A PE router deployed by a customer service provider to provide Internet access, IP VPNs, and, or L2 VPNs may connect directly to a CSC-PE device, or it may back haul traffic within its local ‟site” to the CSC-CE that provides this direct connection. Here, ‟site” means a set of routers owned and managed by the customer service provider that can exchange traffic through means other than the CSC service. The function of the CSC service is to provide IP/MPLS reachability between isolated sites.
The CSC-CE is a ‟CE” from the perspective of the backbone service provider. There may be multiple CSC-CEs at a specific customer service provider site and each one may connect to multiple CSC-PE devices for resiliency/multi-homing purposes.
The CSC-PE is owned and managed by the backbone service provider and provides CSC IP VPN service to connected CSC-CE devices. In many cases, the CSC-PE also supports other services, including regular business IP VPN services. A single CSC-PE may support multiple CSC IP VPN services. Each customer service provider is allocated its own VRF within the CSC-PE; VRFs maintain routing and forwarding separation and allow the use of overlapping IP addresses by different customer service providers.
A backbone service provider may not have the geographic span to connect, with reasonable cost, to every site of a customer service provider. In this case, multiple backbone service providers may coordinate to provide an inter-AS CSC service. Different inter-AS connectivity options are possible, depending on the trust relationships between the different backbone service providers.
The CSC Connectivity Models apply to the 7750 SR and 7950 XRS only.