The Illustrated Network- P71:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 26 MPLS-Based Virtual Private Networks 669 is wildly erratic and thus wasted much of the time. Private networks are designed for peak loads such as end-of-month or end-of-quarter frenzies and sit idle most of the time. The PSTN is no exception by the way and is designed in the United States for the 5 days of maximum calling volume Mother s Day Christmas New Year s Day Thanksgiving and Father s Day. Only unpredictable major disasters can swamp the PSTN at other times. Adding sites can be a problem in this scenario. Organizations with many sites can always contract floor space at some central point and install their own routers and leased lines there in a hub configuration instead of a mesh to cut down on point-to-point mileage costs and the number of ports required on each router. Of course the isolation of the private network is always attractive to customers. But what if the ISP can promise a network that looks like the rented-floor-space router hub solution with leased private line connectivity In other words the ISP provides a solution that looks like a private router network to the customer complete with what appear to be dedicated links and routers that contain routing information for that customer and that customer only. This is of course a VPN. But what we have described is not just any type ofVPN it s a Layer 3 VPN L3VPN because the virtual nature of the network is apparent at Layer 3 the IP layer . It s really a network of virtual routers because in reality the ISP is selling the same router resources to hundreds and even thousands of customers if the router and links are hefty enough to handle the loads. The different L3VPN customers cannot see each other at all or even communicate unless special arrangements are made this is sometimes called an extranet the closed VPN being an intranet . Each can only see the information in its own virtual routing and forwarding VRF tables as if the router were divided into many tiny logical pieces. L3VPNs are .
