IP load testers[1] are a class of protocol analyzers focused on the practical evaluation of router performance.
To test forwarding performance, IP load testers typically surround a router with simulated Internet traffic.
The first method approximates real Internet traffic by using a representative mix of packet lengths, usually referred to as IMIX.
In both cases, the IP load tester measures the performance of the router in terms of loss, latency and throughput.
Scalability typically means how many protocol sessions can be handled by the router at one time, and ultimately is a stress of memory.