200-301 Question #1646: Real Exam Question with Answer & Explanation

Question

When OSPF learns multiple paths to a network, how does it select a route?

Options

• AFor each existing interface, it adds the metric from the source router to the destination to calculate
• BIt counts the number of hops between the source router and the destination to determine the
• CIt divides a reference bandwidth of 100 Mbps by the actual bandwidth of the exiting interface to
• DIt multiplies the active K values by 256 to calculate the route with the lowest metric.

Explanation

Open Shortest Path First (OSPF) uses "Cost" as the value of metric and uses a Reference Bandwidth of 100 Mbps for cost calculation. The formula to calculate the cost is Reference Bandwidth divided by interface bandwidth. For example, in the case of 10 Mbps Ethernet, OSPF Metric Cost value is 100 Mbps / 10 Mbps = 10. The default Reference Bandwidth of OSPF is 100 Mbps and the default OSPF cost formula doesn’t differentiate between interfaces with bandwidth faster than 100 Mbps. These days, 1 Gbps and 10 Gbps links are also common. The according to the default OSPF metric Cost value calculation, the default OSPF Cost for Fast Ethernet interface (100 Mbps) and a Gigabit Ethernet interface (1 Gbps) are same. If you want to change the default behavior, the cost formula can be adjusted using the "auto-cost" command under the OSPF routing process. If you are changing the default OSPF Reference Bandwidth, make sure that you have changed the OSPF Reference Bandwidth in all your OSPF

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