OTV Control plane over Multicast & Unicast Infrastructure

OTV Control plane over Multicast & Unicast Infrastructure

In OTV all edge devices must be adjacent to each other and form adjacency between them and then control protocol run between them in order to exchange the MAC address reachability information.

The above adjacency and MAC address reachability information can be achieved by two ways:

• If the IP transport is multicast enabled, a specific multicast group is used to exchange the control protocol message between OTV edge devices.
• If IP transport is unicast mode, then one or more OTV edge devices can be configured as an Adjacency server and all other OTV edge devices register themselves and form neighbour list to get the adjacency.

Let’s understand how control Plane is built in both cases.

Control Plane adjacency in Multicast enabled Infrastructure:

When transport is multicast enabled the control plane adjacency can be achieved by two method:

• OTV Neighbour discovery
• OTV MAC address advertisement

OTV Neighbour discovery:

In this, all OTV edge devices should be configured to join with a specific ASM (Any Source Multicast) group and by this they can receive and send any multicast packet for this group.

• Each OTV edge device will send the IGMP report to join the specific ASM group which will be used to carry the control plane information. Here we are assuming this ASM group is group “G”. Now these OTV edge device will join this group as host and this works without configuring PIM and this Group is configured and associated to overlay interface.
• Now any OTV edge device , here in this case Edge device of site-1 generates the OTV hello , now this hello message is sent to all edge device who have joined to the specific ASM multicast group. This is required to form the control plane adjacencies. To achieve this the OTV hello is encapsulated to OTV frame and in external IP header source IP would be IP address of Join interface and destination IP is ASM multicast Group IP address which is associated to overlay interface.
• Now these Multicast frame traverse to multicast enabled transport and reach to each OTV edge device who has joined the Multicast group “G”.
• Now the once the Hello is received to OTV edge device these edge device will decapsulate the hello packet and this result in creation of OTV control protocol adjacency between all edge device.  

OTV control plane is transparently enabled in background after creating OTV Overlay interface and does not require any explicit configuration.

MAC address Advertisement:

Once OTV edge is discovered each other now let’s see how MAC address reachability information is shared between them to form control plane adjacencies.