Install guide

2-4 Release Note
Software Version 2.7.5
C613-10454-00 REV A
Types of Filters
The type of filter to use depends on the route source and the point at which you
want to filter. This section describes the available filters, in the following
subsections:
■ About Prefix Lists
■ About AS Path Lists
■ About Route Maps
■ About IP Route Filters
■ About IP Filters
This section describes each of these types of filters and summarises the
circumstances in which you use them.
About Prefix Lists
Description A prefix list is a list of entries, each of which specifies:
■ an IPv4 prefix, and a mask length or range of mask lengths
■ whether those prefixes explicitly match or explicitly do not match the
prefix list
When to use
prefix lists
Prefix lists offer detailed control over which routes you import, export or
advertise.
“Applying Filters” on page 2-20 describes in detail how to use prefix lists, but
this section summarises the uses.
For BGP, you can use prefix lists when:
■ copying routes from an update message to the RIB, by using the prefix list:
• directly as a filter, by making it the infilter on a BGP peer.
• in a route map and applying the route map as the inroutemap on a BGP
peer
■ determining which routes to import from other route sources, by using the
prefix list in a route map and applying the route map to the import entry
■ determining which routes to advertise, by using the prefix list:
• directly as a filter, by making it the outfilter on a BGP peer
• in a route map and applying the route map as the outroutemap on a
BGP peer
For BGP, prefix filtering can reject some of the routes from an update message,
without rejecting the whole update. This enables you to configure the router or
switch to accept only routes for particular networks from a particular peer, and
to send only routes for particular networks to a particular peer.
When you apply a prefix list as an infilter or outfilter on a BGP peer, BGP
looks at the individual prefixes within each update message, and compares
them against the list. If a prefix in the update matches a prefix in the prefix list,
BGP rejects that route. Otherwise, it accepts the route.