when the number of originated or announced routes are "reasonable", prefix lists, as they are called, are generated and applied to BGP sessions. when the originated or announced routes is substantial and/or fluid, there are little to no prefix lists; allowing anything to be announced and accepted.
in an ideal world that has not existed for over a decade, IRR data could be used to automagically generate accurate prefix lists but for all the usual reasons (inaccurate, stale data; obtuse interface; etc) that just isn't done on a wide scale.
You can put a prefix limit on a session (that's the "little" in little to no) but that doesn't stop someone from announcing 10,000 more specific routes to popular (YouTube, Facebook, Twitter, Akamai) destinations.
in an ideal world that has not existed for over a decade, IRR data could be used to automagically generate accurate prefix lists but for all the usual reasons (inaccurate, stale data; obtuse interface; etc) that just isn't done on a wide scale.
You can put a prefix limit on a session (that's the "little" in little to no) but that doesn't stop someone from announcing 10,000 more specific routes to popular (YouTube, Facebook, Twitter, Akamai) destinations.