specific route
has implicitly made a portion of the less specific route unavailable
for use.
If the new route has identical path attributes to an earlier
route contained in the Adj-RIB-In, and is more specific (see 9.1.4)
than the earlier route, no further actions are necessary.
If the new route has NLRI that is not present in any of the
routes currently stored in the Adj-RIB-In, then the new route shall
be placed in the Adj-RIB-In. The BGP speaker shall run its Decision
Process.
If the new route is an overlapping route that is less specific
(see 9.1.4) than an earlier route contained in the Adj-RIB-In, the
BGP speaker shall run its Decision Process on the set of destinations
described only by the less specific route.