Applying Filters; Example Bgp Policies - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

However, you can permit (or deny) routes to subnets within the larger network
by specifying a permitted range of prefix lengths. For example, the filter could
allow all routes to subnets in the 10.1.0.0 /16 network with a prefix length up
to and including 24:
ProCurve(config)# ip prefix-list FilterIn seq 10 permit 10.1.0.0/16 le 24
The ge keyword indicates that the length must be greater than or equal to that
specified in order to match. The le keyword indicates that the length must be
less than or equal to that specified in order to match. If you specify only ge,
the router assumes 32 as the upper limit. If you only specify le the router
assumes the network address's length as the lower limit.
If you want the filter to exactly match a prefix length, enter that length for
both the ge and the le value. For example, you can configure your router to
accept any routes to a /24 subnet in the 10.1.0.0 /16 range, but not to accept a
route to the entire 10.1.0.0 /16 network:
ProCurve(config)# ip prefix-list FilterIn seq 10 permit 10.1.0.0/16 ge 24 le 24

Applying Filters

Move to the configuration mode context for the BGP neighbor to which you
want to apply the filter. Enter this command:
Syntax: prefix-list <listname> [in | out]
To filter the routes that the BGP interface accepts from the neighbor, use the
in keyword. For example, enter:
ProCurve(config-bgp-neighbor)# prefix FilterIn in
To filter the routes that the BGP interface advertises to the neighbor, use the
out keyword.

Example BGP Policies

Prefix list filters help you to regulate which routes the router advertises and
learns, thus controlling to some degree the path traffic takes in and out of your
network. Common uses for prefix filters include:
receiving only routes from remote VPN sites
prohibiting the advertisement of a network
preventing your network from becoming a transit for external traffic when
multihoming
load balancing outgoing traffic
IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring BGP
13-81