Click the exhibit button. In the topology shown, router R1 is an ASBR configured to export external routes to OSPF. How many type 4 LSAs will be present in the network?
A.
One.
B.
One for each of the routers in area 0
C.
One for each of the external routes exported by router R1.
D.
Type 4 LSAs are not generated in this network topology.
LSA Type 4 is a ASBR Summary LSA, and it's generated by the ABR. It is flooded from area 0 into a non-transit area and vice versa - "from" is the key word here.
Basically it represents the ABR's reachability to ASBRs. It tell that he - the ABR - knows how to reach netwroks outside the OSPF domain via ASBR.
So, the ASBR will generate a type 5 into are 0 - Not a 7. Type 7 LSAs are generated if we have a NSSA.
ASBR generates a 5 informing its area that he has routes that were redistributed into OSPF.
The ABR in turn, once he receives type 5 LSAs with its origin in the ASBR, he will know that those are external to the OSPF Domain, and will tell the adjacent area that he knows a way outside the OSPF domain via ASBR. He wont share the networks because there is no need to overload the adjacent area with unnecessary information. They just need to know how to get out.
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