Route filtering

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Route filtering is the concept, especially used on the global Internet with BGP, of filtering (removing from your table) routes for certain prefixes (destination networks). 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).

Usually this is done because each router has finite 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).