To configure a VPN whereby a client can request IP addresses from a DHCP server using a relay agent, you must define a namespace for the VPN and associate a scope with that namespace. Specifically:

1. Ensure that the relay agents that handle DHCP VPN traffic are configured with a version of Cisco IOS that supports the vpn-id suboption of the relay-agent-info option (82) in DHCP.

2. Coordinate with the IOS relay agent administrator whether the VPN is identified by a VPN ID or a VPN Routing and Forwarding instance (VRF) name. It is rarely both.

3. Configure a namespace in Network Registrar to correspond to the VPN. Associate this namespace with the VPN ID or VRF name.

4. Create a scope for the namespace.

http://www.cisco.com/en/US/docs/net_mgmt/network_registrar/6.0/user/guide/14VPN.html

http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6635/ps7180/prod_white_paper0900aecd80617171.html