I am using Cacti to monitor the CPU/Memory/Network traffic usage of Cisco 2821. The router has a public IP address, but not to be revealed.
I want to know any command/method that can generate traffic and changes the CPU/Memory usage.
You could use something like Soasta's CloudTest Lite to generate traffic into your environment. Setup a test web server behind your 2821 and run a load test against it. CloudTest Lite only lets you test up to 100 simultaneous users but it should be enough for you to gather some metrics on your router. If your router is not breaking a sweat under that load you could turn on NetFlow to busy up the CPU a little.