Controller packet-forwarding when hybrid mode is disabled

Controller operation with hybrid mode disabled

Controller operation with hybrid mode disabled

When hybrid mode is disabled (set to "false"), the controller examines and directs the packets in all flows for the given OpenFlow instance. The controller forwarding decisions for flows in a given instance are based on the requirements of the installed applications. The forwarding decision is communicated to controlled switches through OpenFlow. In instances where the controller has not provided the switch with a rule for how to forward a packet type, the switch sends the packet to the controller and waits for the controller to provide forwarding instructions.

Hybrid mode is commonly disabled in networks that are either used for experimental OpenFlow work (such as developing a controller application) or for networks that are completely new and designed to be fully controlled by OpenFlow.

Controller packet forwarding when hybrid mode is enabled

Controller operation with hybrid mode enabled

Controller operation with hybrid mode enabled

When hybrid mode is enabled (the default), the specific packet types for which the controller monitors and overrides switch forwarding rules depends on the applications installed and running in the controller. That is, the controller overrides normal packet forwarding rules in the OpenFlow switch with application-specific forwarding rules, such as:

  • copying ARP request/reply and DHCP offer/ACK packets to the controller so that it can discover end-hosts

  • stealing BDDP packets to the controller so that it can discover inter-switch links

  • changing the priority on Lync packets to improve instant messaging speed

  • monitoring DNS requests to detect dangerous end-host behavior

Packets in flows that the controller does not examine or direct are forwarded through normal switching operations without controller intervention.

NOTE: HP recommends that hybrid mode be enabled when controlling traditional, established networks where applications-related traffic is responsible for only a subset of the overall traffic load on the network. Hybrid mode is commonly enabled in established networks where new applications are installed and running on the controller, creating a need to override normal switching behavior for specific flows.

Learning more about hybrid mode

For more on hybrid mode as it relates to OpenFlow, see the latest OpenFlow Switch Specification on the Open Networking Foundation website. For a list of HPN switches that support OpenFlow operation, see the latest edition of the HP VAN SDN Controller and Applications Support Matrix.