One way to get your arms around it is to organize the various SDN announcements into buckets of common features and functionality. As you can see below, we can classify them into four buckets: 1) legacy proprietary systems; 2) overlay network virtualization; 3) abstracted network management systems; and, 4) open SDN architectures.
Now, let's analyze each of them.
* Legacy proprietary systems need no explanation. Because these systems were built with a distributed control plane architecture, we've been manually provisioning proprietary protocols on each interface of each device for every network service request for the last 20 years. While this is a mature and well-understood way to build and manage networks, it seems to be failing in customer applications where business agility, automated provisioning and best-in-class operational costs are paramount.
* Overlay network virtualization arose to address the fundamental inability of legacy networks to support the dynamic nature of virtualized and cloud data center workloads. Overlay network virtualization products exclusively use network encapsulation or tunnels to dynamically tunnel traffic between virtual hosts, thus circumventing the static nature of traditional L2 and L3 network protocols and addressing schemes.
Overlay network virtualization products implement control of virtual switches operating on virtual hosts using inherently closed, proprietary control protocols to direct traffic. As a result, these solutions tend to be purpose-built applications exclusively built to handle network tunneling for virtual workloads, and nothing else. They are not capable of addressing other limitations of legacy networking systems, since overlay networks are inherently detached from the physical network control plane.
* Abstracted network management systems are a relatively new attempt by legacy vendors to centralize management of multiple types of devices via a management plane abstraction to multiple underlying device operating systems. While it is a step forward in centralizing the management and configuration of devices, it has a number of shortcomings.
First, these management abstractions are inherently vendor-specific. Second, since the abstractions are simply APIs sitting on top of CLI commands, there are inherent limitations that result from different feature sets enabled with different CLI commands on different product offerings. While it is useful for centralizing some basic configurations, abstracted network management systems can't be used to dynamically program or automate a network of multi-vendor devices, which is the ultimate goal of SDN.
* Open SDN architectures represent a fundamental change in networking architectures. An open SDN introduces centralized software controllers that implement a common data plane abstraction that unifies the entire network fabric southbound, and publishes open APIs for software applications northbound. With this open architecture, a fabric of multi-vendor devices can be aggregated into a single policy domain that can be programmed and automated using standard software (not CLI).
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