Creating Prefix Lists: Configuring Filters For Route Exchange - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring BGP
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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 13-96.)
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 13-98.)
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.
Creating Prefix Lists: Configuring Filters for Route
Exchange
Because BGP is designed to run between external networks, it allows admin-
istrators to precisely control the information routers accept from neighbors
and advertise about the private network.
When a BGP router receives a route from a neighbor, it applies an internal
filter before it even considers whether to place the route in its routing table.
And again, before sending a route to a neighbor, the router runs the route
through an external filter.
Filters process routes according to network address and prefix length. They
minimize overhead by keeping routers from learning and advertising unnec-
essary and unnecessarily specific routes.