Consider the following scenario:
- You create domain A, domain B, and an external trust between the domains.
- You create an Active Directory Federation Services (AD FS) server that is running Windows Server 2012 R2 in domain A.
- You add an alternative user principal name (UPN) suffix in domain B.
- You create a user account that uses the alternative UPN suffix in domain B.
- You go to the AD FS server sign-in page on a computer in domain A, and then you log on by using the user account.