Summary

Google Analytics can easily accommodate tracking across different sub-domains and different parent domains
Tracking across sub-domains requires no custom configuration ('it just works')
Tracking across different parent domains requires a minor configuration (you have to tell GA what domains are part of your site)
In GA-terminology, 'Cross-Domain Tracking' refers to user navigation across different parent domains; traversing different sub-domains does not qualify as 'Cross-Domain' from a GA-perspective.

Option 1: Different Sub-Domain (Same Parent Domain)

Example: lodging.destination.com

Default Ripe configuration
Requires no special GA configuration
Navigating across sub-domains will be treated as one session and one site (just as if it all occurred on the same sub-domain)
However, the 'hostname' parameter can be used to easily segment the different areas of the site (e.g. 'www' vs 'lodging')
As an optional enhancement, some clients prefer to prepend the hostname to the primary GA page parameter in order to more easily differentiate the pages.
- This is particularly valuable if both sub-domains have pages with the same URL path (e.g. www.destination.com/search and lodging.destination.com/search)
Additionally, you can configure a custom 'content group' to view aggregate statistics and navigation flow for each site.

Option 2: Different Parent Domain

Example: lodging.visitdestination.com

Optional Ripe configuration
GA can track across domains as if it was all part of one domain
Requires a minor GA admin configuration
- You must tell GA what domains to 'link' together as one site
- This prevents the sessions from being split
- This also prevents each domain from being treated as a referral source for the other
As with option 1, the 'hostname' parameter can be used to easily segment traffic between sites
Additionally, the hostname can be prepending to the page name and/or a custom content group can be configured to enhance the reporting options

Technical Consideration

In order for GA to link sessions across parent domains, a query string parameter ('_ga' or '_gl') is automatically appended to the incoming URL. Please ensure that your website can accommodate the query string parameter to avoid any issues.

Example: www.destination.com/?_ga=2.204321616.1314603355.1675438189-655234177.1675438189