A router is a device that forwards packets across various networks based on the routing table, which informs the decision of path selection per the distance-vector, link-state, or other criteria. A router is typically designed based on a layered architecture that isolates operations, such as data forwarding, route control, and system management so that they won’t interfere with one another.
A layer that shares common design concerns is also known as a plane in the architecture of a Cisco router; that is, the data plane, control plane, and management plane. The planes can be centralized, decentralized, or hybrid. A legacy router typically centralizes the data, control, and management planes. A Software-Defined Network (SDN) decentralizes the control plane to one or more standalone hosts called controllers. A hybrid approach employs controllers and keeps the control plane on discrete routers for performance and availability.
The following is an excerpt from Cisco:
Planes of Operation
A router is typically segmented into three planes of operation, each with a specific and clearly defined objective:
The control plane: The control plane is the brain of the router. It consists of dynamic IP routing protocols (that is OSPF, IS-IS, BGP, and so on), the RIB, routing updates, in addition to other protocols such as PIM, IGMP, ICMP, ARP, BFD, LACP, and so on. In short, the control plane is responsible for maintaining sessions and exchanging protocol information with other router or network devices.
In centralized architecture platforms, the general-purpose CPU manages all control plane protocols. In distributed architecture platforms, routing protocols, and most other protocols, always run on the core CPU in the RPs or Supervisor engines, but there are other control plane protocols such as ARP, BFD, and ICMP that in some distributed architecture platforms have now been offloaded to the line card CPU.
The data plane: The data plane is the forwarding plane, which is responsible for the switching of packets through the router (that is, process switching and CEF switching). In the data plane, there could be features that could affect packet forwarding such as quality of service (QoS) and access control lists (ACLs).
The management plane: The management plane is used to manage a device through its connection to the network. Examples of protocols processed in the management plane include Simple Network Management Protocol (SNMP), Telnet, File Transfer Protocol (FTP), Secure FTP, and Secure Shell (SSH). These management protocols are used for monitoring and for command-line interface (CLI) access.
Figure 3-12 shows how the three planes of operation and how the processes are isolated from each other. In IOS XR, a process failure within one plane does not affect other processes or applications within that plane. This layered architecture creates a more reliable model than one with a monolithic architecture such as IOS, where failure of a single process may cause a failure of the whole system.
Software-Defined Anything
Software-defined technologies rely on decoupling logical architectural elements from physical ones to provide programmability, facilitate virtualization, and render flexibility. Separating the control plane from the data plane is a critical process of decoupling and makes the Software-Defined Network (SDN) come into being. Network Function Virtualization (NFV) simulates physical network devices and provides software ones, e.g., switch hubs, routers, firewalls, etc. Software-Defined WAN (SD-WAN) extends and applies the concept of SDN to WAN. A Software-Defined Data Center (SDDC) relies on technologies, such as Hypervisor, NFV, SDN, Software-Defined Storage, etc., to address dynamic business requirements.