Second, if you want to upgrade the software on a standard network or install a new app you have to upgrade each of hundreds, or thousands, of routers. If you want to upgrade an SDN network you just upgrade the software in the central controller(s). Much faster, cheaper and less prone to errors. Moreover, network functionality can evolve more rapidly when it's tied to software rather than hardware (like ASICs). And centralized control offers the potential for substantial automation and standardization, potentially reducing the demand for scarce (and expensive) network engineers.
Third, centralized control leads to much more efficient use of bandwidth because the centralized controller has a view of the traffic flowing through the network, rather than just knowing what's coming from and going to its immediate neighbor. Google famously increased the utilization of one of its WANs from 60% to 95% by switching to SDN, almost halving its WAN costs per unit of traffic (see "Google's software-defined/OpenFlow backbone drives WAN links to 100% utilization").
Centralized control also permits advanced traffic engineering -- to the point of allowing an enterprise to customize routing by application and time of day -- and faster rerouting if part of the network goes down or becomes congested. And it makes SDN networks a lot easier to scale than traditional networks.
Open source software's cousins in the server world are virtualization and "appliances." They are not the same, but the theory/business case behind both is the replacement of customized, proprietary, single-purpose servers/routers with generic hardware on which you run software -- proprietary or open source -- that gives the box the functionality of a specialized server/router without the price tag. Network Function Virtualization (NFV) is already becoming pervasive in carrier networks, and will dominate them before the decade is out.
Hybrid WANs -- not new, but enhanced
A lot of the early use cases for SDN and virtual routers involved data centers and carrier networks, but given our focus, it is the WAN developments that excite us. When you apply SDN to WAN design, the resulting separation of network control/intelligence from the underlying media paves the way for enhancements that address many pressing WAN concerns -- business continuity/disaster recovery, application (including cloud) performance, bringing new locations online quickly and cheaply, and network reconfiguration.
The "hybrid WAN" as a concept has been energized by the emergence of Cloud services, the lure of low cost, high speed Internet circuits, and SDN. A hybrid WAN combines 'traditional' network transport (including MPLS) with substantial use of dedicated and broadband Internet access. It frequently involves two connections to each site, which can be selected according to traffic type and performance parameters.
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