SCCM Configmgr how to manage clients in untrusted forest

Few weeks ago ,i was assigned with task to manage clients in untrusted forest using Configuration Manager 2012 R2 SP1.Yes, we are still running on Configmgr 2012 but soon (next few weeks) on Configmgr Current Branch . Scenario is ,I have a forest (intranet.asia) where SCCM 2012 R2 SP1 is installed and this is being…