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In the context of network routing, route filtering is the concept, especially used on the global Internet with BGP, of filtering (removing from your table) routes for certain prefixes (destination address ranges). Route filtering can apply both to routes learned from other routers, and routes announced to other routers.
Route filtering is generally carried out by ISPs for security reasons; for example, upstream ISPs generally only permit each of their transit customers to announce a limited range of IP addresses, to prevent route hijacking attacks from within their customer networks. They are also likely to filter the routes they accept from other interconnected networks, to avoid their customers being the victims of route hijacking attacks from other networks.
Route filtering is also done because some routers have an insuffucient amount of RAM in which to store the routing tables; as the global BGP table has grown, older routers (or any router running out of RAM) must filter routes in order to keep from running out of RAM (and crashing). This is usually done by removing of a route for any prefix more specific than a certain length (anything more specific than a /24, for example).
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