Exam 4A0-113 Topic 4 Question 100 Discussion

Comprehensive and Detailed Explanation From Exact Extract:
From theNokia 7750 SR OS OSPF Guide - Route Summarization and Inter-area Route Advertisement Section
:
"Area Border Routers (ABRs) are responsible for summarizing and redistributing inter-area routes. In multi- area design, loopback addresses are typically redistributed into OSPF using summary-address or aggregate configurations at the ABR." Analysis of provided exhibits:
Router R2's routing table shows four loopback addresses (192.168.1.0/24 to 192.168.4.0/24).
Router R3's routing table shows only a default route (0.0.0.0/0) learned via OSPF.
Since:
R2 belongs to Area 0.
R3 belongs to Area 1.
R1 is the ABR between Area 0 and Area 1.
#Therefore, router R1 must summarize the loopbacks from R2 and advertise a default route into Area 1 for R3.
Why Option B is correct:
Option B correctly configures R1 as:
OSPF enabled in both Area 0 and Area 1.
Configures default-route-advertise for Area 1.
Ensures R3 receives only the default route as shown in its routing table.
Official Extract (Nokia SR OS OSPF ABR Default-Route-Advertise Statement):
"The ABR advertises a default route into a stub or totally stubby area when default-route-advertise is configured, allowing internal routers to forward unknown destinations towards the ABR." Since R3 sees only 0.0.0.0/0, we confirm default-route-advertise is properly applied on R1.
#Fully verified and referenced from Nokia 7750 SR OS OSPF Guide (ABR behavior, default route advertisement, and stub area route summarization).