EIGRP Wide Metrics

Hello Christopher

Let’s take a deeper look into this one. Each EIGRP router when advertising a network, will use the EIGRP formula to determine that metric.

  1. So R4 has the destination as directly connected. Using the formula, and taking into account the bandwidth of the link, it measures a metric of 5. R4, in its update to R3, will send that metric which is the advertised distance.
  2. R4 will receive that information, and will rerun the EIGRP formula, and will take into account the characteristics of its link to R4. Based on the formula (including the bandwidth of the link), it generates a metric of 9 for itself to the destination. It shares that with R2.
  3. R2 receives the advertised distance of 9 from R3, and runs the EIGRP formula and takes into account the link between R2 and R3. The result is a metric of 18.

Now this is a simplistic description. In truth we’re not just summing costs, but we’re using the EIGRP formula to determine the cost. The outcome of the formula itself that R4 originally generates has embedded in it, the BW of the link to the destination. Similarly, R3, uses this advertised metric and recalculates the cost using the formula. R3 will take into account the bandwidth of the link to R4 in the metric it sends to R2. So R2, although it doesn’t explicitly receive the information of the lowest bandwidth link to the destination, the advertised metric itself has that information indirectly included in it. Does that make sense?

I hope this has been helpful!

Laz