A network device performs two conceptually different kinds of work.

The data plane handles packets using already-established forwarding state: inspect relevant headers, choose an output action and move the packet at high rate.

The control plane decides what that forwarding state should be. Routing protocols, topology discovery and policy computation belong here.

For example, a routing protocol may determine that prefix 203.0.113.0/24 is reachable through neighbor R. The control plane installs an entry; later packets matching that prefix are forwarded by the data plane without rerunning the routing algorithm for each packet.

Separating these responsibilities explains why forwarding can remain fast while route computation is comparatively sophisticated, and why modern networks can centralize or program control logic without necessarily changing the basic packet-forwarding hardware.