Microsoft Knowledge Base Article 317589 contains the following summary:
This step-by-step article describes how to install and configure a secondary Microsoft Internet Authentication
Service (IAS) server in a domain.
IAS performs the function of a Remote Authentication Dial-In User Service (RADIUS) server.
You can use IAS for centralized authentication and accounting of multiple
Routing and Remote Access Service (RRAS) servers. You can use a secondary IAS server to provide fault-tolerance
and load balancing in your domain.