The _setDomainName method enables Google Analytics to pass cookie information across multiple sub-domains. Here is how you can use this method to track multiple sub-domains for a fictitious website called www.mysite.com which has two sub-domains (e.g. sales.mysite.com and partners.mysite.com).
This method should reference the primary domain (e.g. mysite.com).
You will first need to add the _setDomainName method within your GATC on all pages on all sub-domains of the site. You can either manually add this method into your existing GATC or you can grab the code for sub-domain tracking from your profile settings screen as shown in the following screen shot:
The above modification to your GATC will aggregate all the data from different sub-domains into a single profile. However, your reports may not distinguish between pages coming from mysite.com versus sales.mysite.com. For example, mysite.com/index.html and sales.mysite.com/index.html will be reported as /index.html.
Apply the following filter to resolve this challenge:
Once this filter is applied, Google Analytics will show the following outcome in the Top Content report in regards to the above example: