Hi and welcome to this CertificationKits CCNA training video on troubleshooting router connectivity. We are going to be talking about some interface show commands that you can do to check out the status of the interface. What the status of the interface actually means will be a few different things you will see as far as the interface status. What the clock rate command does and when it’s necessary, some IP naming commands that allow for name resolution from the router and then the ultimate test of connectivity, your ping and trace commands.
I’ve brought up a CCNA slide that shows this scenario I’ve set up in our CCNA sim. We’ve got two routers, Palestra 1 and Palestra 2 connected back to back with a serial cable. So just one little back to back connections simulating the way I’m link. What we are going to do is we are going to go in and look at the routers and see how we would test and verify connectivity between the two of them. Here is Palestra 1 in the CCNA simulator and we are going to g in and take a look at the current situation with some of our show commands. Now the first show command that I love is show running-config. Running config allows me to go in and see how the router is currently configured and I’m worried about interface serial zero because that’s the one that has a direct connection to Palestra 2. Right now it is basically not configured and I can see that the shut down command is administered on the interface; which is a default configuration. So at the bare minimum to get the interface up and up which is the goal I’m after to get it functioning, I have to enter the no shut down command on that interface.
There is one other command that we can do that will allow us to go in and look at the status of serial zero and see what the interface is actually doing. Show interface command and I can just do show interface and hit enter and it will show me all the interfaces on the device and it shows me the status of the interfaces as well. Right now it says serial zero is administratively down, line protocol is down. Now what it is, is basically for the serial interface, this first part refers to layer one like if the cable were unplugged, it is down and things like that. The second part refers to layer two like layer two encapsulation. Again, layer two referring to the OSI model. If it says administratively down, that means the shut down command is administered on the interface so we’re going to have to go to the interface and turn it on by typing in no shut down. So I type go to global mode with configure terminal, type in interface serial zero which is the interface I’m turning on and type no shut down to turn the interface on. If I want to view it, I can do a show running config and make sure that I don’t see that shut down command here. Let me do that again because some syslog messages popped up in the middle of my configuration so it kind of made a little bit harder to see. Show run and right now I can see that the shut down command is gone and if I look at the interface, show interface.
Now maybe I don’t want to see all interfaces. I’m just worried about serial zero. I’ll just type in show interface as for serial zero, hit enter and it show me just serial zero. What this tells me right here, it says “Serial zero, zero” or serial zero is up, line protocol is down. Notice it does not say administratively down, it just say is up, line protocol is down and what this is telling me is that layer one has been turned on. It basically turned on the interface and there is a cable plugged into it. Now, if the router at the other end is down, this could also say is down right here instead of is up. If the cable were unplugged, it would say it is down so the only time that it’s going to say its up is that both routers are up; the cable is connected, everything along with layer one is good. Layer one being the physical layer of the OSI model.
The next step we want is to go in and get line protocol to be up. That would be layer two functioning correctly and so let’s go in and get that line protocol up. Here is the Palestra 2 router now in the CCNA simulator and what I’m going to do is I’m going to go check out everything on this end and see what I can do to get this line protocol up. I do show interface serial zero and that’s the interface that’s connected to Palestra 1. It says “Is up again physical layer” we know Palestra 1 is up, it’s not administratively down or anything like that but line protocol is down. There are two possible things that can make this line protocol down. One being the layer two encapsulation used, now for an Ethernet interface, it’s ARPA or Ethernet two, ARPA would be in the encapsulation. We can’t really change that on Ethernet interface, otherwise it wouldn’t be Ethernet anymore. But for serial interfaces, you have multiple different types of layer two encapsulations. Notice it says encapsulation HDLC. HDLC is the default layer to encapsulation for Cisco routers. Before we go any further, let’s take a look or a little bit closer look at some additional layer two encapsulations and a little bit more overview of what layer in tuned capsulation is just to clear up any question you might have.
On my slide here, I’ve put a few different encapsulation methods. HDLC stands for High Level Data Link Control Protocol. And again a protocol is a set of rules used to carry out a particular task. This is a task for encapsulating traffic at layer two of the OSI model. This is a Cisco proprietary protocol meaning if you are not Cisco you are not going to be using this protocol. Point to point protocol is a multi vender layer two encapsulation so if one router on one end is Cisco, the other on the other hand is Juniper or 3Com. A different type of router then we would use PPP because the 3Com router would not be able to work with HDLC at layer two. A couple of others would be Frame Relay and ATM. Frame relay we would use for a frame relay connection, ATM would be used for an ATM connection. PPP can go out over T1 lines, ISDN has a lot of flexibility in a very popular protocol, plus it has authentication. Let’s look a little bit closer at where these protocols are used just a little bit review of the OSI model.
The OSI model again is a seven-layer mode and it’s really broken down to the use of our protocols. There is an actual four-layer mode with four protocols used to get this to function. Now application layer in the four-layer mode would be an application protocol like FTP or something like that. Transport layer is where TCP functions and again that takes the package from let’s say the FTP application, takes a big packet and breaks it up into smaller packets called segments. Now each one of these segments has to be ordered and the information on how the put the segments back together has to go into the header of each segment so that information would go here on the header of those segments so when the receiving computer got all the segments, you could put them back together. IP at the network layer and this is actually called the internet layer if you are just using the four-layer mode instead of this seven-layer OSI model. This takes each one of those segments and makes it a packet by putting the IP information on it so it makes it a packet at this point.
Now layer two, this is layer two right here, different protocols are used at layer two depending on what type of line you are going out of like Ethernet two also known as ARPA. PPP could be used to package up this packet further. HDLC, again depending on the environment you are using will determine the layer two protocols. So the main differences between a T1 line, ISDN line, frame relay are all at layer two and below. What speed, what hardware, what layer two encapsulation. So HDLC just happens to be the encapsulation we are using when we have all Cisco equipment as the default so the key thing about this is if you have two devices connected, they have to match one and you can’t use PPP in the other end using HDLC. They both have to use the same protocol. If they don’t use the same encapsulation protocol, then they are not going to be able to communicate and that would give us the status of let’s say serial zero interface is up because layer one is good, but line protocol is down because the layer two encapsulations aren’t matching so that would be a big problem. Let’s go back to the routers and take a look at getting that interface up and up. I brought my CCNA simulator back up and we can see Palestra 2 is using HDLC at layer two of the OSI model. Let’s take a look and see what Palestra 1 is using. Do a show interface serial zero and we notice that interface serial zero is up, line protocol is down, encapsulation is HDLC.
So the layer two encapsulations match. Once I verified that, there is on other thing I need to check to make sure that these systems can be up and up and that’s what we are after. Line protocol is up is what we want because that means layer one and layer two are good so there’s one thing stopping us right now and it’s not the layer two encapsulation in a back to back environment. We need a particular command that will allow us to bring our interfaces up and up, let’s talk about that. I’ve brought up the graphic representation of the CCNA slide here and we can see Palestra 1 and Palestra 2 are connected back to back. Now I’ve put in here DCE and DTE. In a back to back environment, meaning a cable directly connecting these two routers normally we would connect to a server provider there would be some modems going on in here and the modems do timing for the link so in a back to back environment, we don’t have anything, time and link. There is no service provider or anything in between these routers so one of the routers has to do the timing for the link. They have to set up some sort of timing scheme on how they are going to send this data out. The DCE router becomes the router that’s going to be doing the timing, Data Communications Equipment, Data Terminal Endpoint or Data Communication Endpoint.
There is a couple different terms you will see for DCE but basically what’s happening is this DCE end need to do the clocking for the link. How do I know or what makes this DCE end as oppose to Palestra 2 being DCE and what it is, it’s the actual cable that determines what the DTE and and DCE end is. There is a little chip in here at the actual end of the cable that marks the cable, one end of the cable being DCE and one end of the cable being DTE. The router detects that little chip at the end of the cable here and determines whether it has a DCE end or the DTE end. We need to know which one is DCE and DTE. If the cable is not labeled and usually it will be, it will have a little label on there saying “Hey, this is a DCE end” then we have to go in and figure out which end is a DCE.
So if it’s not labeled on the cable we are going to have to figure that out. Let’s go back into the router and take a look at how we can figure that out. There is a command that we can us. Show controllers serial zero and what I do I don’t have to type serial zero and will show me control information for both ends. But I’ll just type in show controllers, serial zero and what I do is I’m on Palestra 1 and I look at this third line right here. It says V35, that’s the type of cable it is and DCE cable, that means this end is the DCE, if I got to Palestra 2, and do show controllers, I look up here. Notice here it says V35DTE cable. Really, the cable is what is determining this and all this means is the DTE end of the cable is plugged into Palestra 2. So I have to make Palestra 1 do some clocking for the link. So I will go into interface serial zero, interface serial zero from global mode and the command I use is clock rate and I can put a clock rate in there. Question mark gives me all the different clock rates I have and it’s just basically, it doesn’t affect the bandwidth but just that timing scheme that he’s going to use for communication and I’ll go clock rate; I’ll just pick of them here and 64,000 is one I use quite a bit in my class. Now what that does is since the timing is set up for the link and the layer two encapsulations are the same.
Notice it says line protocol and interface serial zero, it’s changed data up. So if I do show interface serial zero, it says is up, line protocol is up, that’s what I want. I know layer one and layer two of the OSI model are functioning so if I am not able to communicate to the other end, it’s got to be a layer three issue, meaning an IP addressing issue. This is the very first thing I look at if I’m troubleshooting connectivity between routers, I’ll look at the interface and I’ll go and make sure that serial interface that I’m connecting with or whatever interface it is, is up and the line protocol is up. That’s how I verify layer one and layer two. If these were down, meaning layer one down, I’d check cabling and make sure the router at the other end is turned on. If it says serial zero is up, line protocol is down then there are again two things. It’s either the layer two encapsulations is not matching on both sides or if you are using PPP, it might not be authenticating.
If you use an authentication, we’ll look at that a little bit more detail in another CCNA video or the clock rate is not set on back to back environment. Show run, real quick easy way for me to check out the clock right here, I can see clock rate 64,000. Notice I still don’t have an IP address. In the interface it’s saying “Is up, line protocol is up” so again that’s layer one and layer two, it has nothing to do with layer three, I still can’t ping from Palestra 1 to Palestra 2 because I don’t have an IP address to ping. So I’ve got to go in and set up by piece now to verify or to have total connectivity between the two devices. So let’s set up a couple of IPs here and we can go in and test and make sure we can ping.
Let’s say I forgot to set up an IP, show run. I go in, I forget things all the time, that’s what I love about this device and the show run command interface serial zero, I need to set an IP on Palestra 1 and Palestra 2. I forgot the command. So right here I can look and it says “No IP address” so if I want to set an IP address I can simply use the IP address command. Oh yeah, I remember now. I’m in global mode. Everything I see from here is a valid command so… which says interface serial zero so I’m going to type that in first. Interface, serial zero, let me get that… out of the way. Now it says “No IP address” so I want to type IP address and what if I forgot again? Check it out, all I have to do is put a question mark where the next component and it says “Hey man, I want the IP address and I want it in this format A, B, C, D” so I’m going to give it an IP address 192168.1.0, that’s not an IP address, that’s a network address, I’m going to go .1.1 and I’m just going to use a simple class C7 mask or if I forget where it comes next, using my question mark and it says again I want the IP subnet mask now in this format. Now if I do another question mark, I get CR which stands for Carriage Return, meaning hit enter. I didn’t get an error so that means I’ve set the IP address appropriately. I always show my work, show run again we’ll go in and show me what I’ve configured. It says right here, it’s exactly what I typed in. From global mode I type interface serial zero and then I type the IP address, 19216811, and then the subnet mask. So I have an IP here, let’s go set an IP address on Palestra 2.
Just click over to Palestra 2, again maybe I forgot, I keep forgetting. So what I can do is show run, oh yeah its interface, specify the interface and then the IP address command. Let me go in and set that. interface, serial zero, IP address, 192.168.1.2, enter, done. I want to see my work, show running config. If I just hit the tab it will complete it for me and I can go in and I can say “Okay, an IP address is set up.” So we should be able to ping at this point. Show interface, serial zero shows us that is up, line protocol is up, layer one and layer two are good and we’ve set an IP address of 192168.1.2 so unless I messed up typing the IPs in, we should be able to ping Palestra 1. ping 192.168.1.1 and I’ve got a response; so 100%, I now have layer one, layer two and with the IPs I’ve set, layer three connectivity.
Let’s take a look at some other commands that we can use to check out and verify our connectivity as well. All right, so we’ve used the show running-config, we’ve used show interface, there are a couple others, show IP interface will only show me interfaces with IP setup on them and IP statistics, meaning internet protocol shows me IPs, subnet mask, a couple of other things that we will use later. Outgoing access lists which we’ll talk about a little bit later for our CCNA exam as well as all IP information. This can be helpful for monitoring, troubleshooting traffic but it’s not going to tell us… you know it’s not critical for letting us know that the interface, layer one, layer two and layer three are up but it does show us that is up, line protocol is up, the IP and additional IP information so that can be helpful. If I want just real quick information, show IP interface brief command, that doesn’t just shows me real quick brief information about an interface. This can be helpful, if I would just want to make sure up and up, meaning layer one, layer two is good, IP address, everything looks okay, so everything is set up, I got the IP, layer one, layer two and layer three are good. One thing when checking IP addresses out, very important to make sure it’s the right IP.
Always have documentation and verify the IP address line for line or character for character. Any time I’m looking at an IP to verify it, I read it out loud to myself so I’d go… okay 192.168.1.2, I don’t say it very loudly, just kind of mumbling out of my breath so people don’t think I’m talking to myself but I do say it out loud. A big mistake people make, they can mistype an IP very easily and they’ll just overlook it. If I were to change this number eight right here and you are and you were troubleshooting it, it’s very easy just to assume and that’s a 192, you just kind of look at it real quick, okay that looks right. I do troubleshooting with my CCNA students a lot and they always miss that. I’ll make this like 198, 168 or 198, 162 and they almost always overlooked that. So IP connectivity is very important but again I first make sure layer one and layer two were up, line protocol is up before I move on to the IPs because it doesn’t matter if the IP is good or not if layer one and layer two of the OSI model are not functioning.
Another thing that can be helpful if I’m in the router interface a lot and I want to be able to specify instead of remembering the IP for Palestra 1, I want to map the IP, I can go in and basically create my own host file on the router. I go IP host and then if I forget, I just use my question mark word and wants the name of the host, Palestra 1 and then specify the IP address of Palestra 1 and what that does is I can type in Palestra 1 instead of the IP address to be able to ping or test connectivity of Palestra 1. Another problem with this is Palestra 1 is way too long, there’s no way I’d be typing that in here. So what I might do is I’d change my host up a little bit and I might just go Pone, that way I don’t have to type out the whole IP when I want to ping, you just go ping POW one and that should get a response and what it does, is it’s a mapping, it’s a name to IP mapping, it says Pone, but look where it’s pinging, it’s pinging 2192168.1.1 and that’s the IP host command that does that. If I want to see all my name to IP mapping, I can do show host and it shows me right here, okay POW one, it shows me the mapping, I can also do show running config and right here I can see my name to IP mapping. Again if I’m working in the router a lot, that can be helpful. A couple of other minor commands here that we can use, let me X it out. I can use the IP name server command, IP name server and specify the IP address of a DNS server. It’s the public server I use a lot and what that does is it allows me to have name resolution from the router.
So I could go in and maybe type ping www.yahoo.com and what will happen is that I’ll go out to that name server I specified and resolved, yahoo.com to an IP, if I want to test public connectivity. One other thing I can do is I can go IP in global mode IP domain name, dash name, and specify a prefix basically to this or suffix this name so the router’s name is Palestra 2, it might be .pallestra.com or pallestra.com. I won’t put the first dot there so I could go pallestra.com and what that does, it allows me to specify a domain name for the router. Now this command, I assure you is a valid command but the CCNA sim right here is not allowing us to go in and configure it. But it’s IP domain name and as long as I spelled it right it should work. Just sometimes the CCNA sim does not have everything we want. It shows domain lookup but on this version of the CCNA sim. It doesn’t have the domain dash name command available and that is a command that is available with later IOS versions of Cisco.
What that does, if I did have the domain name setup, I could go in from the outside world, let’s say one of my computers and then my own command prompt on my computer. If I want to connect to the router, I could go telnet and then actually put the name Palestra dot… or www.Palestra1.com and I could have it registered with the DNS server somewhere and I can make a connection to this device so I can make it more functional with the public DNS if I want to. Typically though that’s something you would do with the router. I usually just let the router function with IPs and we are not really in the rout of that much once we’ve configured it initially. Those are some things that can help out. One other command that I might need, if I do have a DNS server specified. I would need to turn on the IP domain lookup and this is on by default and what that does is that allows my router to go out and access a web… or a DNS server. If I just type in www.yahoo.com, what will happen a lot of times is it will try to resolve that name to an IP address if the domain lookup is available. If it’s not available, if I want to turn it off, no IP domain lookup, that can be very helpful on a real router because right here I’ve gone in and typed www.yahoo.com and immediately I got invalid input detected at marker. That’s not necessarily what would happen on an actual router, now in the CCNA sim, what could happen is a router could go broadcast out to try to find a DNS server and it would sit here hanging and it would just tic, tic, tic… and sit there hanging for a moment. It will make me wait and turning off domain lookup if I’m not going to be using domain name resolution from the router now. This has nothing to do with your internal computers or external computers, just from the router and then I would want to turn it off so I don’t have that issue of mistyping command and it hangs trying to find a DNS server. So show run again shows me how everything is configured. Best command on this device and there are some other commands to show interface, show IP interface but show run is the best command on this device to see how it’s configured but again it won’t show us a status.
Show interface serial zero, or IP interface serial zero shows us layer one, layer two, as well as IP configuration. One last command before we wrap up this particular video is the trace command. We’ve used ping and I’ll just ping Palestra 1, ping POW one and again the reason I could type POW one here is because I mapped it to an IPs in the IP host command but we can also use the trace command and what’s nice about the trace command in the router is it will tell us the hops we take to go from one place to another, my trace command is… for me so I’m going to have to put the IP but it shows me typically I could put POW one here by the way, again the CCNA simulator, not full functionality, 99% of it but some problems. Trying to escape sequence to abort, tracing route two, 192168.1.1 and what it does is it shows me one hop away, 20 milliseconds, 15 milliseconds, got it so it shows me connectivity. If I had to go through 10 routers to get to the Palestra 1 router, it would show me each one of those hops. You might say big deal, whatever but this trace command would show me where it stopped responding as well so if there was one router down I use a trace command to get from here with Palestra 2 to Palestra 1 in a… you know it’s 10 routers away, if I stop or see a response at the fifth router then I know about where the problem in the network is so it can be very helpful to pin point and there were connectivity issue if you are passing through multiple routers as well as if some of these times seeing… excessive, you know you might see 20 milliseconds whatever and then you see one that’s like 100 milliseconds, that might be a bottleneck in your network so this could be very helpful, so ping and trace are very helpful command when troubleshooting connectivity on layer three on that OSI model. Again if we have up and up, we can ping or use trace route to get to our destination, what that tells us is at layer one, layer two and layer three of the OSI model functions.
So we have talked about the interface show commands, the status messages, what they mean, the clock rate command again in a back to back environment, we need that clock rate, IP naming commands to make it a little easier instead of having to type IPs in all the time and the ping and trace commands to help us troubleshoot this connectivity. Thank you for watching this CertificationKits CCNA training video on troubleshooting router connectivity.