Google Analytics Cross Domain Tracking Made Easy
1/19/2012 A default Google Analytics implementation will break your visits when tracking cross domain websites (such as webshops using an external payment gateway) because the first-party cookies can only be read from the domain that set the cookies. When switching domains a new cookie is created and thus a new visit is registered. This could result in an incorrect report of the number of visits, but even worse, it could also strip the original referrer from your visits.