Configure the Domain cookie attribute which instructs web browsers to only send the cookie to the specified domain and all subdomains

In our environment, we use a few subdomains (xyz.example.com, or abc.example.com) and with an iRule have added additional set-cookie headers when the access policy is started to add the session id cookies and specify the domain (domain.com). With this, we get the cookie on that domain as well as any subdomains. Hope this helps (and that I understood your question right).