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Understanding the Control Plane

3 min read
control-network

I was asked a question off the back of my recent article on “Understanding and Configuring Stackwise Virtual” that I’d like to discuss in more detail. That question was “So, what exactly is a control plane and what does it do?

The Brains of the Outfit

In the most high level terms – the control plane is the chief organiser of a networking device. It is where protocols are interpreted and decisions made on everything from whether there is a loop in the network that needs to be tackled, to whether that new OSPF hello contains the right parameters to form a valid neighbour. It also takes care of device access and management protocol duties; so when you SSH to your device, or set up a syslog to a remote server, these activities are also handled at the control plane.

Crucially, it is here that the best routes are calculated and placed into the master RIB (the routing table). For more information on the process, the articles “What is a RIB and FIB” and “Administrative Distance Cheat Sheet” describes the interplay of these elements.

The Rolling (Control and Data) Planes

The decisions made by the control plane filter into the data plane, influencing the transmission of packets. The job of the data plane is to send (and receive) on an interface, but the intelligence of deciding which interface to send out of, ultimately comes from the control plane. 

Intelligence : Outsourced

The control plane is able to take on so many tasks partly because CPU time is not completely taken serving packets. It is true that some packets do need to be ‘punted’ to the CPU; but in Cisco devices CEF takes care of a lot of this in hardware ASICs. Other manufacturers have similar technologies to CEF. This in turn frees up the device to concentrate on handling other duties. A CEF primer can be found “here“.

The Shared Control Plane

Usually each device has its own control plane, but with  Stackwise Virtual technology, it becomes shared between two separate chassis. This effectively means that a master chassis makes decisions not only for itself, but also for its partner. 

With Cisco Stackwise Virtual (as with the very similar VSS) the master chassis constantly updates the partner chassis – the standby – with information it requires to be able to take over, should the master go offline.

Summary

The control plane does the heavy lifting in deciding the routes that should be entered into a routing table alongside additional ancillary functions expected of a network device such as presenting a CLI, handling SSH connections, sending NetFlow data to a collector, and keeping routing neighbourships active.

I hope this short article has answered your question, check back soon for more news, tutorials and future tech!