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Software-defined networking can use various protocols to route packets and manage traffic. But does it use segment routing?
Short answer: Not necessarily.
Longer answer: Software-defined networking architecture doesn't need segment routing, and most SDN implementations don't use it. They could, however, and more implementations will likely do so steadily over time.
Complete answer: Segment routing is a mechanism for allowing the source of a network packet to specify information about how the packet is supposed to be routed to its destination. It is akin to the idea of label switching -- the technology that underlies MPLS.
In essence, a list of instructions -- unhelpfully called segments -- is attached to the beginning of a packet. The set of instructions is handled like a push-down stack, in which the top instruction is read and acted on by the egress router in the source network. As the packet leaves, the instruction is popped off the stack, and the next router handles the next instruction.
Where segment routing is used
Segment routing has been implemented using IP, IPv6 and MPLS, and using it does not require software-defined networking architecture, per se. Although segment routing doesn't use a specific protocol, it does require extensions and modifications to existing protocols, such as Intermediate System-to-Intermediate System and IPv6 -- e.g., adding a new type of routing header.
Cisco and Juniper routers can handle segment-routing traffic correctly, for example, as both companies have adopted the extended protocols. Their versions of SDN are ready for segment routing, as well. Moreover, Linux has an open source implementation of segment routing, and Cumulus Networks' Linux-based network OS also supports it.
Other prominent SDN players, however -- most notably VMware NSX, but also Big Switch, Pica8 and Pluribus Networks -- do not support segment routing at this time.
Carriers and large-scale service providers are most interested in segment routing, as it is a flexible mechanism for traffic engineering. They are also interested in software-defined networking architecture to allow them more hardware independence and to gain the ability to offer new services on a software timescale, from weeks to months, rather than a hardware timescale, from months to years.
Segment routing provides a control mechanism for SDN traffic paths that is simpler for their purposes and more scalable than traditional SDN protocols, like OpenFlow, Virtual Extensible LAN or Application Centric Infrastructure.
Now, only the largest enterprises are likely to explore segment routing plus software-defined networking architecture. In the future, though, it will be baked into more of the services smaller enterprises deploy.