A hybrid SDN (software-defined network) is a network where both traditional networking and SDN protocols operate in the same environment. A hybrid SDN allows network engineers to introduce new SDN technologies like OpenFlow to legacy environments without a complete overhaul of the network architecture.
In a hybrid SDN, engineers can run SDN technologies and standard switching protocols simultaneously on the physical hardware. A network manager can configure the SDN control plane to discover and control certain traffic flows while traditional, distributed networking protocols continue to direct the rest of the traffic on the network.
The OpenFlow v1.3 standard includes specifications for hybrid interactions between OpenFlow and non-OpenFlow traffic to enable early SDN migration. It specifies two kinds of OpenFlow-compliant switches: pure OpenFlow and hybrid. Hybrid OpenFlow switches support OpenFlow and traditional Ethernet switching technology. Many network hardware vendors who have added hardware support to their switches and routers offer the ability to run them in hybrid mode.