Explanation/Reference:
Explanation:
You can create an external trust to form a one-way or two-way, nontransitive trust with domains that are outside your forest http://technet.microsoft.com/enus/library/cc775736%28v=ws.10%29.aspx Trust types

http://technet.microsoft.com/en-us/library/cc731297.aspx
Understanding When to Create a Realm Trust When to create a realm trust You can establish a realm trust between any non-Windows Kerberos version 5 (V5) realm and an Active Directory domain. This trust relationship allows cross-platform interoperability with security services that are based on other versions of the Kerberos V5 protocol, for example, UNIX and MIT implementations. Realm trusts can switch from non transitive to transitive and back. Realm trusts can also be either one-way or two way.