OSPF (Open Shortest Path First) chooses paths using a metric called cost. Every OSPF-enabled interface has a cost, and the cost of a route is the sum of the outgoing interface costs along the path from this router to the destination. The route with the lowest total cost wins, and when two or more routes tie, OSPF installs them all and load-balances across them (up to four equal-cost paths by default on IOS). Because cost is a simple additive number, you can predict OSPF's choices on paper and tune them precisely, which is exactly what exam questions ask you to do.
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