Both iRule and Cookie persistence?

Detailed description part 1:

I have an application that holds a state in its session. The session itself is first created by another application and then the user navigates to a url with a specific id, so as an example:

1. External application creates a new session in my application and we return id: a12345
2. User navigates to my application with the id, i.e. /a12345

As this is the first time the user access the application we need to ensure that the user ends up at the server that holds the session. We have solved this by the servers creating an id that points to the correct server. So "Server a" always creates an id that starts with a and "Server b" would create an id that starts with b.