Why a user does not authenticate against a local domain controller in the same site when logging on across a forest?

Let's explain the typical scenario in which we see this problem: The scenario starts with two separate Active Directory forests: contoso.com and litware.com. Each forest has a forest (i.e. kerberos) trust to the other. The contoso.com forest has one Active Directory site name CHARLOTTE. The litware.com forest contains two sites: REDMOND and CONTOSO. Administrators in the litware.com forest created the CONTOSO site and subnet to support logons for litware.com users from terminal servers in the contoso.com forest.

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