The BIG-IP persistence cookie is an encoded (not encrypted) form of the pool member IP and port. The cookie name by default is BIGipServer. The encoding is described in SOL6917 (
Click here). This codeshare example (
Click here) shows how to decode it in an iRule.
Are you trying to generate a unique ID for handling sessions or for persistence? If the former, I could see why you'd want to prevent collisions. If the latter, I'm not so clear. Can you explain why you would need a globally unique token for persistence? Why couldn't you just the default cookie insert persistence with a expiration time set according to your requirements? I don't understand why the persistence cookie would need to be globally unique if you're just trying to give the client a token with which you get them back to the same pool member.
Aaron