LSA filtering is done by defining area types in OSPF. These are : Stub:LSA 4, 5 are filtered. Total Stub:LSA 3, 4, 5 are filtered. NSSA:LSA 4,5 are filtered (when ASBR is present in your area. First converts LSA 5 to LSA 7 and vice versa.) Total NSSA:LSA 3, 4, 5 are filtered.