Can I install SDN alongside my existing network?
Yes. One common topology for deployments in a brownfield environment is an "SDN island" where an SDN domain flows through a gateway device to the legacy network. Another topology is that of hybrid switching, where a switch that can handle both OpenFlow and traditional networking splits its ports between the two domains. Hybrid capabilities vary by vendor.
What are overlays, and why are there so many different kinds?
An overlay is used to create virtual network containers that are logically isolated from one another while sharing the same underlying physical network. Virtual eXtensible LAN (VXLAN), Network Virtualization with GRE (NVGRE) and Stateless Transport Tunneling (STT) all emerged at roughly the same time, and all with different vendors leading each effort
If you'll allow for some generalization, Cisco (and others) have pushed VXLAN. Microsoft has driven NVGRE. Nicira (now part of VMware) has championed STT. Each overlay has similar characteristics, but differences in the details that make them the darling of some, but not others. Over time, VXLAN has gained the strongest following (including VMware, interestingly), but it's not yet clear that NVGRE and STT will be deprecated, as both have ardent supporters. In addition, the IETF NVO3 working group has been working on yet another overlay, although the encapsulation type is likely to be one that already exists.
Why are there so many different kinds of controllers?
Vendors early to market with SDN technology have necessarily had to bring a controller as a part of the overall solution. There is no such thing as an SDN controller standard at this time; therefore, each vendor has come up with a controller that best meets the needs of its target market.
Wouldn't it be better if there were SDN controller standards the industry could agree on?
With the creation of the OpenDaylight project, the industry seems to think so. OpenDaylight is a consortium of vendors from across the industry that are contributing code to an open source SDN effort. Time will tell how this translates into vendor products, and what this will mean for the SDN consumer.
Will network engineers have to become programmers?
Network engineers with an understanding of scripting and programming will be able to leverage SDN technology. Will they have to? That remains to be seen. The scenario I see playing out is that vendors will supply corporations with software that enables rich network functionality. Some engineers will use that software interface to provision the network, and will be satisfied as long as the network functions as intended. Other engineers will use that vendor-supplied software, but will also become proficient in a language that allows them to create the unique network applications required by their business. As these network engineers acquire programming skills, they will also maintain their ability to effectively monitor and maintain the network infrastructure.
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