CCNP Data Center Question of the Week: Connecting Multiple Data Centers

Answer: B.

Per Cisco: OTV introduces the concept of “MAC routing,” which means a control plane protocol is used to exchange MAC reachability information between network devices providing LAN extension functionality. This is a significant shift from Layer 2 switching that traditionally leverages data plane learning, and it is justified by the need to limit flooding of Layer 2 traffic across the transport infrastructure. As emphasized throughout this document, Layer 2 communications between sites resembles routing more than switching. If the destination MAC address information is unknown, then traffic is dropped (not flooded), preventing waste of precious bandwidth across the WAN.

OTV also introduces the concept of dynamic encapsulation for Layer 2 flows that need to be sent to remote locations. Each Ethernet frame is individually encapsulated into an IP packet and delivered across the transport network. This eliminates the need to establish virtual circuits, called Pseudowires, between the data center locations. Immediate advantages include improved flexibility when adding or removing sites to the overlay, more optimal bandwidth utilization across the WAN (specifically when the transport infrastructure is multicast enabled), and independence from the transport characteristics (Layer 1, Layer 2 or Layer 3).

OTV provides a native built-in multi-homing capability with automatic detection, critical to increasing high availability of the overall solution. Two or more devices can be leveraged in each data center to provide LAN extension functionality without running the risk of creating an end-to-end loop that would jeopardize the overall stability of the design. This is achieved by leveraging the same control plane protocol used for the exchange of MAC address information, without the need of extending the Spanning-Tree Protocol (STP) across the overlay.