Understanding Planes of Networking

Data Plane:

The term data plane refers to the tasks that a networking device does to forward a message. In other words, anything to do with receiving data, processing it, and forwarding that same data whether you call the data a frame, a packet, or, more generically, a message is part of the data plane.

If you focus on the Layer 3 logic for a moment, the host sends the packet (step 1) to its default router, R1. R1 does some processing on the received packet, makes a forwarding (routing) decision, and forwards the packet (step 2). Routers R3 and R4 also receive, process, and forward the packet (steps 3 and 4). The data plane is often called the forwarding plane.

Some more common actions that the data plane or forwarding plane does are as follows:

■ De-encapsulating and re-encapsulating a packet in a data-link frame (routers, Layer 3 switches)

■ Adding or removing an 802.1Q trunking header (routers and switches)

■ Matching an Ethernet frame’s destination Media Access Control (MAC) address to the MAC address table (Layer 2 switches)

■ Matching an IP packet’s destination IP address to the IP routing table (routers, Layer 3 switches)

■ Encrypting the data and adding a new IP header (for virtual private network [VPN] processing)

■ Changing the source or destination IP address (for Network Address Translation [NAT] processing)

■ Discarding a message due to a filter (access control lists [ACLs], port security)

Control Plane:

routers need IP routes in a routing table before the data plane can forward packets. Layer 2 switches need entries in a MAC address table before they can forward Ethernet frames out the one best port to reach the destination. Switches must use Spanning Tree Protocol (STP) to limit which interfaces can be used for forwarding so that the data plane works well and does not loop frames forever.

The term control plane refers to any action that controls the data plane. Most of these actions have to do with creating the tables used by the data plane, tables like the IP routing table, an IP Address Resolution Protocol (ARP) table, a switch MAC address table, and so on. By adding to, removing, and changing entries to the tables used by the data plane, the control plane processes control what the data plane does. You already know about many control plane protocols — for instance, all the IP routing protocols.

In the figure, Open Shortest Path First (OSPF), the control plane protocol, runs on each router (that is, it is distributed among all the routers). OSPF on each router then adds to, removes from, and changes the IP routing table on each router. Once populated with useful routes, the data plane’s IP routing table on each router can forward incoming packets, as shown from left to right across the bottom of the figure. Some of the control plane protocols are:

■ Routing protocols (OSPF, Enhanced Interior Gateway Routing Protocol [EIGRP], Routing Information Protocol [RIP], Border Gateway Protocol [BGP])

■ IPv4 ARP

■ IPv6 Neighbor Discovery Protocol (NDP)

■ Switch MAC learning

■ STP

Management Plane :

The management plane performs overhead work as well, but that work does not directly impact the data plane. Instead, the management plane includes protocols that allow network engineers to manage the devices.

Telnet and Secure Shell (SSH) are two of the most obvious management plane protocols. To emphasize the difference with control plane protocols, think about two routers: one configured to allow Telnet and SSH into the router and one that does not. Both could still be running a routing protocol and routing packets, whether or not they support Telnet and SSH.

The figure below gives a better distinction between the three planes.