EIGRP Load Balancing

In the previous lesson, R1 chose the path via R3 as its Successor and kept the path via R2 as a Feasible Successor.
That backup path carries no traffic. In this lesson, you make EIGRP use it too.

EIGRP supports Equal-Cost MultiPath (ECMP).
This means you can install multiple routes in the routing table if they have the same metric.

The Default Variance

By default, EIGRP uses a variance of 1.
Think of variance as a multiplier. It decides whether additional paths are allowed into the routing table.

Figure 1 – Default Variance Multiplier (1)

When variance is set to 1, you can only install routes that have the exact same metric as the Successor.

Here is where EIGRP becomes powerful.

Unlike many routing protocols, EIGRP allows you to install multiple paths even if their metrics are different.
If you configure a variance of 2, you allow EIGRP to install routes whose metric is up to 2 times the Successor’s metric.

Determining the Required Variance

To know the minimum variance you need, divide the candidate metric by the Successor metric:
3328 / 3072 = 1.083