Hi,
There are several load balancing methods available on BIG-IP that allow you to specify how to pick an available node in the pool. You can also configure a persistence profile to ensure that repeated requests from the same client are sent to the same node in the pool.
For more information on these functions, you can refer to the BIG-IP Configuration guide:
Load balancing algorithms:
(look for the section titled, "Selecting a load balancing method"
Click here
Persistence methods:
Click here
iRules:
Click here
If you aren't able to find the information you are looking for in the manual, can you post again with more detail on what you are trying to accomplish with an iRule?
Thanks,
Aaron