Understanding Interprovider and Carrier-of-Carriers VPNs
All interprovider and carrier-of-carriers VPNs share the following characteristics:
Each interprovider or carrier-of-carriers VPN customer must distinguish between internal and external customer routes.
Internal customer routes must be maintained by the VPN service provider in its PE routers.
External customer routes are carried only by the customer’s routing platforms, not by the VPN service provider’s routing platforms.
The key difference between interprovider and carrier-of-carriers VPNs is whether the customer sites belong to the same AS or to separate ASs:
Interprovider VPNs—The customer sites belong to different ASs. You need to configure EBGP to exchange the customer’s external routes.
Understanding Carrier-of-Carriers VPNs—The customer sites belong to the same AS. You need to configure IBGP to exchange the customer’s external routes.
In general, each service provider in a VPN hierarchy is required to maintain its own internal routes in its P routers, and the internal routes of its customers in its PE routers. By recursively applying this rule, it is possible to create a hierarchy of VPNs.
The following are definitions of the types of PE routers specific to interprovider and carrier-of-carriers VPNs:
The AS border router is located at the AS border and handles traffic leaving and entering the AS.
The end PE router is the PE router in the customer VPN; it is connected to the CE router at the end customer’s site.