In OSPF multi-area designs with multiple ABRs, when there is temporary inconsistency between the ABRs’ type-3 summary LSAs, microloops can occur during convergence. These loops happen due to slight differences in LSA arrival and SPF calculations across routers while the network is converging.
Microloops are short-lived but can cause brief packet loss.
This becomes more likely as the number of ABRs grows and IGP flooding domains increase.
CCDE v3.1 emphasizes careful ABR design and possible use of loop avoidance mechanisms (e.g., LFA, microloop prevention techniques) in large-scale OSPF designs.
Why other options are incorrect:
A: LSA flooding in OSPF scales well; type-3 LSA replication is not the primary scalability issue.
B: ABRs process all LSAs regardless; load is not split.
D: Multiple ABRs all independently summarize and advertise into the backbone.
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