IP Virtual Private Networks (IP VPNs)

Many carriers provide a managed IP services offering that basically lets customers hook up their CPE IP routers to a service provider’s private IP backbone. Most IP Service providers run an IP network over a Layer 2 infrastructure such as an ATM or Frame Relay network. An example of a conventional IP VPN is shown in the picture below.

The service provider typically configures multiple routing protocols or runs multiple routing processes on its backbone routers for various customers. Typically, the Cisco Routing engine supports the operation of multiple routing protocols in a single router in order to connect networks that use different routing protocols. The routing protocols available are inherently designed to operate independently of each other. Each protocol collects different types of information and reacts to topology changes in its own way. For example, RIP uses a hop count metric and EIGRP uses a five-element vector of metric information.

Customers perceive a private IP VPN by virtue of a combination of access lists, routing protocols, and processes.

The biggest issue facing managed IP service providers is scalability and complexity of implementation. The number of available routing protocols and routing processes supported per router platform sometimes forces service providers to deploy separate routers for each customer VPN at the service provider’s point of presence.

Reference :
“Advanced MPLS Design and Implementation”, Vivek Alwayn, Cisco Press, 2002.