HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 697

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring BGP
Customer A
External traffic
AS3
Router A ISP Router
AS1
External routes
External traffic
ISP Router
AS2
Figure 13-18. Problem with BGP Multihoming
To prevent this problem from arising, you should configure an outbound filter
list that advertises only the null route (the route to your private network's
range of addresses) to the ISPs. For example:
ProCurve(config)# ip prefix-list FilterOut seq 1 permit 10.1.0.0/20
Prohibiting Advertisement of a Network. A special case for a prefix list
is prohibiting the router from advertising a network to a neighbor.
Enter a deny statement for an exact match to the network. For example:
ProCurve(config)# ip prefix-list FilterOut seq 1 deny 172.16.30.0/24
Because prefix lists are applied on a neighbor-to-neighbor basis, you can use
this option to allow the router to advertise a route to one neighbor, but not to
another.
When multihoming, you can configure one BGP interface to advertise one set
of local networks to one ISP and another BGP interface to advertise another
set to another ISP. In this way, you can attempt to force the ISPs to load balance
incoming traffic across your two connections. (However, this method is
uncertain. See "Load Balancing" on page 13-74 for more information on load
balancing.)
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