The Control Plane Explained

The term control plane refers to all functions that provide the information the network devices need to make forwarding decisions, such as routing and CAM table entries, ACL lists, NAT tables, etc.

There are two types of networking environments: traditional and SDN. The traditional networking environment uses a flat network architecture. The SDN networking environment uses a layered architecture. It divides network functionality into three types: data plane, control plane, and management plane.

In the data plane, it keeps all the functions that move data between devices. The following tutorial explains this plane in detail.

The Data Plane Explained

In the control plane, it keeps all the functions that generate and provide the information the data plane needs to move the data. We will discuss, this plane in this tutorial.

In the management plane, it keeps all the functions we use to manage the networking devices. The following tutorial explains this plane in detail.

The Management Plane Explained

The control plane

Forwarding data is the primary job of all network devices. Different network devices use separate logic to make forwarding decisions. For example, routers use the routing table, while switches use the cam table to make forwarding decisions.

The forwarding happens in the data plane. To make forwarding desicision, the data plane needs information. For example, routers need IP routes in the routing table before the data plane can forward packets. Layer 2 switches need entries in the CAM table before they can forward frames.

The control plane provides the information the data plane needs to work. In other words, the information supplied to the data plane controls what the data plane does.

For example, a router uses the routing table to make forwarding decisions. When it receives a packet, it finds an entry in the routing table that matches the packet's destination address. If it finds an entry, it forwards the packet from the interface specified in the entry. If it does not find an entry, it discards the packet.

The entire decision-making process depends on the routing table entries. The control plane creates, updates, and deletes the routing table entries. By manipulating the information the data plane needs to forward the data packets, the control plane controls the flow of data in the network. Since it controls the action the data plane takes, it is called the control plane.

Let us take an example.

data plane

PC1 sends a data packet to PC2. The packet reaches the connected switch. The switch's data plane uses the cam table to forward it to the router R1. The control plane on the switch builds the cam table entries the data plane uses to make the forwarding decision.

R1's data plane receives it from the switch and reads the routing table to make the forwarding decision. R2's data plane gets it from R1's data plane, processes it, and forwards it to R3's data plane. R3's data plane processes and forwards it to PC2.

Control planes on R1, R2, and R3 build the routing table entries, which they use to make the forwarding decisions.

All functions, actions, and steps that add and manage entries in tables that the data plane uses to move the data packet from PC1 to PC2 are part of the control plane. In other words, the control plane controls how the data plane moves a data packet from one device to another device on the network.

Traditional networks use both the data and control planes on the same device. In other words, each device has a data plane and a control plane. The device's control plane is responsible for providing all the information the device's data plane needs to forward data packets.

SDN networks separate planes. It keeps the data plane on the device and moves the control on the SDN controller device. It uses the SDN controller to manage the control plane of all devices.

In other words, each device's data plane gets the information it needs to forward data packets from the control plane stored on the SDN controller device.

Without the protocols and activities of the control plane, the data plane of networking devices will not function properly. Routers are useless without the routes learned by the routing protocols. Without learning the MAC table entries, a switch can still process unicast frames by flooding them. But doing this for all frames would create a lot more load on the LAN network. In simple words, the data plane relies on information from the control plane to perform tasks.

Key points: -

In an SDN network, a control plane is responsible for the following things and functions.

  • Running routing protocols such as OSPF, EIGRP, RIP, IS-IS, and BGP.
  • Learning and adding routes to the routing table.
  • Monitoring network changes
  • Updating the routing table based on the network changes
  • Learning MAC addresses from the incoming frames
  • Managing and updating the CAM table entries
  • Running the STP protocol to avoid loops

ComputerNetworkingNotes CCNA Study Guide The Control Plane Explained