Conroller SDN OpenFlow

Question

Suppose I have a topology like < host1--switch1--switch2--switch3--controller>. So, the physical path between switch1 and the controller consist of switch2 and switch3. Does the control traffic from switch1 to the controller goes from switch2 and switch3? I mean the openflow traffics between switch1 and the controller go to switch2 at first and then go to switch3 until it reach to the controller. Am I right?

Answer 1

Openflow switches have seperate management ports to connect to the controller. If you use outofband connection (direct connection to controller via management port) switch communicates with controller directly. In a such case, for data flows, you can still use the topology you mentioned.

But, if there is no management connection between switch 1 and controller, then it comes to "inband" connection. In a such case, switch 1 sends openflow messages via data port, switch 2 encapsulates this message and send to the controller.

From OpenFlow documentation:

The specication of the networks used for the OpenFlow channels is outside the scope of the present specication. It may be a separate dedicated network, or the OpenFlow channel may use the network managed by the OpenFlow switch (in-band controller connection). The only requirement is that it should provide TCP/IP connectivity.

Dedicated network means outofband connection.

To learn some details about inband you can look this documentation: Important part is:

In this setup, control traffic sent by switch A will be seen by switch B, which will send it to the controller as part of an OFPT_PACKET_IN message. Switch A will then see the OFPT_PACKET_IN message's packet, re-encapsulate it in another OFPT_PACKET_IN, and send it to the controller. Switch B will then see that OFPT_PACKET_IN, and so on in an infinite loop.