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In the context of network routing, route filtering is the process by which certain routes are not considered for inclusion in the local route database, or not advertised to one's neighbours. Route filtering is particularly important for BGP on the global Internet, where it is used for a variety of reasons.
Types of filtering
Input filtering
Output filtering
Reasons to filter
Economic reasons
Security resons
Technical reasons
Route filtering is also done because some routers have an insufficient 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). This is not an ideal practice, as it may result in loss of connectivity if less-specific covering routes are not available for dropped prefixes, and even if connectivity is maintained, may result in sub-optimal routes by frustrating the traffic engineering intentions of the announcing networks.
See also
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