In many common environments (single-tenant, fairly static configs) it probably doesn't make sense to use an SDN. However, if your environment looks more like EC

In many common environments (single-tenant, fairly static configs) it probably doesn't make sense to use an SDN. However, if your environment looks more like EC2's, with lots of VMs/apps/services, lots of tenants, and lots of virtual networks, you need some way create an overlay that provides logical layer 2 switching beyond what simple VLANs can provide. For example, if I want to migrate a running VM from one host to another, it would be nice if the top of rack switches could be made aware of the change as it happens so CAM tables could be updated on the fly. Cisco and VMware do something similar today with the DVS and Nexus 1000v virtual switches, and a lot of the OpenFlow/SDN vendors are hoping to extend that type of dynamic orchestration out to physical switches as well.