When a switch is configured to connect to more than one controller, then the switch should establish a communication channel with all the configured controllers. A switch may connect to multiple controllers for the following reasons:
- To improve the reliability of the system
- To provide load balancing across controllers depending on the role
When a switch is connected to more than one controller then the controller can take either the master role, slave role or the equal role. For more information regarding the controller role, refer to the recipe, Multiple controllers managing switch with different roles in Chapter 1, OpenFlow Channel Connection Establishment (Part 2).
The switch maintains the role of the controller with respect to the controller's connection, as the switch identifies the controller with the channel ID, which is the combination of the Datapath ID and the Auxiliary ID.
During the initialization of the OpenFlow switch, it should initiate a connection to all the configured controllers and should maintain the connectivity to all the controllers concurrently. The procedure to establish an OpenFlow communication channel to multiple controllers is similar to that of establishing a connection to a single controller as was explained in the Connection setup on TCP & TLS recipe. However, as the switch has to process requests concurrently from multiple controllers, the switch could employ mechanisms to read and process the message from multiple channels. This can be done, either by having different threads or different processes, each handling requests from multiple controllers, or should use some of the constructs provided by the operating system such as the select() system call.
In a steady state, the switch should be able to send asynchronous message to all controllers through the channels associated with the controller. Similarly, the switch should be able to process the OpenFlow request message from any of the connected controllers. When a switch receives the OpenFlow request message from any one of the controllers, the switch should send the response only to the channel which is associated with that controller.
The OpenFlow specification doesn't mandate any failure handling while establishing the communication channel. However, it is recommended for the switch to try to re-initiate the connection periodically until the connection is successful.