WEBVTT 00:00:00.000 --> 00:00:10.720 Great news, folks. We now get to move on to routing. We are now officially done our discussions 00:00:10.720 --> 00:00:21.280 on Layer 2 access and distribution layer features and services, so VLANs, trunks, inter-VLAN 00:00:21.280 --> 00:00:25.680 routing, first operand NC protocols. Now we really want to start talking about routing 00:00:25.680 --> 00:00:31.040 and things like OSPF and BGP for the certification exam. So one of the things we have to do is 00:00:31.040 --> 00:00:37.240 we're going to have to go back and do a review of routing just like we had in CCNA. And the 00:00:37.240 --> 00:00:43.360 reason why is because if you are not familiar with what we cover in CCNA, Encore will destroy 00:00:43.360 --> 00:00:49.639 you in regards to what I'm about to cover with you. You really have to know this stuff 00:00:49.639 --> 00:00:52.959 inside and out. And if you don't know this stuff inside and out that I'm about to show 00:00:53.040 --> 00:00:58.480 you, it's going to be difficult to cover or it's going to be difficult to get through the exam 00:00:59.040 --> 00:01:04.080 properly. And the last thing you want is to get a question that is based on, let's say, 00:01:04.080 --> 00:01:07.120 something like administrative distance and you don't fully understand administrative distance 00:01:07.120 --> 00:01:12.000 and what happens to you, you don't get any marks on that. And it could be worth 30 marks 00:01:12.000 --> 00:01:17.040 out of all of your marks. Why? Because Cisco expects you to really understand your routers, 00:01:17.040 --> 00:01:21.360 your routing tables, how your routers learn, how your routers make decisions, 00:01:21.360 --> 00:01:24.800 and all of that stuff. And if you don't know the process, it's going to be very difficult 00:01:24.800 --> 00:01:28.959 to answer the questions the way Cisco wants you to answer them. So, we'll see here that we have 00:01:28.959 --> 00:01:32.080 a routing table. Routing tables are used by our routers to make routing decisions. 00:01:33.919 --> 00:01:39.040 Notice here that we have a multitude of different network IDs that are in our routing table. 00:01:39.040 --> 00:01:44.400 These network IDs have been put in the routing table based on various methods, based on various 00:01:44.400 --> 00:01:52.080 means. The most fundamental way that routes get in the routing table are by having IP addresses 00:01:52.080 --> 00:01:57.440 on your router interfaces. When you put an IP address on the router interface and the interface 00:01:57.440 --> 00:02:06.160 is up, up, what happens is, is the router takes the network ID that that interface belongs to 00:02:06.800 --> 00:02:12.559 and puts it in the routing table. So, what does that mean? It means that this particular network 00:02:13.119 --> 00:02:21.039 is directly connected to me, directly connected. That means that I have an IP address on this 00:02:21.039 --> 00:02:26.960 interface that belongs to that network. And specifically, it is this interface, 00:02:26.960 --> 00:02:33.520 gigabit ethernet 0 slash 0, that would have that IP address in that particular network. 00:02:34.240 --> 00:02:40.000 So, how did that get in there? I put an IP address on that interface, the interface is up, up, 00:02:40.000 --> 00:02:44.720 it gets in the routing table that way. So, if we go to somebody like router number one right now, 00:02:46.160 --> 00:02:48.559 let us type in do show IP interface brief. 00:02:51.279 --> 00:02:55.199 And what we are going to see here is that we have this IP address on our interface of gigabit 00:02:55.199 --> 00:03:01.600 ethernet 0 slash 0. Let us type in do show run interface gigabit ethernet 0 slash 0, 00:03:01.600 --> 00:03:04.720 because I want to make sure I know the subnet mask too. Okay. So, there is our subnet mask. 00:03:05.279 --> 00:03:08.240 So, let us go ahead right now and take a look at a routing table, do show IP route. 00:03:08.800 --> 00:03:16.320 And you will see that for this network, 10.40.23.1, okay, slash 30, 00:03:18.720 --> 00:03:23.199 that would be this network right here for gigabit ethernet 0 slash 0. 00:03:23.199 --> 00:03:31.520 So, I added an IP address to gigabit ethernet 0 slash 0, and the interface is up, up right now. 00:03:32.320 --> 00:03:36.320 Therefore, the route, the network ID gets injected in the routing table. 00:03:36.320 --> 00:03:38.800 So, let us go to interface gigabit ethernet 0 slash 0, 00:03:43.039 --> 00:03:48.160 and let us type in no IP address. And you are going to see now, if we look at a routing table, 00:03:52.559 --> 00:03:56.559 that that route is no longer in the routing table. Why is that route no longer in the 00:03:56.559 --> 00:03:59.759 routing table? Because I do not have an IP address on that interface, 00:03:59.759 --> 00:04:04.880 on that interface anymore. So, let us add that IP address back in. 00:04:06.720 --> 00:04:08.399 We will copy and paste what I have here. 00:04:10.479 --> 00:04:13.679 Right. And you will see, as soon as I put the IP address on the interface and the interface goes 00:04:13.679 --> 00:04:21.359 up, up, right now, we have that network in our routing table. 00:04:23.440 --> 00:04:27.839 All right. What if we shut down the interface? If we shut down the interface, that means the 00:04:27.839 --> 00:04:31.519 interface is no longer available. If the interface is no longer available, then the 00:04:31.519 --> 00:04:35.440 router is like, I cannot reach that network through that interface, so I might as well 00:04:35.440 --> 00:04:38.559 take it out of the routing table then. So, we take a look at do show IP route. You will see 00:04:38.559 --> 00:04:43.600 that it is no longer in there. Why? Because the interface is down. So, two things must match, 00:04:43.600 --> 00:04:48.079 or two things must be true. The interface must be up, and there must be an IP address on that 00:04:48.079 --> 00:04:54.079 interface for the network ID of that interface to be installed in the routing table. So, that is 00:04:54.079 --> 00:04:57.119 how we get our directly connected routes in the routing table. Without directly connected routes, 00:04:57.119 --> 00:05:04.320 nothing else can happen, right? Our router is a router, and it routes packets, and it has to be 00:05:04.320 --> 00:05:08.160 attached to networks. And to be attached to networks, it has to have IP addresses on its 00:05:08.160 --> 00:05:14.880 interfaces for IPv4. So, as a result of that, to build its routing table and get things going and 00:05:14.880 --> 00:05:20.480 to start using dynamic routing protocols, you need IP addresses on interfaces, and therefore, 00:05:20.480 --> 00:05:24.399 your router interfaces have to belong to networks. That is how it happens, right? 00:05:24.399 --> 00:05:29.440 So, the same thing is true with your SVIs. When you create an SVI, you put an IP address on that 00:05:29.440 --> 00:05:32.880 virtual interface. The virtual interface is up, up. You get the route in the routing table. So, 00:05:32.880 --> 00:05:40.079 we type in do show IP route on distribution router 1, right? How do we get 10.0.0.0 slash 24 in there 00:05:40.079 --> 00:05:46.640 for interface VLAN 1? I configured 10.0.0.1, 255.255.255.0 as the IP address on interface 00:05:46.640 --> 00:05:52.160 VLAN 1. So, therefore, it gets put in the routing table for me. You will also see here that we have 00:05:52.160 --> 00:05:57.600 something called local routes. So, look at the local route here. The local route is the actual 00:05:57.600 --> 00:06:04.239 IP address of the interface, but not the subnet mask that we configured. So, if we type in do show 00:06:04.239 --> 00:06:13.679 interface or do show IP interface gigabit ethernet 0.0, we will see here that the IP address and 00:06:13.679 --> 00:06:18.320 subnet mask of this interface is this. But look what is showing up in our routing table as the 00:06:18.320 --> 00:06:23.760 local route. It is actually slash 32. So, a local route is a special route that gets added to your 00:06:23.760 --> 00:06:28.320 routing table, and it is the IP address of the interface, but the subnet mask is slash 32. And 00:06:28.320 --> 00:06:34.959 the reason why that exists, it is not for loopback, okay? The reason why that exists is for traffic 00:06:34.959 --> 00:06:40.880 that is destined to this router. So, for example, like if I ping this router and I ping 10.40.23.1, 00:06:40.880 --> 00:06:47.040 when it arrives at the router, the router can immediately determine that it is for the router, 00:06:47.040 --> 00:06:51.119 because the router knows that this IP address belongs to itself. So, it is not going to route 00:06:51.119 --> 00:06:56.160 it. It is for itself. If I tell it to the router, if I SSH to the router using these IP addresses, 00:06:56.160 --> 00:07:00.720 right, it simplifies the routing table lookup, because the routing table lookup is based on the 00:07:00.720 --> 00:07:06.000 longest match rule. The longest match rule states this. If you find multiple entries in the routing 00:07:06.000 --> 00:07:12.000 table that match, make sure you use the one with the longest subnet mask. So, right now, 10.40.23.1 00:07:12.000 --> 00:07:21.440 matches this, but 10.40.23.1 also matches this, and 10.40.23.1 also matches this. So, by adding 00:07:21.440 --> 00:07:25.920 the local route, it makes sure that if it is 10.40.23.1, it is guaranteed to match this one 00:07:25.920 --> 00:07:32.880 right here, right? Improve the performance of the router for traffic that is destined to the router, 00:07:32.880 --> 00:07:38.399 not to be routed, but to be processed by the router and absorbed by that particular router. 00:07:38.399 --> 00:07:43.679 So, those are the directly connected routes, the local routes. So, we have our connected routes, 00:07:43.679 --> 00:07:46.640 we have our local routes, and the next one we have are, like, static routes. So, what are 00:07:46.640 --> 00:07:51.839 static routes? Static routes are any routes that you manually create. So, for example, on my router 00:07:51.839 --> 00:07:58.000 right now, R1, I could turn around, and I could type in something like exit. I could type in IP 00:07:58.000 --> 00:08:11.200 route, 10.0.0.0, 255.255.255.0. Next top IP address, let me do 10.40.23.2, and if we type in 00:08:11.200 --> 00:08:15.760 do show IP route, what are we going to see? We're going to see in our results that I have a static 00:08:15.760 --> 00:08:21.839 route in order to get to that particular network. And the one above it, you'll see, is also a static 00:08:21.839 --> 00:08:27.119 route, but it's a static default route. So, any time you do IP route, let me type in do show 00:08:27.119 --> 00:08:32.080 run pipe to section IP route, any time you have a route that looks like this, that is your static 00:08:32.080 --> 00:08:37.760 default route, and that is basically your gateway to last resort. So, if your router does not have 00:08:37.760 --> 00:08:41.840 a match, any other match in the routing table, then it's going to go ahead and use that default 00:08:41.840 --> 00:08:46.000 route in order to forward traffic. And then lastly, we have a whole bunch of OSPF routes. 00:08:46.960 --> 00:08:51.840 For example, in this particular case, you could have OSPF routes, you could have EIGRP routes, 00:08:52.320 --> 00:08:58.239 you could have BGP routes, you could have ISIS routes, and the codes up here tell you the type 00:08:58.239 --> 00:09:02.239 of routes you have. You might even have some RIP routes, and the codes will tell you how you 00:09:02.239 --> 00:09:09.679 actually learnt that from its source. So, in this case, this network was learnt via the OSPF routing 00:09:09.679 --> 00:09:14.960 protocol, whereas if there was a D here, it means it was learned via the EIGRP routing protocol, 00:09:14.960 --> 00:09:20.400 and so on and so forth. All right. So, with that in mind now, we need to talk about how the table 00:09:20.400 --> 00:09:28.239 actually gets populated, how it actually gets built, because we so far know that we can learn 00:09:28.239 --> 00:09:34.479 via having an IP address configured on an interface. We know that we can learn via static 00:09:34.479 --> 00:09:40.640 routes. We know that we can learn via dynamic routing protocols like what? Like RIP, BGP, EIGRP, 00:09:40.640 --> 00:09:48.479 OSPF, ISIS. But the thing is, is that we have to recognize that the router has to make a decision 00:09:48.559 --> 00:09:55.919 when it learns networks. It has to figure out first and foremost, you know, what am I actually 00:09:55.919 --> 00:10:00.880 going to install in this table? Just because I learnt it doesn't mean I install it. It has to 00:10:00.880 --> 00:10:05.679 meet certain criteria before I put it in the routing table. So, even if I type something in 00:10:05.679 --> 00:10:09.119 manually, it doesn't mean the router is actually going to use it. Just because I put an IP address 00:10:09.119 --> 00:10:11.760 on an interface and it's up, doesn't mean the router is actually going to use it. Why? Because 00:10:11.760 --> 00:10:15.760 other conditions have to be met. For example, if I put an IP address on the interface, it has to be 00:10:15.760 --> 00:10:20.400 up, up in order for it to get installed. With a static route, I could create a static route, 00:10:20.400 --> 00:10:26.239 but it's not going to get installed in the routing table unless the interface that is used 00:10:26.239 --> 00:10:33.280 to reach the next top IP address is actually up. That's right. Did you know that? If not, 00:10:33.280 --> 00:10:36.479 let me show you. So, we created the static route right now. Why did that get installed 00:10:36.479 --> 00:10:43.440 in the routing table? Well, one, I created it. But two, it's because this next top IP address 00:10:43.440 --> 00:10:50.880 is actually reachable. Reachable how? Out this directly connected interface. So, I have a way 00:10:50.880 --> 00:10:56.479 to get to it. Let's bring down that interface. So, interface gigabit ethernet 0 size 0. If we 00:10:56.479 --> 00:11:00.239 shut down that interface, you're going to see that this is going to get removed from the routing 00:11:00.239 --> 00:11:04.960 table. This static route is going to get removed from the routing table. Now, there might be a 00:11:04.960 --> 00:11:09.679 dynamic route in there from OSPF now, and we'll talk more about that later because I'm running 00:11:09.679 --> 00:11:15.919 multiple sources here. But I'm going to show you that the static route is gone. Okay. The static 00:11:15.919 --> 00:11:20.239 route is gone. Now, there is one for OSPF, you can see, right? Because I am running OSPF on this 00:11:20.239 --> 00:11:26.400 as well. But the static route is gone. Why? Because the interface that is used to get there is gone, 00:11:28.000 --> 00:11:32.799 right? And now we're using gigabit 0 size 1, and OSPF has diverted us that way. 00:11:32.799 --> 00:11:37.200 Let's type in no shut. And you'll see once the interface comes back up, 00:11:37.919 --> 00:11:45.520 because the interface that is being referenced by the static route to reach that destination 00:11:45.520 --> 00:11:49.679 network is now up. Now, the static route gets installed in the routing table. 00:11:52.719 --> 00:11:57.679 Right. So, all these little things are part of the overall decision-making process. But one of 00:11:57.679 --> 00:12:03.520 the most important things is check your administrative distance. Check that administrative 00:12:03.520 --> 00:12:10.799 distance. Because if you learn the same network from multiple sources, you are going to have to 00:12:10.799 --> 00:12:19.840 figure out what source is the better source to use. And if you're using the wrong source, 00:12:22.559 --> 00:12:29.039 then that, well, you can't use the wrong source. It's going to force you to use the, no, actually, 00:12:29.039 --> 00:12:31.679 you know what, you could use the wrong source, which means you would have to figure out how to 00:12:31.679 --> 00:12:36.400 manipulate that. And I teach you that in the NRC course and certification exam to get you through 00:12:36.400 --> 00:12:41.280 that. And then after administrative distance, once you find the best source based on administrative 00:12:41.280 --> 00:12:46.719 distance, then you can look at a metric of that source. And then you can determine what goes on 00:12:46.719 --> 00:12:50.080 the routing table. So, these steps are really important. You're probably going to be testing 00:12:50.080 --> 00:12:56.559 on these on the exam, just like you were for CCNA. So, let's start off with administrative 00:12:56.559 --> 00:13:01.280 distance right now. So, what is administrative distance, folks? It's a ranking system. 00:13:02.080 --> 00:13:08.239 It's a ranking system. Look at the different sources here. 00:13:10.080 --> 00:13:13.280 You see all the sources? Connected, static, 00:13:13.359 --> 00:13:18.960 exterior border gateway protocol, EIGRP, OSPF, ISIS, et cetera. 00:13:30.960 --> 00:13:34.960 And look at the numbering system beside it. That is administrative distance. 00:13:35.679 --> 00:13:43.119 Lower is better, as you can see. Now, what does this mean? It means if I learn the same information 00:13:44.080 --> 00:13:50.479 from multiple sources, we are going to use administrative distance to break that tie. 00:13:50.479 --> 00:14:00.559 You cannot use information about the same network for more than one source. 00:14:01.520 --> 00:14:09.280 And let me diagram this out for you right now. So, for example, let's say that we have EIGRP 00:14:09.280 --> 00:14:17.840 running, and we have OSPF running. So, let's say this is EIGRP, and let's say this is OSPF. 00:14:18.000 --> 00:14:31.280 This is you. So, now you've learned that 10.0.0.0 size 24 is via, we'll say EIGRP path, 00:14:31.919 --> 00:14:37.359 and you've also learned that 10.0.0.0 size 24 is via the OSPF path. 00:14:37.919 --> 00:14:46.479 So, you've learned about the exact same information more than once. You've learned 00:14:46.479 --> 00:14:53.520 it from two different sources. You've learned it from EIGRP, and you've learned it from OSPF. 00:14:54.640 --> 00:15:04.239 All right, folks, you have to chat in your answer. If you learned about 10.0.0.0 size 24 00:15:05.200 --> 00:15:14.719 from both EIGRP and OSPF, which one are you going to use? EIGRP or OSPF? 00:15:16.799 --> 00:15:25.119 Look at the numbers, right? Lower is better. So, 90 is the winner. 90 is the winner. You're going 00:15:25.119 --> 00:15:32.880 to use EIGRP, meaning I'm not even going to consider you. That's right. I'm not even going 00:15:32.880 --> 00:15:50.880 to consider you. Big deal. Why is that a big deal? Well, let's do this. 10 megabits, 10 megabits, 00:15:51.679 --> 00:16:00.719 10 megabits, 10 gigs, 10 gigs. Which one are you going to use now, folks? EIGRP or OSPF? 00:16:02.880 --> 00:16:14.960 EIGRP or OSPF? 00:16:14.960 --> 00:16:28.000 It's still EIGRP. It's still EIGRP. Absolutely. You see, the metrics don't matter yet. 00:16:28.559 --> 00:16:34.080 The metrics don't matter yet. 00:16:46.080 --> 00:16:51.280 So, I learned about the same network from two different sources. The numbers don't matter. 00:16:51.679 --> 00:17:11.359 The numbers don't matter. What matters is who is more believable. That's it. That's it. 00:17:12.319 --> 00:17:23.280 All right, folks. Let's say my wife and I decide we're going to take a trip. 00:17:24.560 --> 00:17:28.959 We're going to go from Phoenix all the way to Vegas. 00:17:28.959 --> 00:17:34.880 Folks, please chat in. 00:17:37.920 --> 00:17:46.560 Sources of directions. 00:17:47.520 --> 00:18:06.479 Keep driving. What are my sources of directions? Come on, folks. Chat in. Good. 00:18:06.479 --> 00:18:28.400 Good. What else? Good. Anything else? 00:18:36.479 --> 00:18:51.040 There you go. Right? Sources of directions. 00:18:55.439 --> 00:19:04.640 Okay. Let's rank them now. Why? Well, I'm going to rank them. Just like who created this list, 00:19:05.280 --> 00:19:08.719 this is Cisco's list. This is Cisco's list. That's right. 00:19:09.439 --> 00:19:14.000 Administrative distance on other devices could be different. This is Cisco's list. 00:19:16.000 --> 00:19:17.199 So, I'm going to create my list. 00:19:17.199 --> 00:19:37.199 What does that mean? It means that I have learned about directions from Phoenix to Las Vegas 00:19:37.199 --> 00:19:42.079 from all of these sources. Who am I going to listen to, folks? 00:19:47.839 --> 00:19:59.920 The lowest number, Google Maps. It doesn't matter. Doesn't matter how much 00:20:00.640 --> 00:20:08.800 I am getting yelled at. That's right. I have to ignore it. I have to ignore it. 00:20:09.839 --> 00:20:14.000 And I must stick to Google Maps because that is my ranking system. 00:20:14.880 --> 00:20:20.400 So, let's say I'm using my phone. All of a sudden, the window starts going down. 00:20:20.400 --> 00:20:25.760 My phone is grabbed. My phone is thrown out the window. Okay. Well, am I going to learn from Google 00:20:25.760 --> 00:20:30.880 Maps anymore? No, I'm not. So, what am I going to use next? The built-in GPS in the vehicle. 00:20:31.920 --> 00:20:38.719 So, again, it doesn't matter. Doesn't matter what anybody else is saying or anything else is going 00:20:38.719 --> 00:20:45.439 on. I'm following the GPS. So, the GPS gets taken out of the vehicle, thrown out the window. 00:20:46.880 --> 00:20:50.640 What's next? I stop at a gas station to ask for directions. 00:20:53.599 --> 00:20:55.359 At that point, the car is probably gone. 00:21:00.160 --> 00:21:02.000 And I'm standing there all by myself. 00:21:02.959 --> 00:21:06.560 Right. What I'm trying to point out to you, folks, is that this is a ranking system 00:21:07.119 --> 00:21:12.959 that is set, but it's not necessarily set in stone. And you'll learn about that in the ANARSI course 00:21:13.599 --> 00:21:18.400 when you get to the advanced routing course. And we teach you how to manipulate it because it's 00:21:18.400 --> 00:21:21.520 very important that you manipulate it under certain scenarios and certain situations. 00:21:21.520 --> 00:21:26.160 If you're not getting the results you want when you learn from multiple sources in complex routing 00:21:26.160 --> 00:21:30.400 environments that use many different sources, then you would have to manipulate the system 00:21:30.400 --> 00:21:35.359 in that particular case. So, you can basically re-rank things if you need to. So, for example, 00:21:35.359 --> 00:21:39.359 let's get rid of this. This could ultimately become four. This could ultimately become three 00:21:39.359 --> 00:21:44.479 if we wanted to. Or maybe, you know, this should be three. That should be two. Or maybe, you know, 00:21:44.479 --> 00:21:49.920 it's better that this is one and maybe this is two. Whatever. Like, you have the ability 00:21:49.920 --> 00:21:53.520 to rank them as you need to for your particular environment. 00:21:54.239 --> 00:22:05.599 So, remember, this is only going to – this is only going to become a major concern, though, 00:22:05.599 --> 00:22:10.959 if you're running multiple sources. If you're running a single source and you're only learning 00:22:10.959 --> 00:22:15.680 from a single source, well, then that source, no matter how many times you learn from it, 00:22:16.239 --> 00:22:20.000 administratively, this is tied. Right? And that's when metrics are going to come into play, 00:22:20.000 --> 00:22:24.719 and we're going to get to that momentarily. So, for example, let's say instead that we had 00:22:24.719 --> 00:22:34.880 another network here, 10.0.2.0.24. And we learned that via OSPF. Well, we learned 10.0.0.0.24 via 00:22:34.880 --> 00:22:43.280 EIGRP. Well, if we go 10.0.2.0.24 via OSPF, are those the same? Are those the same? No, they're 00:22:43.280 --> 00:22:47.839 not. Right? They're not the same. So, because they're not the same, then what happens is they 00:22:47.839 --> 00:22:52.000 both get installed in the routing table from different sources. So, I'd have this in there 00:22:52.000 --> 00:22:56.319 from EIGRP. I'd have this one in there from OSPF. Why? Because they're different. Different 00:22:57.199 --> 00:23:02.479 networks from different sources. Different networks from different sources. Okay? 00:23:02.479 --> 00:23:09.040 So, administrative distance is about if you learn the same piece of information 00:23:09.760 --> 00:23:13.439 from more than one source, we've got to figure out which one's the best. 00:23:14.079 --> 00:23:22.959 So, for example, let's say we draw this as is, put this zero here. Let's say I type in IP route 00:23:22.959 --> 00:23:34.880 10.0.0.0.24 via ABC123. All right? So, now in this case, which one gets installed in the routing 00:23:34.880 --> 00:23:42.160 table? Is it the one via EIGRP? Is it the one via OSPF? Or is it the static route? 00:23:46.000 --> 00:23:51.920 Well, IP route is static route. We got one versus 90 versus 110. So, it's the static route that 00:23:51.920 --> 00:23:57.359 would go in the routing table. And that's what happened over here, folks, in our live environment 00:23:57.359 --> 00:24:02.880 a little earlier. So, in our live environment a little earlier, do you remember I had the 00:24:02.880 --> 00:24:07.680 static route there? What's the administrative distance of the static route? It is one. 00:24:09.599 --> 00:24:14.560 It is one, the administrative distance. Okay, let's go ahead and bring down that interface 00:24:16.560 --> 00:24:21.680 just as an example. Right? And remember, when we bring down that interface, 00:24:21.680 --> 00:24:27.599 then this gets removed. Why? It gets removed because the next hop IP address is not reachable. 00:24:27.599 --> 00:24:30.319 All right? But what I'm trying to show you is if we look in the routing table, 00:24:31.280 --> 00:24:39.359 did I type a no shut? All right. So, if we look at the routing table, you'll see that the OSPF 00:24:39.359 --> 00:24:45.839 route's in there. So, OSPF has administrative distance of 110. So, let's bring back up that 00:24:45.839 --> 00:24:54.800 interface. And when that interface comes back up, right, the static route, there's a static route 00:24:54.959 --> 00:25:01.839 for 10.0.0.0-24. So, that static route gets installed in the routing table. Why? Lower 00:25:01.839 --> 00:25:05.920 administrative distance. So, another way I can show you this is to simply just 00:25:06.880 --> 00:25:13.439 do show run pipe to include IP route. Let's go ahead and remove this right now. 00:25:18.400 --> 00:25:23.520 Type in do show IP route. What do you have? You got your OSPF route installed in the routing table. 00:25:24.479 --> 00:25:31.680 All right. Let's go ahead and put it back in. Because it's a static route, it has what? A lower 00:25:31.680 --> 00:25:36.959 administrative distance. Lower administrative distance means you win. There you go. Static 00:25:36.959 --> 00:25:41.439 routes in the table. That's it. That's what's causing that to happen. Administrative distance, 00:25:41.439 --> 00:25:46.640 same source. I'm sorry, different sources, same information. Administrative distance is used to 00:25:46.640 --> 00:25:51.920 break that. So, again, metric doesn't matter. So, pay attention to that. First things first, 00:25:51.920 --> 00:25:58.880 administrative distance. Okay. What's next? Metric. Okay. So, metric is going to be used 00:25:59.599 --> 00:26:05.839 within the same source. You can't use metric between different sources. And the reason why 00:26:05.839 --> 00:26:09.760 you can't use metric between different sources is because they all have different metrics. 00:26:09.760 --> 00:26:15.199 For example, connected routes have no metric. It's a zero. Static routes on Cisco devices have 00:26:15.199 --> 00:26:20.800 no metric. It's a zero. And you can't do anything about that. Okay. RIP uses hop count. The number 00:26:20.800 --> 00:26:24.880 of routers you have to hop through in order to get to the destination. So, going this way would be 00:26:24.880 --> 00:26:30.079 one, two, three. Three hops going this way. This way is one, two. Two hops going this way. 00:26:30.079 --> 00:26:33.439 So, it's going to choose this path just because of the number of hops. 00:26:35.359 --> 00:26:42.239 With EIGRP, we got bandwidth, lowest bandwidth plus cumulative delay of our path. With OSPF, 00:26:42.239 --> 00:26:49.040 we have the cost of the path that we'll talk about more when we get to our OSPF discussion. 00:26:49.040 --> 00:26:54.880 BGP is based on attributes. None of these are comparable to each other. None of them are 00:26:54.880 --> 00:26:59.199 comparable to each other. They're all different. Right? Just think about an orange and an apple. 00:26:59.199 --> 00:27:04.959 They're both fruits. They're both fruits, but they taste totally different. They have totally 00:27:04.959 --> 00:27:09.920 different skins. Totally different types of seeds. Right? They're not even comparable. Yes, 00:27:09.920 --> 00:27:14.959 they're both fruits, but they're really different. Same thing here. They're all dynamic routing 00:27:14.959 --> 00:27:18.800 protocols. Well, not all, but they're all dynamic routing protocols here, but they're all sources 00:27:18.800 --> 00:27:24.560 of routing information, but they're all different. They calculate things differently. So, that's why 00:27:24.560 --> 00:27:32.160 you can never break the tie using metrics. So, for example, if we said EIGRP versus OSPF, 00:27:33.199 --> 00:27:36.000 you can't do that with a metric. That's why administrative distance is first. 00:27:36.719 --> 00:27:41.439 Who wins? EIGRP. Yay. Done. But what about the metric? Doesn't matter in that case 00:27:41.439 --> 00:27:46.719 because EIGRP is the only option. One path. Okay. But now, let's say they're both EIGRP. Oh, 00:27:46.719 --> 00:27:51.199 well, there. Now, that's different. Right? Now that they're both learned via EIGRP, 00:27:52.160 --> 00:27:55.599 EIGRP, we look at administrative distance. Administrative distance is 90. Administrative 00:27:55.599 --> 00:28:00.880 distance is 90. Administrative distance is tied. Good. Yay. Now, we can look at the metric to break 00:28:00.880 --> 00:28:06.719 that tie. And so, let's say this was a one-meg path, one-meg path, one-meg path, and this was 00:28:06.719 --> 00:28:12.719 a two-meg path, two-meg path. Okay. With EIGRP, higher bandwidth better. So, we would more than 00:28:12.719 --> 00:28:18.800 likely take this path. Same thing would be true with something like OSPF. So, if we learned the 00:28:18.800 --> 00:28:24.800 same information from OSPF in both paths, well, then OSPF, we're using cost. So, if the cost of 00:28:24.800 --> 00:28:32.400 this is one, one, one, well, that'd be a total cost of three. If it's one, one, total cost of two. 00:28:32.479 --> 00:28:36.800 So, lower is better. We go that way. Right? So, cost is based on bandwidth. We'll talk about that 00:28:36.800 --> 00:28:42.880 when we get to OSPF if you don't know how that works. Right? So, metric is just something that 00:28:42.880 --> 00:28:45.920 is different amongst all of these. And if we go back to our Google Maps example, 00:28:47.520 --> 00:28:50.959 right, it's easy to portray that. Why is it easy to portray that? 00:28:51.599 --> 00:28:54.719 Because if you think about right now, if we had Google Maps 00:28:55.520 --> 00:29:02.800 and we had my wife, let me ask you this. What is Google Maps metric, folks? Chat in. What is 00:29:02.800 --> 00:29:08.479 Google Maps metric? Just throw some things out. What might Google Maps be using to determine 00:29:08.479 --> 00:29:22.719 the best path based on this diagram? Okay. Distance, right? How far is it? 00:29:22.719 --> 00:29:32.319 Distance, right? How far is it away if I go this way versus this way versus this way versus this 00:29:32.319 --> 00:29:39.920 way? Right? So, Google Maps knows about many different ways. Google Maps knows about many 00:29:39.920 --> 00:29:46.000 different ways. And what it's going to do is it's going to figure out what is the shortest way to go 00:29:46.000 --> 00:29:52.880 based on things like, yeah, distance, traffic, speed limits, all of that stuff. Perfect. Perfect. 00:29:53.439 --> 00:29:59.199 Okay. What is my wife going to use as her metric? 00:30:08.319 --> 00:30:12.800 Okay. My wife is going to use the number of crystal shops that are along the path. 00:30:13.680 --> 00:30:20.880 The number of crystal shops that are along the path. So, for us, right, if I'm using my wife as 00:30:20.880 --> 00:30:26.640 my source of information, we're probably not going to spend a lot of time in Vegas. 00:30:26.640 --> 00:30:33.520 We're probably going to spend a lot of time over here. So, more than likely, the trip is going this 00:30:33.520 --> 00:30:42.239 way. We'll eventually get to Vegas. But that's what her metric would be based on as an example. 00:30:42.719 --> 00:30:47.599 So, that's a really great scenario, folks, for all of you to realize that, yeah, you know, 00:30:48.640 --> 00:30:54.400 Google Maps could be EIGRP. My wife is OSPF. Right? Totally different metrics. Therefore, 00:30:54.400 --> 00:30:59.520 uncomparable when it's all said and done. That's why you, right, could pick your metrics based on 00:30:59.520 --> 00:31:03.760 administrative distance, manipulate things as you need to in the real world. Or, you know, 00:31:03.760 --> 00:31:07.280 if you're just using one metric, well, then, whatever its decision-making factor is is what 00:31:07.280 --> 00:31:12.079 you're going to use in that case. So, if I just decide, hey, the only option we have is my wife, 00:31:12.479 --> 00:31:17.359 well, then, whatever she chooses, she chooses. That's what we're doing. There is no other option 00:31:17.359 --> 00:31:21.520 available at that point in time. So, that would just be a network with one dynamic routing protocol 00:31:21.520 --> 00:31:26.160 running in it. But we'd still have static and connected. Right? So, maybe I could override 00:31:26.160 --> 00:31:30.959 her with my static routes because my static routes would have a lower administrative distance. 00:31:30.959 --> 00:31:37.359 Maybe. Maybe. Probably won't do that, though. All right. So, hopefully, folks, you love that 00:31:37.359 --> 00:31:41.040 scenario. You love that story. And, yes, I have permission from my wife to give that 00:31:41.040 --> 00:31:46.479 scenario, that story, as is. So, but I hope it gets the point across. This is one of the biggest 00:31:46.479 --> 00:31:53.040 challenges I had many, many, many, many years ago. Too many. I've ran out of fingers and toes now 00:31:53.760 --> 00:31:59.359 to count how many years ago that is. But, yeah, I struggled with administration distance in my 00:31:59.359 --> 00:32:03.760 early, early years and this whole concept. So, hopefully, something like that really helps 00:32:03.760 --> 00:32:08.640 clarify it for you. And now it makes a lot more sense. And then, now that you think about it, 00:32:08.640 --> 00:32:13.599 we figured out what the best source is. And if there is multiple paths within that same source, 00:32:13.599 --> 00:32:18.000 then we break that up based on metric. And then, in the end, we're down to one potential 00:32:18.000 --> 00:32:22.719 route or two potential routes or three, whatever it is. And we install whatever is left over in 00:32:22.719 --> 00:32:28.640 the routing table that matches our overall criteria. So, now, let's test your understanding 00:32:28.640 --> 00:32:38.560 of this. So, you are here, right? This is the network that we're talking about. Which route, 00:32:39.199 --> 00:32:44.719 the EIGRP route, the RIP route, or the OSPF route, folks, is going to be used 00:32:44.719 --> 00:32:48.800 by you to get here? The EIGRP, RIP, or OSPF? 00:33:05.599 --> 00:33:11.040 Is everyone sure? Are you sure? Are you sure it's EIGRP? 00:33:14.719 --> 00:33:25.359 Are you sure? If you are sure, tell me why you chose that. 00:33:32.479 --> 00:33:40.479 Yes. Administrative distance, that's it. Three different sources, right? Three different 00:33:40.479 --> 00:33:51.199 sources. EIGRP, 90. OSPF, 110. RIP, 120. Who's lower? EIGRP, you're the winner. 00:33:51.199 --> 00:33:58.239 Metric doesn't matter, remember? Administrative distance first. Administrative distance first, 00:33:58.239 --> 00:34:02.239 that's it. We're done. The metric doesn't even matter in this scenario right now. 00:34:04.959 --> 00:34:06.560 Important for your exam success. 00:34:10.639 --> 00:34:21.360 Okay. So, let's build upon this discussion now. Look at how I've changed it for you. 00:34:22.639 --> 00:34:27.840 Now, tell me which path is going to be used. Is it going to be path number one, 00:34:28.639 --> 00:34:34.080 path number two, or path number three? Would you like door one, door two, or door three? 00:34:41.120 --> 00:34:50.159 Okay. So, first things first. Administrative distance. So, we have 110, we have 120, 00:34:50.879 --> 00:34:58.879 we have 110. Okay. So, you are out of the picture automatically because your administrative 00:34:58.879 --> 00:35:08.479 distance is too high. We don't care about you anymore. 110 versus 110, tied. Let's break the 00:35:09.040 --> 00:35:17.600 tie. How do we break the tie? We break the tie with the metric now. So, lower is better. 50 is 00:35:17.600 --> 00:35:22.239 lower than 70. This is the winner. That's the one we're going to install in our routing table. We're 00:35:22.239 --> 00:35:33.919 going to go this way. Let's modify it just a little bit more. Now, which one's going to be used? 00:35:33.919 --> 00:35:47.360 One, two, or three? 00:36:04.080 --> 00:36:09.360 So, 00:36:16.399 --> 00:36:23.439 load balance on two and three. They're equal metric paths, and you load balance 00:36:23.439 --> 00:36:27.600 on equal metric paths automatically. Let's see if I can get you an example of that. 00:36:28.080 --> 00:36:37.120 There you go. There's an example of load balancing right there. I have two equal metric paths to get 00:36:37.120 --> 00:36:47.040 to that network. One is out this interface. The other one is out that interface. 00:36:47.600 --> 00:37:02.159 All right. So, hops don't matter anyways when it comes to OSPF. Hops don't matter at all. 00:37:03.360 --> 00:37:09.040 Number of routers don't matter. It is purely based on bandwidth, which is the metric. 00:37:10.719 --> 00:37:14.879 So, in the end, if the metric was calculated based on the bandwidth as 50, 00:37:14.879 --> 00:37:18.560 and the metric is calculated based on bandwidth of 50, it doesn't matter the number of routers, 00:37:18.560 --> 00:37:21.439 doesn't matter the number of links, doesn't matter the number of hops. 00:37:21.439 --> 00:37:24.399 50 is 50, and therefore, we would load balance. 00:37:30.879 --> 00:37:37.199 Yes, Andrew, it is really load balanced, okay? So, back in the day, we used to do 00:37:37.199 --> 00:37:47.840 packet-by-packet load balancing, right, by default on our routers. But our routers now, 00:37:47.840 --> 00:37:55.760 and Cisco routers use flow-based load balancing instead, meaning that if my traffic, 00:37:56.320 --> 00:38:03.600 let's say my traffic, this is me, okay? This is me, and this is you. It would ensure, 00:38:03.600 --> 00:38:07.520 instead of packet load balancing, because if it's packet load balancing, that means my one 00:38:07.520 --> 00:38:12.399 packet of mine would go this way, the other packet would go this way. That could cause problems, 00:38:12.399 --> 00:38:17.919 potentially, in the network because packets could arrive out of order, and if we're not using a 00:38:17.919 --> 00:38:21.840 protocol that can reassemble that properly, then in the end, it causes problems, all right? 00:38:22.399 --> 00:38:29.120 So, instead, we do flow-based load balancing. So, if my traffic arrives here, and the router 00:38:29.120 --> 00:38:34.959 chooses to send my traffic this way, then all of my traffic will go that way, and when yours 00:38:34.959 --> 00:38:42.239 arrives, if it chooses to go this way, then all of your traffic will go that way for that session, 00:38:42.239 --> 00:38:48.000 for that flow of traffic that is all coming from the same source, going to the same destination, 00:38:48.000 --> 00:38:52.479 using the same port numbers, using the same quality of service markings, and all that stuff. 00:38:53.360 --> 00:38:54.399 So, it's flow-based. 00:38:58.959 --> 00:39:04.000 John says, isn't it cost? Yes. So, the name for OSPF is cost, but it's still the metric. 00:39:04.000 --> 00:39:11.040 So, we come back here to this page, right? The metric for OSPF is called cost. 00:39:12.639 --> 00:39:16.080 So, whether you say metric, whether you say cost, same idea. 00:39:17.040 --> 00:39:22.639 So, cost is the cost of the interfaces that are then added together, which equal your metric. 00:39:23.679 --> 00:39:26.239 We'll talk about that when we get to our OSPF discussion on that. 00:39:27.439 --> 00:39:32.479 All right. So, now that we have a routing table built, right? So, let's make sure we 00:39:32.479 --> 00:39:38.800 understand what just happened. Based on administrative distance, based on metric, 00:39:39.199 --> 00:39:48.159 based on metric, we built our routing table. We built our routing table. That's all we've done. 00:39:54.399 --> 00:40:00.800 Now, we can use it. So, when we use it, let's pretend this is R2, right here in my diagram, 00:40:01.360 --> 00:40:11.919 traffic arrives. The router now has to use this table to make a decision. What is it going to do? 00:40:16.479 --> 00:40:21.600 Find the network entry that matches the destination IP address. So, 00:40:21.600 --> 00:40:27.280 let's say the destination IP in the packets 10.0.10.25. 00:40:28.239 --> 00:40:36.639 Then, find the network entry or entries that the destination IP address lives in. So, which of all 00:40:36.639 --> 00:40:42.639 of these does this fall into? Which one? Which one? Which one? Which one? Which one? And then, 00:40:42.639 --> 00:40:50.719 if there's more than one you find, use the one with the subnet mask closest to 32. That's what 00:40:51.360 --> 00:41:01.040 we call the longest match rule. The longest match rule. Match the longest subnet mask entry, 00:41:01.679 --> 00:41:08.479 the one closest to 32. So, a packet arrives. 00:41:08.479 --> 00:41:21.280 A packet arrives, destined at 10.0.10.25. That is somewhere over here. 00:41:23.760 --> 00:41:27.679 So, the router has to figure out, which of these entries am I going to use to forward it? 00:41:29.679 --> 00:41:35.520 So, 10 doesn't match, doesn't match, doesn't match, doesn't match, doesn't match. So, 00:41:35.520 --> 00:41:41.439 none of these match, just based on that. 10.0, okay, all of these match, including this one up 00:41:41.439 --> 00:41:45.840 here. 0, 0, 0, 0, size 0 matches everything. So, we already know this one's a match. We already 00:41:45.840 --> 00:41:49.439 know that. Meaning, if none of the other ones match, that's the one we're using. 00:41:52.080 --> 00:41:58.080 10.0.10. 10.0.10. So, this is greater than 10, greater than 10, greater than 10, greater than 00:41:58.080 --> 00:42:05.120 10. So, none of these are going to match. So, we can take them off. 10.0.10.25. 00:42:05.679 --> 00:42:09.919 Greater than 25, so I can take that off. Greater than 25, I can take that off. So, we're left with 00:42:09.919 --> 00:42:18.719 all of these. Now, I look at this one here, because I see 32. 32 means this exact IP address. 00:42:18.719 --> 00:42:25.439 So, this is a 2. This is a 25. So, not the exact IP address. So, what does that mean? We can take it 00:42:25.439 --> 00:42:34.800 off, right? Any slash 24 network you have, any slash 24 network you have, 0 will be the first 00:42:36.479 --> 00:42:41.840 address in that range. It's also the network ID. It's a non-usable address. It's the network ID, 00:42:42.479 --> 00:42:48.080 and the last one will always be 255, and that will be the directed broadcast address. So, 00:42:48.080 --> 00:42:55.040 the question is, this is your range. This is your range for a slash 24. So, if it's 10.0.10, 00:42:55.600 --> 00:43:00.399 dot zero slash 24, it's easy to figure out the range. Zero is the first one, because it's a 00:43:00.399 --> 00:43:07.280 slash 24. 255 is the last one, and then just put those numbers in. So, question is, does this, 00:43:07.280 --> 00:43:12.959 folks, fall within that range? Yes or no? Does that fall within that range? 00:43:17.679 --> 00:43:22.879 All right. It absolutely does. Yes, it does, right? We can see the numbers. So, we know that 00:43:22.879 --> 00:43:27.919 this is a possible match. We know it's a possible match, but we still have one more to check. 00:43:28.560 --> 00:43:32.159 All right. So, I'm going to use some skills I teach in CCNA to figure out this one right here. 00:43:33.040 --> 00:43:42.560 Okay. So, slash 28 would equal 8, 8, 8, 24, 25, 26, 27, 28, 29, 30, 31, 32. Okay. This is 128, 00:43:43.439 --> 00:43:55.600 128, 64, 33. So, 16. So, I'm counting by 16 in the fourth octet. So, if I'm counting by 16 in 00:43:55.600 --> 00:44:04.320 the fourth octet, the first network ID would be 10.0.10.0. And if I'm counting by 16, the next 00:44:04.320 --> 00:44:10.320 one would be 10.0.10.16. So, that would be a slash 28. That would be a slash 28. The next one would 00:44:10.320 --> 00:44:19.120 be 10.0.10.16 plus 16 is 32. So, that's 28. Okay. I can stop because you see this? This is that 00:44:19.120 --> 00:44:25.439 network ID. I already figured it out. So, what do I know now? I know that the range of addresses 00:44:25.439 --> 00:44:36.320 are going to be from 10.0.10.16 to 10.0.10.31. There's my range. There's my range now. So, 00:44:36.320 --> 00:44:41.040 the question is, does this fall in that range we just figured out? Yes or no? 00:44:45.120 --> 00:44:52.399 Yes, it does. Ding, ding, ding. Awesome. Okay. So, we now know that all three of these match. 00:44:54.320 --> 00:45:01.600 All three of them match this IP address. So, what does that mean? It means now we got to find the 00:45:01.600 --> 00:45:09.840 subnet mask closest to slash 32. The one closest to slash 32. That's the longest match rule. 00:45:11.840 --> 00:45:18.479 Right? So, is this one closer to slash 32? Is this one closer to slash 32? Or is this one closer 00:45:18.479 --> 00:45:26.719 to slash 32? Right? Absolutely, it's this one right here. Here's our winner in this scenario. 00:45:27.520 --> 00:45:33.600 Okay. So, what do I want to point out to you in this case? And I point this out on the CCNA exam 00:45:33.600 --> 00:45:39.760 as well. If you ever get a question like, which of the following entry will be used 00:45:40.560 --> 00:45:46.320 to forward the packet destined to 10.0.10.25, you need to be able to do this. Right? So, we learn 00:45:46.320 --> 00:45:51.280 all these skills in CCNA. They take a long time to learn. So, if you struggle figuring out ranges 00:45:51.280 --> 00:45:57.199 of IP addresses, you definitely want to go back and look at session three of CCNA, where I spend 00:45:57.199 --> 00:46:02.560 the entire session going over IP addressing and subnetting. You'll definitely want to brush up 00:46:02.560 --> 00:46:08.800 on that if you struggle with that. Right? But just imagine now on the exam if they turned around and 00:46:08.800 --> 00:46:19.760 said, what dynamic routing protocol or what source of information was used to populate the routing 00:46:19.760 --> 00:46:26.639 table for the network that matches that destination IP address? Okay. So, now you're sitting there 00:46:26.639 --> 00:46:31.439 going, okay, I've got to figure out which one wins, and then I have to look for the source, 00:46:31.439 --> 00:46:41.280 and your answer would be OSPF. Okay? Or what is the subnet mask of the network ID that will be 00:46:41.280 --> 00:46:47.199 used for the entry for this destination? Well, okay, slash 28. Like, you see how they are kind 00:46:47.199 --> 00:46:54.399 of testing you on your ability to figure things out, and then asking you to figure out what value 00:46:54.399 --> 00:46:59.919 of that line is the actual answer. Makes it really hard to guess. It really does. Because they could 00:46:59.919 --> 00:47:04.479 turn around and say, what is the administrative distance of the entry that matches the packet 00:47:04.479 --> 00:47:11.199 that's tested at 10.0.10.25? Well, it's 110. What is the metric of the packet, or what is the, yeah, 00:47:11.199 --> 00:47:16.719 the metric of the entry that matches the packet for 10.0.10.25? It's four. Okay, what is the next 00:47:16.719 --> 00:47:22.000 top IP address? How long has it been installed in the routing table based on the image? What 00:47:22.000 --> 00:47:31.280 interface is going to be used? Just look at this. You've got 00, 20, 10, 30 all listed. Could you 00:47:31.280 --> 00:47:38.320 imagine if they said, what is the interface that will be used for the packet to 10.0.10.25? And 00:47:38.320 --> 00:47:44.000 then you look at the answer options, it says gig 00, gig 01, or sorry, gig 00, gig 10, gig 20, 00:47:44.399 --> 00:47:48.959 they're all listed there. If you guess, you've got a 25% chance of getting it right. Don't, 00:47:48.959 --> 00:47:56.159 don't, figure it out. Figure it out. All right. Yeah, so that's another way you can figure out 00:47:56.159 --> 00:48:04.399 the block size as well. So if you know that the subnet mask is 255, 255, 255, 240, then you can 00:48:04.399 --> 00:48:10.320 take 256 minus 240, that leaves you with 16. It's just another way to figure it out. Just another 00:48:10.320 --> 00:48:15.120 way to figure it out. Yeah. And I teach both those methods in my CCNA course. All right. So 00:48:15.120 --> 00:48:19.439 just something to remember. So we reminded you about administrative distance. We reminded you 00:48:19.439 --> 00:48:24.800 about metric. We reminded you about longest match rule. And one thing I want to cover here just 00:48:24.800 --> 00:48:33.120 before we move on, and that is going to be this one right here. The time to live of our packet. 00:48:33.120 --> 00:48:37.760 Do you remember what the time to live of our packet is for folks? The time to live of our packet 00:48:38.399 --> 00:48:46.000 is used to time out the actual packet if it doesn't reach its destination. 00:48:47.360 --> 00:48:53.439 How is the time to live used in modern day equipment? It is basically just a hop count. 00:48:53.439 --> 00:49:00.399 That's what it is. So your PC creates the packet and sets the time to live, let's say to 64. 00:49:01.199 --> 00:49:09.040 It's a value that can be from 0 to 255. Packet gets sent out. What does the router do? The router 00:49:09.040 --> 00:49:13.280 is going to route the packet, but it modifies the time to live. It modifies the time to live. 00:49:14.000 --> 00:49:20.320 It will drop it down to 63. So every time a router touches a packet, it drops down the time to live. 00:49:21.360 --> 00:49:28.239 Now it's 62. Now it's 61. Next router, and so on and so forth. So the idea here is your packet 00:49:28.239 --> 00:49:33.840 must reach the destination before the time to live expires. So let's say the destination address 00:49:33.840 --> 00:49:40.320 is 8888 out on the internet over here somewhere. So, and I set the time to live to 2. 00:49:44.080 --> 00:49:50.239 What's going to happen here? Well, this router gets it, decrements it to 1, forwards it on. 00:49:51.120 --> 00:50:01.919 This router gets it, decrements it to 0. The router that decrements it to 0 will drop the packet 00:50:04.000 --> 00:50:14.959 and then it's going to send an ICMP time exceeded message to your client. 00:50:21.199 --> 00:50:24.719 I want you to know that. That's why I'm bringing it up, just in case it shows up 00:50:25.600 --> 00:50:32.959 on your exam. So which router decrements it? So which router is going to send the ICMP time 00:50:32.959 --> 00:50:39.199 exceeded message? The one that decrements it to 0. The one that decrements it to 0. 00:50:39.199 --> 00:50:44.320 So you're going to get it in with a time to live of 1. You decrement it to 0. You're like, well, 00:50:44.320 --> 00:50:49.120 it's dead. You drop it and you send it back. Okay, it's not this one here. It's not the 00:50:49.120 --> 00:50:53.439 one that has 1 in it. No, no, no, no. It's the one that decrements it to 0. 00:50:55.760 --> 00:51:03.679 So the router that makes it 0 drops it and sends the ICMP exceeded message back. Keep that in mind. 00:51:07.840 --> 00:51:14.959 All right. Make sure you know what we've covered there for the exam. Good stuff, everybody.