Carrier's Carrier (CSC) Layer 3 VPNs

This VPN service enables a service provider to offer IP-VPN service to another service as the end customer. The following are the two main types of carrier-of-carriers VPNs (as described in RFC 4364):

• Internet service provider as the customer: The VPN customer is an ISP that uses the VPN service provider's network to connect its geographically disparate regional networks. 
• VPN service provider as the customer: The VPN customer is itself a VPN service provider offering VPN service to its customers. The carrier-of-carriers VPN service customer relies on the backbone VPN service provider for intersite connectivity.