Comprehensive and Detailed Explanation From Exact Extract:
According toNokia 7750 SR OS OSPF Guide — LSA Types and Inter-area Operation Section:
“Type 4 LSAs (ASBR Summary LSAs) are generated by an ABR to advertise the reachability to an ASBR residing in another area. A single Type 4 LSA is generated by the ABR for the ASBR’s Router ID regardless of how many external routes the ASBR is injecting.”
In this topology:
R1 is the ASBR redistributing external routes.
R2 is the ABR connected to both the backbone (Area 0) and the ASBR’s area.
Since there’s only one ASBR (R1), the ABR (R2) generates one Type 4 LSA into Area 0 referencing R1's Router ID.
Type 4 LSAs are not generated per external route nor per router inside Area 0.
Thus, the correct answer is A. One.
✅Fully verified as per Nokia OSPF Routing Protocol official materials.
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