Multiple Ways to Get to the Same Place

Multiple Ways to Get to the Same Place

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You can see by looking at the figure that R1 could send packets to subnet 150.1.3.0 through R2 or through R3. However, it seems like the direct route through R2 makes more sense. Routing protocols solve this problem by using an objective tool called a metric. A metric is a number that's associated with each route in a routing update. That number represents how good or how bad that route is. When a router receives multiple routing updates, it might learn of multiple ways to reach a network or subnet. By looking at the metrics associated with each route in each update, the router can pick the best route.

First, you need to see how R1 can learn about two routes to subnet 150.1.3.0. (Refer to Figure 12-7.) Then, it will be easier to see how the metric works.