Discussion

(called longest-match lookup), so the order of the prefixes in the list does not matter.
This is different from how the JUNOS software handles policy evaluation, which is
to look at policy terms and chained policies in order, from beginning to end, and perform
the action immediately when a match occurs.
The then clause in this recipe has a reject action, which rejects the route. When
applying this policy with a set import command, the routes will not be installed into
the routing table. When applying it with a set export command, the routes will not
be advertised to peer routers. The set import command in this recipe applies the policy
to routes received from neighbors in an EBGP peer group, so the routes are not
installed in the inet.0 routing table.
9.4 Filtering Long Prefixes
Problem
You do not want to install IP address prefixes longer than 172.18.20.0/19 in the
routing table.
Solution
Create a filter that identifies the long prefixes:
[edit policy-options policy-statement long-prefixes term 1]
aviva@router1# set from route-filter 172.18.20.0/19 longer
aviva@router1# set then reject
Then apply the policy to an EBGP group:
[edit protocols bgp]
aviva@router1# set group external-group import long-prefixes
Discussion
A second way to filter routes based on their IP address prefixes is to create a route
list. Unlike prefix lists, route lists are embedded in the routing policy, not maintained
in a separate list, so it can be somewhat harder to maintain them because the
same prefixes may be used in different policies. This recipe creates a simple policy
that an EBGP group uses to reject all incoming prefixes longer than 172.18.20.0/19.
This policy keeps longer prefixes out of the routing table and is somewhat similar to
aggregating routes.
In the recipe, the set from route-filter command defines the prefix (172.18.20.0/19)
and how to match it (longer). The set then command is a simple action clause to
reject matching prefixes. We apply the policy with a set import command to an
EBGP group to prevent BGP from installing the long prefixes into the routing table.
Route lists have two advantages over prefix lists. The first is that route lists match
prefix ranges instead of the exact matching performed by prefix lists. This recipe uses
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