CompTIA Network+ N10-008 – Troubleshooting

1. Network Troubleshooting Methodology

Methodology. As a network technician, you will now spend the majority of your time answering trouble tickets or troubleshooting problems in your networks. Now there are three basic steps that are going to occur as you start going through this troubleshooting. The first is a problem report; the second is problem diagnosis; and the third is problem resolution.

Now when we look at the problem report, this is going to be an issue that has been reported to you either by the end user, an administrator, or through some automated system. For example, you may receive a call from the service desk informing you that the internet is down. Now you and I both know that the internet is probably not down, but that users’ connections to the internet may be down. They may have been trying to go to Facebook and gotten the error message here on the screen that says there’s no internet connection. As a result, the internet is unavailable to them. But we all know the internet is made up of many, many different networks, and the chances of the entire internet being down are pretty darn slim. Now what are you going to do about it? Well, this is where the problem report transitions into diagnosis, and we begin to look at what all of these symptoms are and what issues the customer is experiencing. In the case of my internet is down example, the symptomis really just that the user can’t get to Facebook.

Now there can be lots of different problems that are leading up to that, and the majority of your effort is going to be spent diagnosing the problem and figuring out where the problem is. You’re going to collect information, you’re going to examine that information, you’re going to eliminate potential causes, hypothesise an underlying cause, and then verify your hypothesis. So in my example of the internet being down, I might hypothesise that the cable was unplugged from the machine. And so if there’s no network cable plugged into your machine, you have a layer-one issue, and you’re not going to have any connectivity. Or maybe there are some thunderstorms in your area and your internet connection went down. Or if you rely on satellite internet, maybe it’s snowing outside, there’s lots of different problems.

However, as I proceed, based on the information I collect from the user, I can begin to consider what might be the problem and then begin looking at those and verifying if they are. That will bring us to our third step in problem resolution. Now we’ve figured out what we think the problem is. Now we’re going to fix it. And once we fix it, our work isn’t done. We need to document it in the trouble ticket system, stating the cause of the outage, and we need to confirm with the user that they’re satisfied, because if I said, “I replugged in the cable,” and then walked away and said, “You’re good now,” but the user still can’t access Facebook, we need to document it. Well, guess what? In the user’s mind, it’s not solved. And so you want to make sure you follow through all the way through the problem to ensure your customer is happy and gives you that five-star review that you’re ready and done with the problem. Now, this is the basic troubleshooting method that we just went through.

But for the exam, you need to use a structured approach to troubleshooting. Now, in the real world, why would we want to use a structured approach? Well, using a structured approach is going to save us time, and it’s repeatable. Sometimes these problems aren’t going to be solved by a single technician. Instead, I might start fixing it on my shift, and then when I go home, John, my relief, is going to take over, and he’s going to start fixing that problem. We can all operate in the same way if we use a standard method to fix this, and there will be much less downtime. Now this is also going to prevent you, the technician, from doing what we call “hunting and pecking” for a solution, which is just randomly guessing. Instead, you start doing things logically and you start thinking about the fact that if the Internet is down, that can be anything from a layer one problem to a layer seven problem.

And so as we start figuring out exactly where that problem is, we can start digging in a little bit deeper and start going through a methodical method of doing it. Whether verifying that the router is on, the DSL modem is on, or that the cable is plugged in, all of these things are important. If we do it the same way every time, it makes it very easy for someone to come behind us and take over. Now, there are many different approaches to troubleshooting. For example, if the Internet is down, you might go and do a ping. Now, would you ping your local interface first, or would you ping the distant end of Facebook first? Well, in the CompTIA Network Plus books, they tend to tell you that you should ping your loopback address first, then your own IP address, then your router’s IP address, and then the distant end. Personally, in the real world, I tend to do the opposite. The first thing I do is ping Facebook, and if I can reach Facebook, the problem is solved. If not, I go to my router. And if that works, I know that everything is fine between me and my router. And now I’ve realised that it’s a weak connection, right? And for me, that’s a quicker method. But for the exam, I want you to use CompTIA’s method. Now, what is their seven-step troubleshooting method? Well, their method looks like this. You need to define the problem.

Then you’ll investigate the possible source of the problem. Next, you’re going to test that guess. After that, you’re going to create an action plan, and then you’re going to implement the action plan, and then you’re going to verify the problem has been resolved, and then you’re going to create a post mortem report, which is what I was talking about when you document this in your trouble ticket system. Now, you’ll notice I have this little arrow between steps three and two. If I guess that the problem is A, and I test that A is the problem, and it turns out that A isn’t the problem, then I go back to step two and create B, which is now the problem, or C or D until I figure out what that problem is. Now, these seven steps really do fit nicely into the three steps that I showed you before. But the three steps are just a lot simpler. For the exam, though, you do need to memorise these seven steps and the order they’re in. You may get a drag-and-drop that says, “Put these steps in order,” or you may get asked, “After you have taken the steps necessary to test your hypothesis, what is your next step?” And the answer would be to create an action plan. So I want you to make sure you understand these steps and remember them for the exam.

2. Troubleshooting (Layer 1)

Troubleshooting layer one Now, in the last lesson, we talked about these seven steps of the CompTIA Network+ troubleshooting method. And we also mentioned the fact that, as a network administrator or technician, resolving network issues is one of your main priorities. There are numerous issues that can arise in your networks today. But if we use the OSI model as our guide, we can start with layer one, then layer two, then layer three. And then on the exam, they’ll also cover what we call “wireless networking,” which is just kind of a mixture of the different layers because with wireless layer 1, 2, and 3, they tend to blend together a lot. Now, as we go through this in this lecture, we’re going to focus on the physical layer, or layer 1. Nothing else will work if the physical layer is not functioning properly. There are many different common issues here. It can be something like a bad cable or connector. The cable placement is wrong. Your distance limitations have been exceeded. You’ve split pairs in a cable, you’ve had EMI interference or crosstalk, or you’ve transposed transmit and receive.

We’re going to look at each of those and see what we can do to solve them in this lesson. Now the first one is bad cables or connectors, and this again is a physical layer problem. This can be because you have a faulty cable or connector. You’ve used the wrong category of cable for the purpose. For example, if you’re trying to push 4K video over an old Category 3 cable, it’s just not going to work. It doesn’t have the bandwidth for it. You need to be using Cat 6 or Cat 7 or something of that nature. Now look at these two cables on the screen. On the one on the left, the tip has been completely ripped off, and that could be because somebody has abused this cable by having it get caught in a cabinet drawer or run over it with their chair. The one on the right has really been abused. It looks like it’s been melted, almost with fire or something else like that. One of the places I’ve seen a lot of bagged cables and connectors is when people have cables lying across the floor. Most of us in our offices have chairs with wheels on them, and people will roll over those cables, which will cause them to break and fray. One of the things I look for when I first arrive on the scene of a network problem is the cabling, because if the cable has been abused or destroyed in some way, the rest of it’s not going to work either.

And it’s really easy to change out a patch cable between the wall jack and your desktop computer. Now, when you start having problems inside the walls, that gets a little more difficult and a little more expensive to have to repair. For instance, in one of the buildings I worked in, we had a rat problem. And the rats kept chewing through my fiberoptic cables because they liked the sheathing on them to make their nests out of. and that became a big issue. And we could replace the fibre all day long, but the rats would keep coming back until we got the pest control issue under control. And so you want to make sure you figure out what the root cause of this is. Is someone wheeling over it with their chair? Well, maybe you need one of those plastic strips or you need to run that cable differently so that doesn’t keep happening. If rats are eating your cables, get a pest control sergeant out there and get rid of those rats. Next, we have cable placement. This can be an issue as well.

Many people like to use the same jack box for their networks and to keep it all conveniently located. Well, that’s actually a bad idea because if your cable is too close to high-voltage cables, generators,  motors, or radio transmitters, it can cause interference and drop the signal for you. So if you have something that has a power outlet, you want to have a couple of feet between that and your network outlet. Now, this diagram at the bottom is actually my home office. A couple of years ago, we built our house, and as you can see here in red, I’ve circled the two power outlets on the wall where my computer is, and in the centre of the blue circle, you can see my data outlet. Notice that they’re about 3 feet apart. That’s going to ensure that I don’t have cable placement issues in the future.

If I put them all on the same strip and had the blue and the red next to each other, I could have crosstalk and interference, which can cause issues for my data networks. Next, we have the distance limits being exceeded. If you remember back in Ethernet, we talked about how categories three, five, five E, six, six A, and seven all have a limitation because they’re copper cables of about 100 meters. What I would recommend is that you don’t go over about 80 metres with your cabling. That will make sure that your cable is always going to work at peak efficiency. If you exceed those distance limitations, you’re going to have problems because the signal does attenuate over time, which is going to cause intermittent drops for you. Now, if you need to run longer distances, that’s when you want to start thinking about using fibre for those applications. And you can convert your copper networks into fibre networks using things like media converters.

So I can go from a cat-5 switch to a media converter, then back into a media converter that brings it back into a cat-5 copper network over a long distance of fiber. Now, if you’re running high-bandwidth applications, such as a video system that supports 4K video over Ethernet, You want to make sure you’re keeping that distance small, something around 70 meters. That’s why I say if you’re using 70 or 80 metres as your standard of design, you’re going to be future-proof in the long run. Then there’s pair splitting, and some technicians are slackers. I know shocker people are lazy sometimes. So when you run Ethernet, you’re only using two of the four pairs of wires in there. So we all know there are eight wires twisted into four pairs each. Now when we use them, we’re using pins 1, 2, 3, and 6. The other eight pins aren’t used. Well, what some technicians will do is actually split the pair and use those other four pins to make a second jack out of them. Now this seems like a great idea. You don’t have to run another cable, you don’t have to fish it through the ceiling or through the walls, and it’s a really quick way to get a second jack up. The problem is that all of those cable pairs are so close together that you’re going to have a lot of interference if you use splitting pairs. So I do not recommend using splitting pairs. And if you come across them in your network, you want to replace them with dedicated cables for each of those jacks. This, when you split up pairs, is going to lead to nonstandard wiring of the jack and a lot of EMI and crosstalk. Notice the theme here. When we’re dealing with physical, it’s either EMI, crosstalk, or a destroyed cable.

So try to prevent both of those things from happening. Next, we have EMI interference and crosstalk. And we’ve been saying over and over that all these things can cause this. Well, there are a lot of things that cause EMI and crosstalk. If you have split pairs, there’s going to be crosstalk between those wires because those internal wires are not shielded well. Crosstalk can occur if cables are run over a fluorescent light and placed on the same cable tray as other electric high-voltage wires. Crosstalk can happen inside the wire bundles themselves. For instance, as you see on the bottom left here, I have a really big network cable that has 50 or 100 pairs of wires. That could be something I use in my networks to connect one large patch panel to another. Well, if I don’t shield them properly and I run too many pairs through the same network, that’s going to give me crosstalk.

This was very common when running cat-3 networks because they had lower bandwidth, so crosstalk wasn’t as big of an issue, and it was also heavily used in phone networks. However, as we’ve progressed into 5, 6, and 7 networks, using these large bundles without shielding is no longer a good idea. So if you’re trying to eliminate crosstalk, make sure you’re using shielded wires, especially if you have to run everything in the same tray, as you see here on the right. Next we have the transpose, transmit, and receive, and this is our final physical layer issue. Now there are two issues here with transposition. The first is when you’re dealing with older switches that don’t support MDIX. If you remember, MDIX is the medium-dependent interface crossover, and this allows the switch port to automatically configure itself as either a crossover or a normal port. Now, if you don’t have that and you use an Ethernet cable to go through, that’s going to cause issues at layer one and layer two of your network. Older switches don’t support MDIX, and you have to use a crossover cable that will make your transmit talk to your receive and your receive talk to your transmit. If you don’t use a crossover cable, you’re sending your transmit to transmit and your receive to receive.

And that’s not going to be good for network communications. Now the second place I’ve seen transposition happen is when you’re dealing with fibre optic cables. Now, if you look at the fibre cable on the right, this is an ST connector. One of those is used for transmission, and the other is used for reception. Now, if you plug them in backwards into this media converter, transmit to transmit and receive to receive, that’s also a transposition. So if you’re ever dealing with fibre networks, one of the first things I try is checking that my transmit is plugged into my transmit and my receive is plugged into my receive. To aid in this, the cables are typically color-coded with red and black. Now that’s to help you, the technician. But not all organisations use a standard colour convention. One organization, for example, used red for transmit and black for receive. Another method was to use red for receiving and black for transmitting. So those organisations work backwards. But by having that color, you can check what’s on the switch, and then you can plug it in appropriately to your network card or media converter.

3. Troubleshooting (Layer 2)

Troubleshooting layer two Now as we move up the OSI model, we get to the data link layer, which is really focused on things like switches and bridges, and we have to understand those layer-two operations because they’re critical to troubleshooting a lot of our local area network issues. We’ll start looking at Wan and routing issues once we get to layer three and beyond. But for right now, we’re focused internally on our network work.

Some common issues we can have are things like bad modules, layer-two loops, port misconfigurations, and VLAN configuration issues. So the first issue we’re going to look at is a bad module. Now what is a bad module? Well, it’s a module that’s gone bad. I know that’s silly, but we’re talking about things like our GBICS. We’re talking about our SPFs and our SPF pluses now. When we put these into our switches to be able to allow fibre or copper to be installed to them for our uplinks, when we swap the modules in or out, we can damage those connectors, and if they’re damaged, nothing’s going to transmit over them. So when you have a bad module, the simplest solution is to buy a new one and replace it. Use your electrostatic strap to ensure you don’t short out the card because it contains sensitive circuitry.

Next we have our layer two loops, which we talked about back in our spanning tree lecture. So if you don’t remember this one, make sure you go back and review the spanning tree protocol lecture earlier in this course. But if your spanning tree protocol fails, you’re going to start getting a broadcast storm, and you’re going to have this loop of traffic that’s just going to take down your switches because it’s going to overwhelm them. If your spanning tree protocol is misconfigured or you haven’t configured it, your traffic can start looping, taking suboptimal paths, and causing significant network degradation, even to the point of taking your switches offline. So if you have this issue, make sure you check your spanning tree protocol configurations. Next, we have port misconfigurations.

Now you might have something that is working but not well, and what I mean by that is that you might be connected to the network, but you’re only seeing speeds of ten megabits per second or 50 megabits per second when you should be seeing 100 or 1000. This is usually due to a mismatch in speed duplex or an incorrect MDIX setting on a switch, port, or the workstation itself. Now if you go and check your Windows settings, you can go in under your device properties, click on Link speed and duplex as I’ve done here on the screen, and you’ll see that you can set it to auto negotiation. Sometimes auto negotiation doesn’t work, so instead you’d want to make sure you’re setting it to 1000 full, 100 full, 100 half, or whatever your switch is going to support. If you’re using a gigabit switch, set it to 1000 full. And so if you’re seeing poor performance, you can go in and manually set your device to do this as opposed to letting it do the auto negotiation.

Next. We have VLAN configuration. And when you deal with VLANs, you have to remember that all traffic has to be routed between the VLANs. And so if you set the purple VLAN and the blue VLAN here to different VLANs, you need to make sure they’re being routed through that router to connect between the two. If you don’t, traffic is not going to leave the subnet. And so in this case, we’d have to be using layer three switches or configure it to push it down to the router and back to the other switch. Now, if you start having devices that cannot communicate with each other, like one of the IT devices and one of the HR devices here in the diagram, it’s probably a VLAN issue. They’re each on a separate VLAN, and there’s no routing of traffic going on. So make sure you check that as your configuration because it will be the most common source of problems when dealing with VLANs.

4. Troubleshooting (Layer 3)

Troubleshooting layer three So now we’re going to be concerned with routing. When we start dealing with the network layer, we’re going to have to understand routing and logical IP addressing because that’s where a lot of our issues are going to reside. When dealing with LAN Town issues, it is possible that routing, subletting, and other services such as DNS and DHCP are causing you problems. Duplicate IP addresses, where you have a duplicate logical address, and incorrect default gateways, where you can’t find your router or the way off the network, are two common issues. incorrect DNS, where you can’t make names and numbers translate between each other, or mismatched MTUs, which are maximum transmission unit sizes. We’ll talk about that here in this lecture as well.

Now, the first one is a duplicate IP address. This is when two hosts on your network have the same IP address assigned. and this can cause some unusual behavior in your network. This could be because you’ve been using static IP addresses and you haven’t kept track of your IPS. Or it could be a problem with your DHCP server, where it’s handing out the same IP twice. Generally, when you have duplicate IP addresses, it’s either a static issue or somebody trying to jump onto your network and spoof somebody else’s IP. How will you know this is happening? Well, on your workstation, if you’re using Windows, you’re going to get the error message shown here at the top of the screen saying that it’s detected an IP address conflict. If you’re using a Macintosh machine, you’re going to get the message that’s shown here on the bottom of the screen, which is a very similar message. Also, you may lose network connectivity on and off and experience other strange things like that.

The best thing to do is check your DHCP scopes and check your client to figure out what address it has and if that was a free address in the first place. Next, we have the incorrect default gateway. This is when a host is unable to communicate outside of its own local subnet. So in the example here on the screen, I have two of them on top, and you’ll see that I have an IP address assigned of 192 168 1200.and look at its router. Its default gateway is 100 0 1. Are those on the same subnet? Well, of course not. And so that’s going to cause it a problem where it cannot leave the subnet because it can’t even find that router to be able to leave the subnet. Now, let’s look at the bottom one. This one’s a little bit trickier. Take note of my IP address, which is 100 1200. My subnet mask is 255-255-2550, which means that my router should be somewhere on the 100-something subnet. But it’s not. It’s on line 100, zero, one. So, once again, I won’t be able to leave the network because my 200 machine can’t find that one machine because they’re on two different networks, and I’d have to go through a router first. So how do I solve this problem? Well, in the case of the first example, we would want to change that router to the one on that same subnet.

Something like 192, 168, dot one, dot one. If there was a router there. In the bottom example, it’s just a simple misconfiguration. Here we typed in “ten,” “dot zero zero,” and “one,” when we probably meant to type “one.” By changing these two IPS to the correct routers, you’ll solve this incorrect default gateway and get that person back online quickly. Now the next one we have is incorrect DNS configuration. How do you determine this? Well, if the person isn’t able to browse to something like Facebook or Google using a domain name, but they can do it using an IP address, that means your network is up and your DNS is just down or misconfigured. So you want to check your DNS server. If your DNS server is up, then you should be able to get this working. Now the way to do this is to run Ipconfig All, or ipconfig, depending on if you’re on a Windows or Linux machine. In Windows, you do Ipconfig All, and you’ll get a screen like this, and you can see what your DNS server is. In this case, my DNS server is set to eight.

Now, if you’re not familiar with that address, I would recommend you remember it, not for the exam but because it’s really helpful in troubleshooting. If you’re ever having DNS issues, you can set your DNS server to 8: 8: 8, which is the public DNS that’s hosted by Google. It is always up and online, and it will solve a lot of your DNS issues if you start pointing your DNS there as opposed to a local server that you may be running yourself. And lastly, we have mismatched MTUs, or Maximum Transmission Units. Now, an MTU is going to define the largest packet size that the router is going to forward on for you. If your MTU is too small and it gets something that’s larger than that, it simply drops the packet. The default for most routers is 1500 bytes, but you can configure that if you’d like to. For example, you can see my CiscoASA router in the bottom right. It allows you to configure it anywhere from as little as 64 bytes all the way up to 65,535 bytes. Anything over a 1500-byte frame is what we refer to as a “jumbo frame.” Now, this is a common issue if you start doing things like VPN tunneling, because if you already have a large packet like 1400, 1450, or almost 1500 and then you wrap it inside another wrapper by adding an additional header that makes the size go above that 1500, you can start seeing drops in your packets through the router. To fix that, go check what your MTU is, and you can increase the MTU size. Or you could set your VPN to fragment down to lower sizes so it won’t have this issue.

5. Troubleshooting (Wireless)

Troubleshooting wireless. Now in this final deep dive into troubleshooting, I want to talk about some issues that are specific to wireless networks. Now, when you deal with wireless troubleshooting, this can require a variety of skill sets. There’s layer one issues like radiofrequency, there’s layer two and layer three issues like switching and routing.

All of these issues can happen simultaneously. Layer one, two, and three issues are all possible. You can have radio frequency issues, placement of access points and coverage issues, and signal strength issues. All of these are different problems that you can experience when dealing with wireless networks. Now, the first of those is radio frequency interference. Radio frequency issues, or RFI, are going to come from things like cordless phones, baby monitors, microwaves, and even other wireless networks in your area. Because we’re all competing for the same radio frequencies in the same local area, we start interfering with each other’s signals, which causes problems for us and could result in drops from the network.

How do you solve this problem? Well, you want to pick an unused channel in the 2.4 GHz spectrum if you’re using something like wireless BG or N, if possible. You’d be better off upgrading your network to something like N for the 5 GHz spectrum or AC because both of those operate in the 5 GHz spectrum, which is much less crowded and therefore there’s less interference. And finally, you want to determine the best placement for your access points as well. If you’re putting your access point next to the microwave, it’s going to drop every time the microwave goes on because it’s a 2.4 GHz signal as well. So the best way to figure out what is in your area is to use a wireless sniffer, as shown here on the screen. This will allow you to determine what frequencies are in use in your vicinity and see what might be causing interference for your wireless networks. Next, we have the misconfiguration of devices.

Now, there are a lot of different settings that you have to use to make your wireless network connect properly. You need to have the right SSID or server-set identification; you need to have the right channel; you need to have the right encryption. All of these things work together to give you access to the network. So in the example here on the screen, you can see my network at home. It’s called deon. I’m using WPA2 encryption, the personal version, which uses a pre-shared key. If I don’t have the right name, the right encryption method, and the right password, I’m not going to be able to connect. Now, in my case, you can also look here and see all of the different settings. You could see that I had to get the right IP address, the right gateway, or router. I needed to make sure that I had the right encryption and the right channel number. In my case, 36 are operating on the 5 GHz spectrum. And then you can see my RSSI, which is my signal-to-noise ratio. And we’ll talk about what you do if you have a weak signal later.

And then you can see the noise, which is very low. It’s about half as low as my signal, giving me a nice, strong signal to my access point. And then you can see how fast I can transmit. You can see I’m using wireless AC and all of this information, but each and every one of these settings could be misconfigured, and any of them can give you a bad day when you’re trying to connect to wireless. Now the next one we have is: where do you place your access points? We’ve talked about this before as well, but I want you to remember that for the 2.4 GHz spectrum, you should be using channels 1, 6, and 11 because they provide the best distance from each other and the least amount of radio frequency interference. When you place them, they should have a 10% to 15% overlap coverage so that calls don’t drop as you move from one zone to the next. If you’re dealing with 5 GHz, though, you need at least two cells between each of the same channel numbers, with the two 4s needing one cell of difference. So notice that none of my ones are touching each other, none of my sixes are touching each other, and none of my elevens are touching each other in this network diagram. Now, the last thing we want to talk about here is weak signal strength. What do you do if you’re in a foggy environment and your signal is too low? Well, I showed you before that we have this thing called the RSSI, which is our received signal strength indicator. In my network, I was currently attached at -56 decibels, and my noise floor was -96, which meant I had plenty of signal over the noise because the lower number here is better. Now if I started putting metal between me or other physical objects between me, or if I moved further away from the antenna or the access point, that signal could drop. So how can I boost that signal?

Well, there are three main ways of doing it. One is using a signal booster, which you can see in the middle of the screen here. It receives the signal on one antenna, amplifies it, and rebroadcasts it out. The second thing you can do is use an ESS mode or an extended server set and use additional wireless access points. On the left here, you can see the circle access point called “ambiguous.” That’s what I use in my network. We have three of those to COVID our building and ensure that we have a good, strong signal no matter where we are in the building. And finally, we have the external antenna. If you have an external card, like I have my Alpha Networks card here, you can actually switch out that five-decibel antenna for a nine-decibel antenna, getting you almost twice the gain and allowing you to pick up signals from further away. You can also swap out the antennas on your access point if you have screw-on or scroll-able antennas.

6. Troubleshooting Problems

Troubleshooting problems. So in the last couple of lectures, we talked about the methodology for troubleshooting. And then we went through layers one,  two, and three and wireless networking issues. Now, in this lesson, I’m going to give you a couple of problems to test your knowledge. After I present the problem, pause the video, try to come up with the solution on your own, and then restart the video and see if you got the right answer. The first problem I have is that I have a PC here that can’t talk to a server one.

They’re both on the same network because they’re both to the left of the router. Notice that I have a Cat 5e patch cable going between my PC and the switch. And I have Cat 6 patch cables between switch one, switch to the server, and router one. But whenever a PC tries to connect to the server, it gets an error. Why can’t the client reach the server? Go ahead and pause the video here and come up with your solution. Welcome back. So what did you come up with? Let’s take a look at this problem and see where the issue might lie. Now, one of the key words in this problem is that those switches are old and don’t support MDIX. Do you remember what MDIX did? That’s right. It eliminates the need for a crossover cable. But because these switches don’t support MDIX, we have to have a crossover cable between switch one and switch two.

And that’s the issue here. So to solve this problem, I need to replace that patch cable with a crossover cable. Whenever you hear the word “patch cable,” remember that a patch cable is a straight through cable. When you’re dealing with old switches, you need to use a crossover cable anytime you connect two things that are the same, like a PC to a PC, a switch to a switch, or a router to a router; you have to use a crossover cable or they have to support MDIX. Let’s look at the second problem. My second problem is the same type of network. But now that I’ve replaced that cable with a fibre patch cable, I can do a crossover. PC one is still unable to reach server one, though you verify there is communication between switch one and switch two. Now, because we replaced that fibre cable and switched the transmit and receive, Now, why can’t PC one talk to server one? Go ahead and pause the video and come back when you’re ready to answer the question. All right, so let’s take a look at what is causing the problem. Well, notice in this one we have three VLANs. We have VLAN 100, VLAN 200, and VLAN 300.

So what do you think the problem is? That’s right, the VLANs aren’t being routed. Anytime you have different VLANs, you have to have routers there. And since this is a logical diagram, we should have some method to show that there is a logical router. Either these are multilayer switches like Layer 3 or they are routing the traffic through the router. In this case, we have neither of those. And so our problem is that VLAN 100 only gets to switch one, VLAN 300 only gets to switch two, and VLAN 200 only goes between switches one and two. And because we have three different VLANs, we have to have routers to route that traffic. All right, now we have our third problem. This one is going to be a layer-three issue. So PC One still can’t reach server 1. We verified that we have communication between switches one and two, but we still can’t get all the way from PC One down to Server One. What could be the problem this time? I’m going to go ahead and pause the video and let you think about this. All right, you’re back. Are you ready to give me the answer? Let’s see if you get this one. This one was a little bit tricky and a little bit hard. Notice I have three IP addresses on the screen. I have on the router one interface: 100 165 26.

Then there’s PC 1100, 162 26. And server one is at 100 167 26. Now notice the special thing here: that cider notation. They are all Slash 26 subnets. But are they on the same subnet? Well, maybe. Let’s take a look at it. What are your Slash 26 networks? As long as you were within the 256-scope and had a slash 24, all of these machines would be in the same network. But because it’s a 26, we’re actually splitting up that network into four subnets. Those four subnets are 100. So the problem is that all three of these IPS are not in the same subnet. The 62 is in one subnet, and the dots 65 and 67 are in the other. And you can see that here illustrated with PC-1 being 62 in blue and the other two being 65 and 67 in red. How are we going to solve this problem? Well, we’re going to have to move PC One onto the second subnet by assigning it an address of something over 64 and less than 128. So in this case, our router is 65. If 66 or 68 were available, we could use that and give it a new IP of 100 168 26. And now all three will be on the same subnet, and they’ll be able to communicate with each other. Our fourth problem is a wireless issue. I have a two-story building with one access point on each floor. They do have a proper overlap of about 15% of coverage, but users keep complaining that their WiFi service is dropping.

What do you think is the design flaw? Go ahead and pause the video, and then we’ll come back and explain it. Welcome back. All right, so have you figured out the design flaw? It’s not a placement issue here because we do have proper coverage and we’re covering the entire floor. Instead, it has to do with the configuration. Let’s look at access point one. It’s 8211 N, operating in the 2.4 GHz spectrum. It’s operating on channel 1. It’s using WPA 2 encryption, and its SSID is Office. When I look at access.2, its type matches; it’s 800 and 211 N. Its band matches its 2.4 GHz frequency. Its encryption matches; it’s WPA II, and its SSID matches, which is Office. But its channel is Channel Five. Now, why is that a problem? If you remember back to our frequencies in WiFilecture, you remember that we said channels 1, 6, and 11 gave you the widest distribution of channels and the least amount of interference. If you use something like one in five or one in four, that would be too close together in the channels. And that can cause RFI, or radio frequency interference, and cause drops for your users. In this case, that’s the problem. Because we’re using channels one and five in a small overlapping space, we’re getting RFI issues. And so to solve this, we would change channel five to channel six or to channel eleven, and that would solve our issues.