Networks are a collection of network devices (such as routers and switches) connected using links. At their simplest, their purpose is to forward data packets from their source to their destination. This is known as routing.
As its name suggests, routing is performed by routers, and routers are divided into two logical parts (planes):
Control Plane
Data Plane
These planes describe how packets travel to, from, and through the router. It must be stressed that the planes do not describe the data packets themselves but rather only how the router handles them.
An important distinction that a router must make is between packets that the router must process itself and those that it must forward to another router. This is achieved entirely using the planes mentioned earlier.
The Control Plane is the "brain" of a network. Therefore, it comes as no surprise that it's the more important of the two planes.
The Control Plane is responsible for managing traffic sent to or from the router instead of through it. To elaborate, traffic generated by the router or received by the router for itself (such as SSH requests) is dealt with by the Control Plane.
Most importantly, the Control Plane handles all the traffic concerned with the routing protocol between routers, such as messages sent for OSPF or BGP. The router's Control Plane takes all these messages and constructs a routing table that stores the routes to all nodes in the network.
The Control Plane performs all the above mentioned tasks using the router's CPU.
The Data Plane can be referred to as the "messenger" of the network. This is because passing data on quickly and accurately is the sole responsibility of the Data Plane. The preceding is also why the Data Plane is referred to as the Forwarding Plane.
It must be noted that the Data Plane would have no idea where to pass data if the Control Plane hadn't performed routing and computed the routing table. Hence, the functioning of the Data Plane is entirely dependent on the Control Plane.
Unlike the Control Plane, however, the Data Plane doesn't use the router's CPU as it doesn't have to compute anything! Rather, it uses specialized hardware, called ASICs, to forward the data packets quickly and efficiently.
Network devices are split into planes because of a few reasons, such as:
The ability to develop Control Plane and Data Plane technologies independently.
To separate the main functions that the planes carry out entirely. For example, if a router gets overwhelmed with traffic and its CPU hits 100% (rendering the Control Plane unusable), then the Data Plane will still be able to forward traffic as it doesn't use the CPU!
A separate Control Plane means the router can be used in an SDN with various benefits.
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