Communication between the domains happens through trusts. Trusts are needed for users in one domain to access the resources in another domain. There are two types of default trusts that are automatically created as shown in the figure.
Parent and Child type – By default, when a child domain is added to parent domain tree, a transitive two way trust is created. When new child domains are added, the trust path flows upward through the domain hierarchy.
Tree-root type – When a new domain tree is added to an existing forest, then a two way transitive trust is created.
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