Explanation/Reference:
Explanation:
http://www.cisco.com/en/US/docs/net_mgmt/ip_solution_center/4.0/mpls/user/guide/9_iscqsg.html Since the CSC-PE routers do not have to carry external routes in the VRF routing table, they can use the incoming label in the packet to forward the customer carrier Internet traffic. Adding MPLS to the routers provides a consistent method of transporting packets from the customer carrier to the backbone carrier.
MPLS allows the exchange of an MPLS label between the CSC-PE and the CSC-CE routers for every internal customer carrier route. The routers in the customer carrier have all the external routes either through IBGP or route redistribution to provide Internet connectivity.
When a backbone carrier and the customer carrier both provide BGP/MPLS VPN services, the method of transporting data is different from when a customer carrier provides only ISP services. The following list highlights those differences.
*When a customer carrier provides BGP/MPLS VPN services, its external routes are VPN-IPv4 routes.
When a customer carrier is an ISP, its external routes are IP routes.
*When a customer carrier provides BGP/MPLS VPN services, every site within the customer carrier must use MPLS. When a customer carrier is an ISP, the sites do not need to use MPLS.