I’d Rather Fight Than Switch

My maternal grandfather smoked Tareyton cigarettes, which carried the famous ad line of “I’d rather fight than switch” that I thought was a perfect lead-in for discussing LAN switching (clever, right?)  To the uninitiated, the term switch conjures up a whole set of images, usually relating to electrical components and/or lights.  In terms of networking, switching was a quantum leap forward, especially in terms of network congestion and bandwidth.  Think of a hub just like a parking lot after a sporting event–there is only one exit, and everybody is crowding to get out of that one opening, and coming from all directions as well.  Not a pretty picture, especially if you have waited seemingly hours to get out…

Wouldn’t it be amazing if there were a whole block of separate exit points from that parking lot?  Imagine how much faster things would go and how much more smoothly would the lot empty.  That’s the essential idea behind switches…instead of all stations sharing a single network entry point, separate data channels are created for all of the attached devices.  Sounds almost magical, but there is a rather simple logic that makes it all work, and it all based at Layer 2…the data-link layer of the OSI model.  Hubs just took electrical signals and retransmitted it out all ports because it had no way of distinguishing traffic.  Switches are different because they are examining the MAC (hardware) addresses of the frames passing through their ports.  If the frame is one it has never seen before, or a broadcast (ffff.ffff.ffff), it sends it out all ports except the one it arrived on (termed flooding).  The switch then records the source address contained in the frame, as well as the interface it came in on, and when a frame destined for that address arrives again, it send it out the port contained in the table entry (termed forwarding).  If it arrives on the same port in the table, it simply drops the frame (termed filtering).  The reason for the term switch is simple, because it takes a frame from one port,and then switches it to another port and sends it on its way.

In the next blog, I will drill into the specific types of switches in the Cisco product line and how they are best used in real-world settings.

– Joe

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