Example Prefix List Configuration - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring BGP
For example, Router A in Figure 13-19 connects to ISP 1 and ISP 2 through
two PPP interfaces. You permit PPP interface 1, which connects to ISP 1, to
receive routes for networks 1.0.0.0 /8 through 126.0.0.0 /8. PPP interface 2
receives routes for networks 128.0.0.0 /16 through 223.255.255.0 /24. The
router would learn to forward traffic to the first set of networks through PPP
interface 1 and traffic to the second set of networks through PPP interface 2.
You would configure the router as follows:
ProCurve(config)# ip prefix-list ExternalRoutes1 seq 1 permit 0.0.0.0/1 ge 8 le 8
ProCurve(config)# ip prefix-list ExternalRoutes2 seq 1 permit 128.0.0.0/1 ge 16 le 24
ProCurve(config)# router bgp 1
ProCurve(config-bgp)# neighbor 1.1.1.1
ProCurve(config-bgp-neighbor)# prefix-list ExternalRoutes1 in
ProCurve(config-bgp-neighbor)# exit
ProCurve(config-bgp)# neighbor 2.2.2.2
ProCurve(config-bgp-neighbor)# prefix-list ExternalRoutes2 in
The same general idea would hold if you were using two different routers to
connect to the two ISPs. The first router would learn the first half of the routes
and the second router would learn the second half. Because the BGP algorithm
prefers eBGP routes to iBGP routes, each router will choose the other for the
routes it did not learn. The network will send traffic destined to the first set
of addresses through the first router and traffic to the second set through the
second, effectively balancing outbound traffic.
N o t e
You can also enable load sharing to enable the router to balance outbound
traffic. See "Configuring Load Sharing" on page 13-120.

Example Prefix List Configuration

Router A in AS 1 connects to the Internet. It uses a default route for typical
Internet traffic, but needs routes to the private networks at a remote VPN site.
Each site in the VPN uses addresses in the 10.1.0.0 /16 range. To minimize the
number of routes routers must learn, the organization has decided that each
site should advertise its range of subnets as a 20-bit network. For example,
the local site uses subnets in the 10.1.0.0 /20 range, Site 2 uses subnets in the
10.1.16.0 /20 range, and so forth.
Router A connects to two ISPs. Your organization would also filter BGP
updates so that the router does not advertise external routes from one ISP to
the other, turning your private network into a thoroughfare between them.
13-85