An OSPF AS-external LSA describes a route to an AS external network. When the field in the LSA is 0.0.0.0, the traffic destined for the external network is sent to the ASBR that imports this external route. (Enter the acronym in uppercase.)
An OSPF AS-external LSA is used to describe routes to networks that are external to the OSPF Autonomous System (AS). This external route information is typically imported into OSPF by an Autonomous System Boundary Router (ASBR).
If the Forwarding Address field in the AS-external LSA is set to 0.0.0.0, this indicates that traffic destined for the external network should be forwarded directly to the ASBR that advertised the external route.
The specific context mentioned here relates to the Not-So-Stubby Area (NSSA), a type of OSPF area designed to handle AS-external routes in a more controlled way. In NSSAs:
Type 7 LSAs are used to describe AS-external routes.
The ASBR within the NSSA converts the Type 7 LSA to a Type 5 LSA before advertising it to the backbone area.
Hence, the correct acronym for the described functionality is NSSA.
[References:, Huawei HCIA-Datacom Study Guide, Chapter on "OSPF Area Types"., RFC 3101 – OSPF Not-So-Stubby Area (NSSA) Option., , , ]
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