Intelligent Services Gateway (ISG) is a Cisco IOS software feature set that provides a structured framework in which edge devices can deliver flexible and scalable services to subscribers. ISG supports IP sessions for subscribers who connect to ISG from routed Layer 2 or Layer 3 access networks. This module describes how to configure ISG to bring up IP subscriber sessions, manage subscriber IP addressing, and configure dynamic Virtual Private Network (VPN) selection.
For complete configuration guide, visit below link:
http://www.cisco.com/en/US/docs/ios/isg/configuration/guide/isg_acess_sub_sessns.html