HP 7102dl - ProCurve Secure Router Configuration Manual page 811

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring BGP
Preventing the Router from Advertising External Traffic. A common
BGP application is multihoming. Multihoming allows you to connect to two
ISPs and advertise certain routes to one ISP and certain routes to the other ISP.
An unintended consequence of multihoming is that the ISPs can advertise
routes to each other through your local network, which can then become a
transit network for external traffic. (See Figure 15-18).
Customer A
External traffic
AS3
Router A ISP Router
AS1
External routes
External traffic
ISP Router
AS2
Figure 15-18. 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.
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