An ABR injects a default route into a standard stub area so routers in the area have a default to reach external destinations. What LSA type does the ABR use to advertise that default into the stub area?
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Correct answer: Type-3 summary LSA for 0.0.0.0/0.
Why this is the answer
An Area Border Router (ABR) advertises a default route into a standard stub area using a Type-3 summary LSA for 0.0.0.0/0. Stub areas do not allow Type-5 external LSAs, so the ABR summarizes all external routes into a single default route. This ensures routers within the stub area have a path to destinations outside their area without needing to store the entire OSPF routing table. Type-5 external LSAs are not permitted in stub areas. Type-7 NSSA external LSAs are specific to Not-So-Stubby Areas (NSSAs), not standard stub areas. Type-1 router LSAs describe a router's directly connected links and their states within its own area, not default routes for other areas.
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