Virtualizing the Campus

The design principles discussed so far must be preserved as we virtualize the campus. To preserve the hierarchy and path symmetry provided by a multilayer routed core, this routed core must be kept in place. It is tempting for the network architect to associate the concept of virtual networks (VNs) with that of VLANs. Although a VLAN is a form of a VN, it is not by itself a hierarchical or modular VN and therefore does not preserve all the characteristics of resiliency and scalability desired in the enterprise. VLANs are an important component of a VN because they allow the virtualization of the Layer 2 portion of the network. However, to preserve the desired scalability and resiliency, a routed core is necessary. Therefore, the VLANs in the Layer 2 access must be combined with Layer 3 VPNs in the routed core. By combining VLANs with Layer 3 VPNs, you can create an end-to-end hierarchical VN in the campus. Therefore, an overlay of VPNs must be added to the routed core to virtualize it. The choice of VPN technology to use depends significantly on how well the VPN technology accommodates a hierarchical routing structure, the efficient use of redundant paths for load balancing, and failover and support for the different types of traffic present in the enterprise. The technologies available to create such an overlay and the subtleties of their deployment in the enterprise campus are discussed in Chapters 4 through 6. Chapter 4, "A Virtualization Technologies Primer: Theory," discusses the technologies in detail. Chapter 5, "Infrastructure Segmentation Architectures: Theory," discusses the different VPN architectures available. Chapter 6 "Infrastructure Segmentation Architectures: Practice" explains the caveats and best practices for the successful application of these virtualization techniques in the enterprise campus and metropolitan-area network.

Another important part of an enterprise network is the wide-area network. The design of the WAN has also evolved significantly over the years to achieve the desired level of resiliency and modularity. The remainder of this chapter discusses the general guidelines for designing a WAN.