Load balancing for non-equivalent routing using EIGRP

How to achieve the load balance of non-equivalent routing in EIGRP

EIGRP Load Balancing

Each routing protocol supports load balancing of equivalent paths. In addition, IGRP and EIGRP also support load balancing of unequal paths, using the variance command. The variance command advertises an n value to the router, and the n value is specified with the variance command. N values are 1-128, and the default is 1.

Network topology

Variance

In the figure above, router E has three paths to network X

e-b-a with A metric of 30

e-c-a with A metric of 20

e-d-a with A metric of 45

Router E selects the second path, e-c-a metric is the minimum value for 20,20 three paths, and if you want EIGRP to select a priority e-b-a path, configure the variance value to be a multiplier of 2

Router EIGRP 1

Network x.x.x.x

Variance 2

This increased the metric to 40 (2*20=40). This eigrp includes all routes that are less than 40 metric, in which the router uses two paths to reach the network x,e-c-a and E-b-a, because two paths have metric values below 40. Because E-d-a's metric is 45, greater than 40, EIGRP does not select this path to reach network X. Also, Router D reports a metric of 25 to network X, which is larger than a viable metric value of 20. This means that even if variance is set to the 3,e-d-a path, it will not be selected as a load-balanced path because router D is not a viable successor.

Traffic sharing

EIGRP not only supports load balancing of unequal paths, but also supports intelligent load balancing, such as traffic sharing. When multiple routes reach the same destination network with different cost, you can use the Traffic Sharing command to control the traffic on different routes. Using the Balanced keyword, routers distribute traffic proportionally according to the metrics ratios of different routes. This is the default setting

Router EIGRP 1

Network x.x.x.x

Variance 2

Traffic-share Balanced

Here is the traffic share calculation case

for Path e-c-a:30/20 = 3/2 = 1

for path E-B-A:30/30 = 1

If the division is not divisible, we calculate it by its integer (note: not rounded). In this case, the EIGRP sends the packet through the ratio of e-c-a and e-b-a to 1:1, which achieves the goal of load balancing.

Now we assume that the metric between the E-b is 15 between the metric and the 25,b-a. This e-b-a metric is 40 because 40 is not less than 2x20 (20 is a viable path metric,2 is a variance value), so that load balancing between e-c-a and e-b-a is not possible. If you want to achieve load balancing, we need to set the variance value to 3. In this way, the traffic share ratio of the two links is

for Path E-C-A:40/20 = 2

for path E-B-A:40/40 = 1

The flow rate of EIGRP in E-c-a and E-b-a is 2:1. In this way, EIGRP not only supports the unequal routing load balancing, but also supports the intelligent load balancing

When keyword min is used, traffic is sent only through the minimum cost path, even if there are multiple routing paths in the routing table

Router EIGRP 1

Network x.x.x.x

Variance 3

Traffic-share min across-interfaces

As a result of this configuration, EIGRP is only sent via the e-c-a path

This article also applies to IGRP