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Notes:
This local-pref settings corresponds to the economical relationships between the various ASes.
Since AS1 is paid to carry packets towards Cust1 and Cust2, it will select a route towards those networks whenever possible.
Since AS1 does not need to pay to carry packets towards Peer1-4, AS1 will select a route towards those networks whenever possible.
AS1 will only utilize the routes receive from its providers when there is no other choice.
It is shown in the following papers that this way of utilizing the local-pref attribute leads to stable BGP routes :
Lixin Gao, Timothy G. Griffin, and Jennifer Rexford, "Inherently safe backup
routing with BGP," Proc. IEEE INFOCOM, April 2001
Lixin Gao and Jennifer Rexford, "Stable Internet routing without global
coordination," IEEE/ACM Transactions on Networking, December 2001, pp.
681-692
The RPSL policy of AS1 could be as follows :
RPSL policy for AS1
aut-num: AS1
import: from Cust1 action pref=100; accept Cust1
from Cust2 action pref=100; accept Cust2
from Peer1 action pref=500; accept Peer1
from Peer2 action pref=600; accept Peer2
from Peer3 action pref=700; accept Peer3
from Peer4 action pref=800; accept Peer4
from Prov1 action pref=1000; accept ANY
from Prov2 action pref=1000; accept ANY