HP 7102dl - ProCurve Secure Router Configuration Manual page 806

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring BGP
15-80
You configure which routes the router accepts and advertises by configuring
prefix lists and applying them to neighbors. Apply a prefix list to outbound
data to restrict the advertisement of certain routes; apply a prefix list to
inbound data to prohibit the router from accepting a route. See "Creating
Prefix Lists: Configuring Filters for Route Exchange" on page 15-81.
While an inbound filter can work fairly well for balancing outbound traffic,
advertising different routes to different neighbors may not balance inbound
traffic as effectively. You will always find it more difficult to load balance
inbound traffic because you cannot control your ISPs' policies.
In this case, even through you advertise only certain routes to one neighbor, the
ISPs' routers will probably aggregate these routes when they advertise them.
For example, you configure one interface to advertise routes to networks
192.168.1.0 /24 through 192.168.127.0 /24 and the other to advertise routes to
networks 192.168.128.0 /24 through 192.168.255.0 /24. Both ISP routers then
advertise routes to network 192.168.0.0 /16.
Even though you advertised different routes to the two different neighbors,
they advertise the same route, and other external networks then have to select
one or the other.
You can attempt to force the ISP router to prefer one connection over the other
by adding a private AS number to the advertisement to one neighbor. Routers
will prefer the route through the other connection because it will seem to have
fewer hops. (See "Prepending Private AS Numbers for Load Balancing" on
page 15-99.)
You can also set different multi-exit discriminators for routes advertised over
different connections. The discriminators assign routes a higher or lower
metric depending on the neighbor to which the routes are advertised, thus
influencing which connection ISP routers select for inbound traffic. (See
"Setting a Multi-Exit Discriminator Metric for Load Balancing" on page
15-100.)
Because a route's metric and AS hop count are only two of the many factors
BGP uses to select a best route, these strategies may not always have the
desired effect.