4.1.h Describe basic L2VPN — LAN services

4.1.h [ii] OTV general principals

overlay transport  virtualization

OTV is a “MAC address in or over IP” technique for supporting Layer 2 VPNs to extend LANs over any transport . The transport can be Layer 2 based, Layer 3 based, IP switched, label switched, or anything else as long as it can carry IP packets. By using the principles of MAC routing, OTV provides an overlay that enables Layer 2 connectivity between separate Layer 2 domains while keeping these domains independent and preserving the fault-isolation, resiliency, and load-balancing benefits of an IP-based interconnection.

The core principles on which OTV operates are the use of a control protocol to advertise MAC address reachability information (instead of using data plane learning) and packet switching of IP encapsulated Layer 2 traffic (instead of using circuit switching) for data forwarding. These features are a significant departure from the core mechanics of traditional Layer 2 VPNs. In traditional Layer 2 VPNs, a static mesh of circuits is maintained among all devices in the VPN to enable flooding of traffic and source-based learning of MAC addresses. This full mesh of circuits is an unrestricted flood domain on which all traffic is forwarded. Maintaining this full mesh of circuits severely limits the scalability of existing Layer 2 VPN approaches. At the same time, the lack of a control plane limits the extensibility of current Layer 2 VPN solutions to properly address the requirements for extending LANs across data centers.

OTV uses a control protocol to map MAC address destinations to IP next hops that are reachable through the network core. OTV can be thought of as MAC routing in which the destination is a MAC address, the next hop is an IP address, and traffic is encapsulated in IP so it can simply be carried to its MAC routing next hop over the core IP network. Thus a flow between source and destination host MAC addresses is translated in the overlay into an IP flow between the source and destination IP addresses of the relevant edge devices. This process is called encapsulation rather than tunneling as the encapsulation is imposed dynamically and tunnels are not maintained. Since traffic is IP forwarded, OTV is as efficient as the core IP network and will deliver optimal traffic load balancing, multicast traffic replication, and fast failover just like the core would. OTV also supports detection of multi-homing.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4795-4800).  . Kindle Edition.

The transport can be Layer 2 based, Layer 3 based, IP switched, label switched, or anything else as long as it can carry IP packets.

OTV also supports detection of multi-homing.