vSphere 5.1 introduces VMware Single Sign-On (SSO), which is a required component for all vCenter Server installations. SSO includes three installation options: Basic mode, Multi-site mode, and High Availability Cluster mode. If SSO is installed in Multi-site mode or HA Cluster mode, VMware prevents the use of any Local OS identity sources and, therefore, prevents users from adding vSphere permissions for any local users.