When you plan a load test, one of the first things you need to know about the backend configuration is if it includes a load balancer. This is important because most load balancers distribute new user sessions by the client IP address. Some of them allow changing the distribution method. However this may be achievable only by a significant configuration change. You will hardly persuade the site admins to make such changes in the production environment.
What is a bit embarrassing in this respect is that even if you select the “distribute to the least loaded” option in LB settings, this may still mean that it will use this rule only for new IPs. After the initial connect, each IP is remembered for a certain time (from 1 hour to few days) and all new connections from the same IP will be directed to the same web server.
This creates a very big problem for load testing. If you use WAPT Pro, or any other efficient load testing tool, by default all your virtual users will share the same IP address and will be directed to the same web server behind the LB. As a result, you will not be able to test the whole system correctly.