TYPO3 menu link to subdomains

When you have a site setup like this:

example.com [Root level with own domain]
          |- sub1.example.com [Root level with own domain]
          |-- Page 1
          |- sub2.example.com [Root level with own domain]
          |-- Page 1

And you want to have a shared menu like:

Main Site
Sub 1
Sub 2

which works correctly between the sites simply set

config.typolinkCheckRootline = 1

in your top level template and it should work correctly.