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.