Cisco IOS Cookbook, 2nd Edition

Using NAT for Server Load Distribution

Problem

You have several application servers and you want to use NAT so that users can connect to them by a single IP address, distributing the load across all of the servers.

Solution

The rotary keyword allows you to do simple NAT-based load balancing of servers:

Router#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#interface FastEthernet0/0
Router(config-if)#ip address 192.168.1.1 255.255.255.0
Router(config-if)#ip nat inside
Router(config-if)#exit
Router(config)#interface FastEthernet0/1
Router(config-if)#ip address 192.168.2.1 255.255.255.0
Router(config-if)#ip nat outside
Router(config-if)#exit
Router(config)#ip nat pool WEBSERVERS 192.168.1.101 192.168.1.105 netmask 255.255.255.0 type rotary
Router(config)#access-list 20 permit host 192.168.1.100
Router(config)#ip nat inside destination list 20 pool WEBSERVERS
Router(config)#end
Router#

Discussion

This example is more applicable to an Intranet than to the public Internet. It shows a simple way of doing load balancing among a group of application servers that all perform the same function. We would like to stress that this load balancing is extremely limited, and if you want a fully featured load-balancing device, you would be much better off with one of Cisco’s Content Services Switches, such as the CSS 11500 series. However, we understand that these devices are expensive, so if your requirements are fairly basic, this recipe provides a simpler ...