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Notes:
Compared with the utilization of the selective announcements, the main advantage of using more specific prefixes is that if link R11-R31 fails, then the packets towards 10.0.0.0/8 will still be sent by AS3 through the R32-R12 link since they are part of the 10.0.0.0/7 router learned from R12.
An important drawback of this solution is that it unnecessarily increases the size of the BGP routing tables of all routers on the Internet. For this reason, several ISPs block prefixes that are too long. For example, some ISPs do not accept prefixes longer than /22, and other try to filter prefixes based on the allocation rules of the regional IP address registries.
For more information on this filtering, see :
S. Bellovin et al., Slowing routing table growth by filtering on address allocation policies, preprint available from http://www.research.att.com/~jrex , June 2001
Note that if AS1 wants to use the more selective prefixes only to control the traffic on its links with AS3 and not beyond, then, the more specific prefixes should be advertised with the NO_EXPORT community while 10.0.0.0/7 would be advertised without community values. With this community value, the two more specific prefixes will not be advertised by AS3 and thus will not contribute to the growth of the global BGP routing table.