When you are configuring multiple realms for cross-realm authentication, you need to decide how to tie the realms together. You can establish a hierarchical relationship among the realms, which provides automatic paths to the related domains. Of course, all realms in the hierarchical chain must be configured properly. The automatic paths can ease the administration burden. However, if there are many levels of domains, you might not want to use the default path because it requires too many transactions.
You can also choose to establish the trust relationship directly. A direct trust relationship is most useful when too many levels exist between two hierarchical realms or when no hierarchal relationship exists. The connection must be defined in the /etc/krb5/krb5.conf file on all hosts that use the connection. So, some additional work is required. The direct trust relationship is also referred to as a transitive relationship. For an introduction, see Kerberos Realms. For the configuration procedures for multiple realms, see Configuring Cross-Realm Authentication.