Install guide

Filtering IP Routes 2-5
Software Version 2.7.5
C613-10454-00 REV A
For OSPF, you can use prefix lists in a route map, and then use the route map:
■ to filter OSPF routes before adding them to the RIB
■ when importing static routes into the OSPF LSA database
About AS Path Lists
Description In BGP, the AS_path attribute lists the AS numbers of every Autonomous
System that the routing information in an update message has passed through.
It shows the path the update message has taken, and how “close” the routes are
to the router or switch.
AS path lists let you filter to accept or reject update messages on the basis of all
or part of their AS path. They look at the AS_path attribute in BGP update
messages. If the attribute in the update message matches the filter criteria then
the whole update message is filtered out (or accepted, depending on what
action the filter entry has been configured to carry out).
When to use
AS path lists
You can only use AS path lists with BGP. “Applying Filters” on page 2-20
describes in detail how to use AS path lists, but this section summarises the
uses.
For BGP, you can use AS path lists when:
■ copying routes from an update message to the RIB, by using the AS path
list:
•as the inpathfilter on a BGP peer.
• in a route map and applying the route map as the inroutemap on a BGP
peer
■ determining which routes to advertise, by using the AS path list:
•as the outpathfilter on a BGP peer
• in a route map and applying the route map as the outroutemap on a
BGP peer
About Route Maps
Description Route maps are the most powerful route filtering option, and allow you to
configure complex flexible filters. They achieve this by having several levels of
structure:
■ each route map consists of multiple entries
■ each entry consists of an action (include or exclude) and at least one clause:
• zero or one match clause, which determines which routes or BGP update
messages match the entry. If you do not specify a match clause, every
route or update message matches.
• zero or more set clauses, which change certain features of matching
routes or the attributes of matching BGP updates.