# How to Maximize Bandwidth Utilization with EIGRP Variance Command

## Introduction

EIGRP routing protocol can automate routing decisions and configuration based on the routing table. It has the feature of EIGRP variance command which allows a router for load balancing over unequal cost paths. Thus it sends routes on those paths that the router considers as backup paths. In other words, it sends the packet over backup paths which is not the best path, but EIGRP enables it to send packets over it.

EIGRP allows only one best path to be added to its routing table. However, the variance command enables EIGRP-speaking routers to include those paths in the routing table that are equal to or less to a multiple of the best metric value. These paths exist in the EIGRP topology table but not in its routing table. The routing table installs only the best paths.

**How variance command works**

The variance command is a multiplier argument in the EIGRP configuration. EIGRP multiplies the best route’s metric by the variance value. A route having a metric less than or equal to this calculated value is considered a feasible path and it will be used for load balancing. A famous condition, known as feasibility condition states that the reported distance must be less than the successor’s feasible distance. If the feasibility condition is not met, then the variance command will not apply to it. So the route will not be used for load balancing.

If there are two redundant links to the destination. If primary link has a bandwidth of 2 Mbps and the secondary link has a bandwidth of 1 Mbps. Then, the EIGRP variance command will send 2 packets on the primary link (2 Mbps link) and 1 packet on the secondary link (1 Mbps link).

**The variance command formula**

We can describe the EIGRP variance command in the following way. Suppose if we denote the successor with M and the variance with V, then a route with a metric less than or equal to M*V, will be included in the routing table of an EIGRP-speaking router and it will be used for load balancing. The variance value ranges from 1 to 128. It has a default value of 1.

Check the routing table of R1.

The network 192.168.20.0/32 from R4 to R1 is coming through two ways i.e. 1.1.1.2 and 2.1.1.2. If you want to prefer one path over the other, then change the bandwidth of the above links as shown in below.

But first, check the bandwidth of both the links.

Now change the bandwidth of the links. We will consider Fastethernet 0/0 as the primary link, and Fastethernet 0/1 as the secondary link by changing their bandwidth.

Again check the routing table of R1, if some changes occur.

All the redundant paths are removed from the routing table. Now, it prefers fastethernet 0/0 over fastethernet 0/1 of R1. Also, networks 3.0.0.0/8 and 4.0.0.0/8 are affected by the bandwidth command. So, it also sends its traffic over R1’s fastethernet 0/0.

**Variance Calculation**

**Calculate the variance formula for the following three networks.**

__Network 192.168.2.0__

Formula = Maximum Feasible Distance** ÷** Minimum Feasible Distance

Division= 2632960 **÷** 1382912

= 1.9, which is approximately 2.0

__Network 3.0.0.0__

Formula = Maximum Feasible Distance** ÷** Minimum Feasible Distance

= 2507520 **÷ **1254912

= 1.9, which is approximately 2.0

__Network 4.0.0.0__

Formula = Maximum Feasible Distance** ÷** Minimum Feasible Distance

= 2504960 **÷ ** 1257472

= 1.9, which is approximately 2.0

As 1.9 is approximately equal to 2.0, so multiply 2 by the EIGRP variance command under EIGRP router mode.

After applying the above command, It will also add those paths in the routing table that exist in the topology table.

Now, the router will do unequal cost load balancing by sending different numbers of packets over different paths. Now It will also utilize those paths that were considered as backup paths.