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| 6.4 |  | Troubleshooting Complex Network Systems |
| 6.4.5 |  | TCP load distribution with NAT |

NAT on Cisco routers allows network engineers to provide TCP Load Distribution among hosts. A graphical representation of TCP Load Distribution is given in Figure . Steps to configure TCP load distribution with NAT are in Figure .

Although TCP load distribution can improve the performance of some types of network transactions (such as accessing a corporate intranet web service), it can also be a source of network complexity, resulting in intermittent fault behaviors when something goes wrong.

Example 1 – host fault
Assume that the network is configured as depicted in Figure . The router is distributing TCP connections evenly among the hosts, as it should. Overnight, power to Host 1.1.1.2 fails and the device is no longer on the network. Because the router does not know this, it continues to forward TCP connection requests to the failed host. The result of this is that one in three connection attempts fail, presenting a seemingly intermittent fault.

After a little investigation, the network engineer realizes that every third connection attempt is failing and immediately suspects that one of the hosts may have failed. Power is restored to Host 1.1.1.2 and network performance returns to normal.

Example 2 – host misconfiguration
Using the same network configuration from Figure , assume a new network engineer is instructed to build a new server. Because the new network engineer does not understand how the TCP load distribution system works, the new server is configured with the IP address assigned to the virtual host on the NAT router. When the new server is powered up, it detects an IP address conflict and cannot establish a connection to the network. The new engineer examines the interface configuration on the router to locate and confirm the duplicate IP address, but can not find it in the interface configuration.

After discussing the problem with a more experienced engineer, the new engineer reconfigures the new server with correct IP settings, reboots, and connects to the network.