Traditional VPNs, Interprovider VPNs, and Carrier-of-Carriers VPNs
As VPNs are deployed on the Internet, the customer of a VPN service provider might be another service provider rather than an end customer. The customer service provider depends on the VPN service provider to deliver a VPN transport service between the customer service provider’s points of presence (POPs) or regional networks.
If the customer service provider’s sites have different autonomous system (AS) numbers, then the VPN transit service provider supports carrier-of-carrier VPN service for the interprovider VPN service. If the customer service provider’s sites have the same AS number, then the VPN transit service provider delivers a carrier-of-carriers VPN service.
The sections that follow provide an overview of traditional VPNs, interprovider and carrier-of-carriers VPNs, and the differences in how external and internal routes are handled in each of these environments.
In traditional IP routing architectures, there is a clear distinction between internal routes and external routes. From the perspective of an Internet service provider (ISP), internal routes include all the provider’s internal links (including BGP next hops) and loopback interfaces. These internal routes are exchanged with other routing platforms in the ISP’s network by means of an interior gateway protocol (IGP), such as OSPF or IS-IS. All routes learned at Internet peering points or from customer sites are classified as external routes and are distributed by means of an exterior gateway protocol (EGP) such as BGP. In traditional IP routing architectures, the number of internal routes is typically much smaller than the number of external routes.
