When a trust exists between two forests, the authentication mechanisms for each forest trust the authentications that come ...

When a trust exists between two forests, the authentication mechanisms for each forest trust the authentications that come from the other forest. Trusts help to control access to shared resources in a resource domain (the trusting domain) by verifying that incoming authentication requests come from a trusted authority (the trusted domain). Trusts between forests can be external trusts or forest trusts. External trusts can exist with Windows 2000 Server or Windows Server 2003 domains, regardless of their functional level, and they use NTLM authentication. Forest trusts can exist with Windows Server 2003 or Windows Server 2008 forests that operate at the Windows Server 2003 forest functional level or higher, and they use Kerberos authentication.