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What is the essential difference between the flow tables in SDN-based switches and the conventional routing tables in the existing routers? In fact, I understood that the difference is in the routing mechanism so SDN uses flow-based routing mechanism while in the conventional network it is merely packet-based routing. If my understanding is correct, kindly can an expert elaborate this in details?

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First note that all of these technical terms are subject to distortion by Marketing, so sometimes you have to read between the lines.

The term switch refers to a layer-2 device, so it is forwarding on layer-2 info. On an Ethernet this would be the 48 bit MAC. With MPLS it would be the Label. The term router officially refers to a layer-3 device, so it is forwarding on layer-3 info (the 32/128 bit IPv[46] address, unless you have a very strange network). That's the primary difference between the terms.

How each device actually implements that forwarding is partly controlled by the protocols defined to exchange the relevant data, and the engineering choices made by the device designer, and is mostly opaque to the outside. So, to get more detail on how a particular product does it's thing, you need to get more detail from the manufacturer than the legal department wants them to divulge, so under NDA (Non-Disclosure Agreement). Unless the device you care about is just a general purpose computer running Free or Open Source Software, in which case, you look at the source.

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