Connecting Ethernet scenarios

The current exam has a lot of questions that talk about different types of scenarios. So in this episode, I want to concentrate on the different situations we can run into when we're connecting ethernet, in particular, the many ways we can mess up when it comes to connecting lots and lots of switches. So let's go ahead and get started in a few scenarios. I'm thinking like the first one might be something that has to do with loops. All right, now I've got a pretty standard setup here. What I've got is I've got a router with a built-in switch. It's going out to the cloud. Now you guys watching this video don't see it, but the people in my production think my clouds are so terrible, they're literally giggling at me. It's a cloud, trust me. So this is going out to the internet, but since we're talking about loop scenarios, I really want to concentrate over here. So what I've got is I've got two switches that are connecting to a switch. So this switch could be in my main distribution frame. This will be connected to my customer premise equipment, whatever it might be. But these guys are going to say maybe different floors, it doesn't really matter. So this setup's pretty straightforward. All these individual connections are going out to individual computers. Now let's just say I add a little smaller switch here. Now this switch is, maybe I'm just using it for my own IT uses or something. Now I can go ahead and connect that. And the beautiful part about switches is you can connect in pretty much any way you want with one big exception. What I would need to avoid is creating a loop like this. What I do when I create something like this, I create what's known as a bridging loop. And without anything else to protect us, data will start zooming around infinitely, and it will literally take down the entire broadcast domain. So this is something we want to avoid. Now luckily for us, many, many years ago, a protocol called Spanning Tree Protocol or STP was developed expressively to avoid these types of bridging loops. In most switches, it's simply a matter of just turning it on. In fact, in a lot of managed switches, you can't even really turn it off easily. And what will happen in these scenarios is that before a bridge comes along, one of these switches, usually this switch, will become what's known as the root switch. This is all done automatically. All we have to do is connect the switches. And as long as they're all STP, this takes place without any interference from us whatsoever. So he'll call himself the root switch, and will then watch any type of data that goes through in any type of situation. And what'll take place is that a bridge loop is a pretty obvious thing. You have a phenomenal amount of data that's pouring through, and what would take place, well, let me reconnect that. So what'll take place is that the root switch will then automatically, without any interference from us, simply turn off one of its ports. So it's actually pretty cool the way STP works. And if you look at this scenario carefully, even with this turned off, the switches are still properly connected, and all the switches keep on working. So STP is the way we avoid bridging loops. Now, the one problem we run into is that there is a very similar type of functionality. It's not really looping functionality, but it comes in at about this level. So I want to mention it right now. So let's just say I've got some evil box on my network right here. Now this evil box has a problem, and the problem is is that he is doing naughty things to my broadcast domain. This naughty box is sending out spurious and evil information. He is using MAC address information to create, in essence, denial of service situations, he's flooding my network with all kinds of evil stuff, and this is usually done for denial of service or man in the middle type of attacks. Now also keep in mind, this is a layer two attack. This isn't something that's being happened by some guy way overseas. This type of attack requires a physical connection to your local area network. So it's particularly nefarious. Anyway, in these types of situations, we create what's known as a flood. So a flood is a bad thing that happens within our networks. What we need are smart switches that provide a type of flood guard to prevent this type of thing. Now, these types of controls are very, very similar, at least in terms of function as Spanning Tree Protocol because their main thing that they do is turn off ports. So in this type of scenario, if we have smart enough switches, they can actually detect incoming MAC address flooding, other types of denial of service attacks, and simply turn the port off. So while protection from bridging loops and protection from floods are in essence two very different technologies, the reality is is they both work the same way, they turn off ports until somebody can come in and turn it back on once the system is back up and running normally. CompTIA cracks me up with some of the types of scenarios they bring up. In one type of scenario, it really wouldn't happen anymore, but since it's on the exam, we're going to go ahead and cover it. Let's take a look at a situation where you're trying to take a very old switch and plug it into a very new network. Here's a situation where I've got a, we'll say this guy on the left is a very new switch. It's a 10 gigabit ethernet switch. Now the downside to this switch, and this is fairly rare, but in this particular switch, the only thing it can do is 10 gigabit ethernet. That's it, it can't do anything else. Most switches today are auto-sensing. So if I have a gigabit ethernet switch and I plug a 10 base T switch into it, in most cases, that high speed switch will slow down whenever it's talking to the slower switch to take care of the speed differences. But there are situations where this can come into play. For example, in this particular example, I've got a very old 100 base T switch, and in this particular case, I mean they're all RJ45 connections, and I just want to plug them in. So as we would normally do, we're going to plug them in. In this particular situation, we have, this is just a regular switch, regular port to regular port. These are not auto-sensing ports, so we'd use a crossover cable in this particular scenario, and all of a sudden it doesn't work. What you're looking at here is a speed problem. This is a rare thing today, and with so many switches auto-sensing and auto-speeding, this is a rare scenario. However, for the exam, this is something you want to watch out for. This can be a very tricky problem to diagnose because most of the time when you have a speed mismatch, either absolutely nothing is going to happen, you won't even have a link light, that would be a interesting one. But the only other real way to tell is that the speed lights on these two different ports will show up as different speeds. In that particular case, there's nothing you can do, but you probably want to junk this old guy and get something a little bit more modern, okay? Here's the situation where I have four switches. Now in this particular case, two of these switches are regular 100 base T switches, no problem there. These two switches up here are also 100 base T, but there's a little bit of a trick in this case. If you look very, very carefully, you'll see each one of these has a dedicated one gigabit port. Now, dedicated high speed ports, while they're becoming less common today, you'll still see them on a lot of different types of switches. The trick is is being able to wire these properly. What you don't want to do in these scenarios is end up with speed differences that you don't want to take advantage of. Even if this one gig can go down to 100, if you're spending the money for a high speed port, you might as well be using it. So in a situation like this, what I would probably be doing is I would be connecting the two one gig ports on each of these switches and treating that as like a little mini backbone. And once I have that backbone, then I can go ahead and make interconnections between switches, something like this. Personally though, if I had a setup like this, what I would be doing is I'd get rid of these, I'd put in four of these switches with the one gig, and then have another, a fifth switch act as my backbone, that's a one gigabit switch, and plug all the one gigabit ports into that one backbone switch. Not because it wouldn't work, but because we want to take advantage of the speed that we have. We don't want to be wasteful of a good gigabit port. One thing I love about modern switches is the concept of auto-sensing is pretty much built into just about all switches today. If you connect two switches together, the days of worried about uplink ports and crossover cables has pretty much faded. However, this is a common question that CompTIA likes to ask. So let's go over a scenario where uplink ports and crossover cables might be the solution to a scenario. We've got a situation here where I've got four computers that are connected on a network. These two computers connected to this switch are able to communicate just fine. These two computers connected on this switch are also able to communicate just fine. However, they can't talk across the switches to each other, so these guys can't talk to these guys, and these guys can't talk to these guys. First of all, when you're running into network scenarios, not just on CompTIA exams, but in the real world, take advantage of big clues like this. Where would the problem be if these guys can talk to each other, but not to these guys? You should be automatically thinking about the interconnection between those. So not to digress too much, but a lot of times on these CompTIA exams, thinking about where the weak spot can be, where the fault can be is often the big first step to getting that answer right. Anyway, taking a look at this, in this particular case, I'm going to tell you that this is just a regular straight through cable that we've plugged in. In this particular situation, these switches don't like that. If you take a look, these are kind of older school switches that have uplink ports on it, in these types of situations, you're going to have to make some choices. If you run into a switch that still has an uplink port, you can take a straight through cable, go to any regular port to an uplink port. That's one way to do it. Secondly, you could go to the uplink port to a regular port here using a straight through cable. I'm going to use an uplink port if it's there for me. But if you don't want to use the uplink port, then just use a crossover cable. In that case, take from one regular port to another regular port with a crossover cable, and those two switches will start to talk. And yes, because somebody always asks this question, Mike, could I take a crossover cable and run from uplink port to uplink port? You can, but if I ever see it, I will tease you mercilessly. The great thing about modern ethernet switches is that everything today is full duplex. When you are working with these switches, whenever you have an individual computer, you just plug it in and it will auto-negotiate the best possible speed it can get, and pretty much all the best speeds today will go full duplex. However, there's one situation left where you might run into what we call duplex problems, and to show you that, we don't even need a switch. Here's a simple situation where I've got two computers. This actually happened to me not that long ago. I was on a flight and I wanted to talk to my buddy's computer who was sitting right next to me and he's like, "Well how can we network these things? We're at 36,000 feet." And I said, "Not a problem." And I pulled out a piece of cable, but it wasn't just a regular piece of cable, it was a crossover cable. What I did is I connected the two computers together, no switch, no nothing directly together with a crossover cable knowing that it would work, but it didn't work. The problem I ran into, and this happens a lot these days with windows 10 and such, is that these guys will try to negotiate the best possible speed, which is always going to be full duplex. And here's one situation where we're actually going to intentionally change the duplex from full to a half duplex. In this one scenario, you need to go into Device Manager, find your network card, click on its properties, and scroll through. And instead of just letting it go to auto-negotiate, put it on half duplex. By putting it on half duplex, each computer will automatically show up in the other computer's network neighborhood. Pretty neat feature.