cookie persistence profiles in conjunction with an iRule

From my experience if you have a Virtual Server that has multiple pools the BigIP will give you a persistence cookie for each pool that your traffic is being routed to.

 

 

We keep our static content on a different set of servers, so anything that's "/images" goes to one pool and anything that's "/app" goes to another. You will be issued a cookie for each pool in the format of:

 

 

BIGipServer=HashedServerIPAddress.HashedServerPortNumber.0000

 

 

You can decipher the hash by looking at "SOL6917: Overview of BIG-IP persistence cookie encoding" to see which server in each pool you are going to.

 

 

We also have multiple levels of BigIP's and the others just ignore a cookie that isn't for them (although I do wonder what would happen if you had identical pool names on different load balancers).

 

 

I guess this leads to my question of, why set a persistence cookie yourself via an iRule when you can use the default functionality of the BigIP? Is there a special circumstance where you have to?