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

Figure 2 – Variance Multiplier Calculation for Unequal-Cost Load Balancing

This means your variance must be greater than or equal to 1.083.

If you set variance to 1, the second path will not be installed.
The multiplier must be a whole number between 1 and 128, so you round 1.083 up. Variance 2 is the smallest value that installs both paths.

Setting Variance to 2

Figure 3 – EIGRP Variance Modified to 2 to Enable Load Balancing

In this scenario, you configure variance high enough so that R1 installs both paths toward 10.0.23.0/24.

Here is how you enable it on R1:

R1(config)# router eigrp 1
R1(config-router)# variance 2

Traffic Share Behavior

Once multiple paths are installed, EIGRP does not split traffic equally by default.
Instead, it distributes traffic proportionally based on the metric.

R1# show ip route 10.0.23.0
Routing entry for 10.0.23.0/24
  Known via "eigrp 1", distance 90, metric 3072, type internal
  Redistributing via eigrp 1
  Last update from 10.0.13.2 on GigabitEthernet0/1, 00:00:35 ago
  Routing Descriptor Blocks:
  * 10.0.13.2, from 10.0.13.2, 00:00:35 ago, via GigabitEthernet0/1
      Route metric is 3072, traffic share count is 16
      Total delay is 20 microseconds, minimum bandwidth is 1000000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2
    10.0.12.2, from 10.0.12.2, 00:00:35 ago, via GigabitEthernet0/2
      Route metric is 3328, traffic share count is 14
      Total delay is 30 microseconds, minimum bandwidth is 1000000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2

Look at the traffic share count:

• Metric 3072 → traffic share count 16

• Metric 3328 → traffic share count 14

The lower the metric, the more traffic that path carries.

In the next lesson, the Successor path fails, and you watch how fast EIGRP recovers.