 Layer 2 VPN 2.3 Network Virtualization (L2VPN), and Layer 3 VPN (L3VPN) [23, 24, 28, 29]. While L1VPN technology is under development, the other two technologies are mature and have been widely deployed. Also, based on their network- ing requirements, enterprises can connect their corporate locations together in many different ways. These networking services can typically be viewed from three perspectives, which are demarcation point (or enterprise/service provider hand-off), the local loop (or access circuit), and the service core. Choosing Layer 2 or Layer 3 VPN will make a different impact on these three network service [30]. In an L2VPN, the service provider's network is virtualized as a Layer 2 switch, whereas it is virtualized as a Layer 3 router in an L3VPN [23]. In the former, the customer sites are responsible for building their own routing infrastructure. Put differently, in an L3VPN, the service provider participates in the customer's Layer 3 routing, while in an L2VPN it interconnects customer sites using Layer 2 technology. As listed in Tables 2.2 and 2.3, both Layer 2 and Layer 3 services have their advantages and disadvantages. These are basically related to the differences of router and switch in computer networking; some of them are highlighted in Table 2.4. 2.3.3 Virtual Sharing Networks VPNs and overlays are not the only types of virtual networks implemented SO far; there exist other networks that do not fall into these two categories. Virtual local area networks (Virtual LANs) are Table 2.2 Layer 2 VPNs: advantages and disadvantages. Advantages Highly flexible, granular, and scalable bandwidth Transparent interface - no router hardware investment is required Low latency - switched as opposed to routed Ease of deployment - no configuration required for new sites Enterprises have complete control over their own routing Disadvantages Layer 2 networks are susceptible to broadcast storms - due to no router hardware No visibility from the service provider - monitoring services can be difficult Extr