Q. How can I enable high availability for Microsoft RADIUS services?

A. RADIUS (part of Network Policy Server, NPS) is a stateless service and therefore does not need to be clustered. Instead simply deploy multiple instances of RADIUS in the environment with the same configuration (by exporting the configuration from the master and import to the others as I document at http://windowsitpro.com/windows-server/bulk-create-nps-policies-0) and deploying a load balancer to front the multiple RADIUS instances.