Naming The List; Assigning The Entry An Order; Discarding Or Allowing Routes; Specifying The Network Address - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring BGP
13-80

Naming the List

You apply a list to a neighbor by its name. You can apply only one list to each
neighbor for an inbound filter and one for an outbound filter. You should
therefore give the same name to every entry that applies to the routes that
your router will receive from a neighbor. Conversely, give the same second
name to every entry that applies to routes the router will advertise to that
neighbor.

Assigning the Entry an Order

The router processes the prefix list from the lowest entry to the highest entry.
The router stops processing the prefix list as soon as it finds a match. You
should therefore take care when assigning entries a sequence number. For
example, you may generally want to discard routes with a prefix longer that
24, but allow a route to a specific /30 network. Give the default entry a higher
sequence number:
ProCurve(config)# ip prefix-list FilterIn seq 1 permit 10.3.3.0/30
ProCurve(config)# ip prefix-list FilterIn seq 10 deny 0.0.0.0/0 ge 24

Discarding or Allowing Routes

Permit statements allow the router to accept or advertise a matching route.
Deny statements configure the router to drop the matching route.

Specifying the Network Address

Identify the route that you want to filter by its network address. Remember
that the prefix length is an essential part of this address. Network 172.16.0.0 /
16 is different from network 172.16.0.0 /20.
If you want to filter by prefix length only, without regard to the network, use
the default network address, 0.0.0.0 /0.

Specifying the Range of Prefix Lengths

If you enter only a network address without specifying a range for prefix
lengths, the router assumes that the route must be an exact match. For
example, if you enter ip prefix-list FilterIn seq 5 permit 10.1.0.0 /16, the
BGP interface will only accept routes to the entire 10.1.0.0 /16 subnet. It will
not accept routes to a network such as 10.1.1.0 /24, which was subdivided from
the /16 network.