9 – Overlay Transport Virtualization (OTV)

Cisco has recently introduced a new feature called OTV that extends Layer 2 traffic between multiple sites over a Layer 3 network. The edge devices that interconnect data centers are known as OTV edge devices.

OTV dynamically encapsulates Layer 2 packets into an IP header for the traffic sent to the remote data centers. Routing Layer 2 traffic on top of a Layer 3 network is known as “MAC routing” transport. MAC Routing leverages the use of a control protocol to propagate MAC address reachability information, this is in contrast with the traditional data plane learning done in technologies like VPLS.

In the next example, MAC 1 sends a Layer 2 frame to destination MAC 2. On the MAC table of the OTV edge device (DC1), MAC 1 is a local address (Eth1), while the destination MAC 2 belongs to a remote location reachable via the IP address B (remote OTV Edge device).

The local OTV-ED encapsulates the Layer 2 frame using an IP header with as IP destination “IP B”. The remote OTV-ED removes the IP header and forwards the frames to its internal interface (Eth 5). Local Layer 2 traffic is treated like any classical Ethernet switch (i.e. MAC 2 <=> MAC 3 on DC2).


A control plane protocol is used to exchange MAC reachability information between network devices, extending the VLANs between the remote sites while the learning process inside the data center is performed as in any traditional Layer 2 switch. This mechanism of advertisement destined to the remote OTV edge device differs fundamentally from classical Layer 2 switches, which traditionally leverage the data plane learning mechanism based on L2 source MAC address discovery: if the destination address is unknown after a MAC lookup on the MAC table, the traffic is flooded everywhere.

With OTV, the process for learning MAC addresses is performed by advertising the local MAC tables to all remote OTV edge devices. Consequently, if a destination MAC address is not known, the packet destined to the remote data center is dropped.

This technical innovation has the advantage of removing the risk of broadcasting unknown Unicast addresses from one site to another. This technique is based on a routing protocol, and provides a very stable and efficient mechanism of MAC address learning and Layer 2 extension while maintaining the failure domain inside each data center.

While OTV natively maintains the STP and the failure domain within each local data center, it provides the ability to deploy multiple OTV edge switches in the same data center in active mode. This function is known as Multi-Homing.

OTV works across any type of transport (Fiber, TCP/IP, MPLS) extended between the remote sites with the reliability and effectiveness of the Layer 3 protocol.

In addition to these primary functions, which are essential for the cloud networking, OTV offers several very important innovations:

OTV connects two or more sites to form a single virtual data center (Distributed Virtual Data Center). No circuit states are required between the remote sites to establish the remote connection. Each site and each link are independent and maintain an active state. This is known as “Point to Cloud” service, which allows a data center to be securely attached or removed at any time without configuring the remote sites and without disturbing cloud services.


OTV offers a native multicast traffic optimization function between all remote sites. OTV is currently available on Cisco Nexus 7000 Series Switches and the Cisco ASR 1000 Series Aggregation Services Routers.

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