Network switches are essential layer 2 devices that interconnect end devices by learning MAC addresses and forwarding traffic only to intended recipients. Understanding how switches evolved from hubs and bridges helps explain their role in modern network design.
Network Switches
One of the most critical intermediary devices that we have is the network switch. The network switch interconnects our end devices, and it's critical for communication.
The thing is, you have to realize we've evolved over time. At one point in time, hubs were the way that we set up networks; that's just what we had available to us. Then we started running into these collision problems, so we created bridges to reduce the amount of collisions we had on the network. And the more bridges that we could incorporate into our network, the fewer collisions we would have. So we break things apart into these smaller segments.
Over time we came up with a multiport bridge. What that looked like is that we had our hubs sitting out here on the edge of our network, servicing all of our access ports to our end devices, but then at the center we had this multiport bridge. Essentially, that's what became our switching. The reason why we set it up this way is because hubs were a lot cheaper, and when switches and these multiport bridges first came out, they were very expensive. So this is how we would set it up with the hubs, and then in the center of it all we would have a bridge.
Over time, switches became less and less expensive. So rather than having a central switch and then hubs on the outside, we just started replacing all the hubs with switches, and that's what we use nowadays. We don't really use hubs all that often. The concept is still the same, though.
We have a switch here, and the first job that a switch does is learning. It associates these different MAC addresses when information is coming in or being sent from these devices right here. It's sent with the source MAC address, so it records that source MAC address and the port it came in on. That way it can develop this MAC address table.
What can happen then is that when information comes into the switch, it will read that information, where the destination is, and it can send it directly to that device. This has two big advantages here. Number one is we don't get the collisions that we used to have. And number two is there's a lot less security concerns. You're not sending out traffic to all of the devices. So everybody can listen to what traffic is going across this network.
I thought I'd just show you what a MAC address table looks like. I'm on a Cisco switch right here and I typed in the command to show the MAC address table. As you can see here, there are several MAC addresses and the associated ports. I'm not going to get in depth into what all of this means, but I just wanted to show you that we have a MAC address and it's mapped to certain ports that are on this machine. So there you go, there's a MAC address table.
Here are some examples of some switches. Down here at the bottom, we've got a Cisco 3750 with lots of features. Here we have an old Extreme Networks switch, something that you'd also find in a business. And then we have a TRENDnet switch. This would be something that you would find in your home.
So what are some of the differences here? This expensive switch right here is packed full of features. It's managed, so you can actually get on it and manage it and do some configuration on it remotely. It also has the capabilities of doing VLANs and spanning tree protocol. It has power over Ethernet. It'll even do routing, so this is a layer 3 switch. It has lots of features right here, versus this one, which looks like it's a managed switch and probably has things like spanning tree protocol on it, but not as many features as this bottom one right here.
And then this one right here is just for home use. This one actually has the fastest speeds. Initially, when you look at the stats, this is a gig switch, so I can provide gig connections throughout my home. But it actually is not a true, it's not the same as this. Even though the line rates are smaller on this, this is 100 meg, and it provides some capabilities of expanding it to gig connections. But this one has true one gig or 100 meg between every single port on this switch right here, versus this gets shared with that gig. The back plane is only a gig, and so it gets shared amongst all of those ports. So you've got to kind of split that gig connection up amongst however many computers you have connected to it and communicating at that given time.
So what I would do is I would just take my equipment here. This is plugged into my laptop, and I'd plug it into here. And then this would then plug into my server, and then the two devices could talk with each other. And then I can plug in up to eight ports on here, and on this bottom one I have 48 ports. So that's another difference: how many ports are on each one of these devices.
We consider a switch a layer 2 device because it has to look at layer 2 for both the learning and the forwarding functions of this switch.
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