The design goal is to reduce the impact of branch and remote-office instability while using the fewest possible ABRs. In OSPF, topology changes inside an area trigger SPF calculations for routers in that area. Creating separate areas behind aggregation points contains those changes and prevents every branch link flap from affecting the entire backbone. The minimal-ABR approach is to place the area boundary at the higher aggregation layer, where fewer routers connect the new areas to area 0. In the described topology, R5 and R6 can serve as ABRs for both sets of downstream aggregation connections. That supports a new area for the R1
/R2 branch connections with R5 and R6 as ABRs, and a new area for the R3/R4 remote-office connections with R5 and R6 as ABRs. Making R1, R2, R3, and R4 the ABRs increases the ABR count and spreads area- boundary functions deeper into the network. A single new area for all downstream connections would reduce isolation between different failure domains. The selected design improves stability and keeps the OSPF hierarchy clean.