3 Software Defined Networking (SDN)
3.4 Comparison to distributed control plane of traditional networks

Traditional network equipment incorporates the control plane, network applications and other higher level capabilities in device firmware. This means all network nodes make routing decisions locally - effectively using a fully distributed control plane, as displayed in Fig. 4. This can be contrasted to the centralized control plane in SDN architecture shown in Fig. 3.

Integration of many functions provides greater functionality of the every node and makes them more independent, almost eliminating single point of failure.

But it is achieved at the expense of the increased complexity. That correlates to more expensive equipment; higher power consumption and can lead to delays in the traffic handling caused by the need to process it by various applications. Furthermore, the configuration or modification of the network requires manual configuration of many devices and often the use of complex management tools. Network management is additionally complicated by implementation and configuration disparities between different vendors causing interoperability issues.

As it is stated in [7], all that complexity leads to static networks what is in contrast with the need for a dynamic environment with greater user mobility and server virtualization. Inconsistencies in network-wide policies and limited network scalability are further effects of traditional network intricacies in large networks.

Centralized control plane in SDN allows network administrator to shape traffic without the need to manually configure many devices, thus providing easier network management and greater flexibility.

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Fig. 4 - Traditional packet network architecture [8]

While MPLS is superficially similar to SDN with fast switching of traffic flows established by the control plane, it does so in a different manner. There is no centralized control plane, which is in every MPLS router with every LER able to create a path and distribute it using Label Distribution Protocol (LDP).