How to configure radius authentication for users who connect with Cisco VPN client ?

Let's talk about the clients for the radius server. As an example windows VPN. The client for the radius server are not the users who are connecting remotely via Windows VPN. The Routing and Remote Access server that acts as the VPN server for the remote users, is a client for the Internet Authentication server.

With Cisco, let's say that VPN users connect to an ASA, the ASA is considered as VPN server and a radius client.

A Cisco VPN device can be an ASA or a PIX, it can be also a router..

Almost there is not big difference in VPN radius authentication configuration between PIX firewall and different versions of ASA. Also not big difference regarding RADIUS AUTHENTICATION CONFIGURATION between IOS 7.x and IOS 8.x.As an example, Let's explain the configuration of ASA 5520 (IOS 8.4) for authenticating Remote VPN users via Radius server.