We have two ISP's connected to my Cisco 2600 WAN serial ports. Each ISP has given us 64 live IP addresses each. My question is: I wanted a few of my potential clients to be routed through the ISP A (which is more reliable) and a few other clients through ISP B. (And if any one of the ISP link goes down, the traffic should be automatically diverted through the other ISP.) How do I achieve this?

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There's several slightly different ways to do this depending on the routing protocol type RIP, OSPF, or IGRP and the type of set up configuration: equal or unequal load balancing.

As an example, IGRP is able to handle multiple routes with equal or unequal metrics. The traffic can then be split across the multiple routes depending upon the routes metric.

How is this accomplished? Equal-cost load balancing is controlled by the maximum-paths command. It's located in the router IGRP configuration. To separate traffic using unequal metrics IGRP uses the bandwidth statement. Below is an example of unequal routing using IGRP. 

interface Ethernet0
 ip address 192.168.x.x 255.255.0.0

interface Serial0
 ip address x.x.x.0 255.255.255.192
 bandwidth 768

interface Serial1
 ip address y.y.y.0 255.255.255.192
 bandwidth 512

This was first published in February 2003

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