In general, load balancing is the capability of a router to distribute traffic over all the router network ports that are the same distance from the destination address. Load balancing increases the utilization of network segments, and so increases effective network bandwidth. There are two types of load balancing:
Equal cost path
Unequal cost path
This document explains how unequal cost path load balancing works in Enhanced Interior Gateway Routing Protocol (EIGRP).
This document requires a basic understanding of IP routing protocols and EIGRP routing protocol. In order to learn more about IP routing protocols and EIGRP, refer to these documents:
EIGRP is supported in Cisco IOS® Software Release 9.21 and later.
You can configure EIGRP in all routers (such as the Cisco 2500 series and the Cisco 2600 series) and in all Layer 3 switches.
The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.
Refer to Cisco Technical Tips Conventions for more information on document conventions.
Every routing protocol supports equal cost path load balancing. In addition, Interior Gateway Routing Protocol (IGRP) and EIGRP also support unequal cost path load balancing. Use the variance n command in order to instruct the router to include routes with a metric of less than n times the minimum metric route for that destination. The variable n can take a value between 1 and 128. The default is 1, which means equal cost load balancing. Traffic is also distributed among the links with unequal costs, proportionately, with respect to the metric.
Note: If a path is not a feasible successor, the path is not used in load balancing.
This section provides an example. In the network diagram, there are three ways to get 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 chooses the path E-C-A with a metric of 20 because 20 is better than 30 and 45. In order to instruct EIGRP to select the path E-B-A as well, configure variance with a multiplier of 2:
router eigrp 1
network x.x.x.x
variance 2
This configuration increases the minimum metric to 40 (2 * 20 = 40). EIGRP includes all routes that have a metric of less than 40 and satisfy the feasibility condition. In the configuration in this section, EIGRP now uses two paths to get to Network X, E-C-A and E-B-A, because both paths have a metric of under 40. EIGRP does not use path E-D-A because that path has a metric of 45, which is not less than the value of the minimum metric of 40, because of the variance configuration. Also, the reported distance of neighbor D is 25, which is greater than the feasible distance (FD) of 20 through C. This means that, even if variance is set to 3, the E-D-A path is not selected for load balancing because Router D is not a feasible successor. (c) Cisco Systems 2008
Practical Work
Download the Packet
Tracer file. This file has 5 routers and 3 LANs. The routers
A,B, D, E
are already configured to route using EIGRP. The bandwidth set for each
serial link is 1000 kbps.
Scenario
The company wants to implement load balancing between router E
and Network X. They have decided to add another path between Router E
and Router A.
The Router C has already been added but the contractors have
gone out of business.
Your Task
You must configure the addressing for the new serial links to
match the existing addressing scheme for the rest of the existing
network.
Once the serial links have been configured, you should
configure EIGRP on router C.
Make sure that you use the same AS number
as in the rest of the network (why?).
Be sure to configure the
bandwidth as 1000 kbps for each serial link.
Configure DHCP on each of the routers that have a LAN using a
well-defined naming convention for the address pools.
Exclude the first
20 available addresses on each LAN from being available to DHCP
clients.
These are reserved for servers. Check that each PC can obtain
an IP address automatically.
Check continuity across the network. Do not proceed until you
can ping all destinations and all points in the network are reachable.