1 1 00:00:00,000 --> 00:00:03,370 So how does a router make its routing decisions? 2 2 00:00:03,370 --> 00:00:05,680 Well, it uses a routing table. 3 3 00:00:05,680 --> 00:00:08,000 Just like our switches used a MAC address table 4 4 00:00:08,000 --> 00:00:10,820 or a CAM table we're going to use routing tables 5 5 00:00:10,820 --> 00:00:13,250 to decide where our packets need to go inside 6 6 00:00:13,250 --> 00:00:14,920 and outside of our networks. 7 7 00:00:14,920 --> 00:00:16,970 Now, these routing decisions are going to contain 8 8 00:00:16,970 --> 00:00:18,210 layer three information, 9 9 00:00:18,210 --> 00:00:21,100 and they're going to do a layer three to layer two map. 10 10 00:00:21,100 --> 00:00:24,410 The router is going to use an ARP cache to map an IP address 11 11 00:00:24,410 --> 00:00:25,890 to a given MAC address. 12 12 00:00:25,890 --> 00:00:28,830 And this way it knows inside it's local area network 13 13 00:00:28,830 --> 00:00:31,980 which IPs are going to relate to which MAC addresses. 14 14 00:00:31,980 --> 00:00:34,630 Now each packet forwarding decision is then going to be based 15 15 00:00:34,630 --> 00:00:36,560 on it's internal routing table. 16 16 00:00:36,560 --> 00:00:39,170 Those internal routing tables are really focused on the 17 17 00:00:39,170 --> 00:00:42,280 logical address side of things or that IP address. 18 18 00:00:42,280 --> 00:00:44,380 So let's dig a little bit deeper into these routing tables 19 19 00:00:44,380 --> 00:00:46,440 to really see how they work. 20 20 00:00:46,440 --> 00:00:48,170 These tables are kept by the router 21 21 00:00:48,170 --> 00:00:49,720 and they help them determine which route 22 22 00:00:49,720 --> 00:00:50,950 is best fit for them, 23 23 00:00:50,950 --> 00:00:52,600 as they're trying to route the traffic 24 24 00:00:52,600 --> 00:00:54,250 throughout the different networks. 25 25 00:00:54,250 --> 00:00:57,406 Now a route entry has a prefix and the longer the prefix is 26 26 00:00:57,406 --> 00:01:00,170 the more specific that network is. 27 27 00:01:00,170 --> 00:01:01,600 So if we look here at this chart, 28 28 00:01:01,600 --> 00:01:03,730 I have three different networks showing up. 29 29 00:01:03,730 --> 00:01:06,490 There's 125.0.0.0. 30 30 00:01:06,490 --> 00:01:09,260 I have 161.5.0.0. 31 31 00:01:09,260 --> 00:01:12,420 And I have 134.7.0.0. 32 32 00:01:12,420 --> 00:01:15,160 Now, which of these would have the longest prefix? 33 33 00:01:15,160 --> 00:01:18,490 Well, it's the bottom two because they're the most specific. 34 34 00:01:18,490 --> 00:01:19,950 These are the ones that are most specific 35 35 00:01:19,950 --> 00:01:22,370 because they have the first two octets specified. 36 36 00:01:22,370 --> 00:01:26,220 Whereas the first one with the 125.0.0.0 37 37 00:01:26,220 --> 00:01:28,410 only has the first octet specified. 38 38 00:01:28,410 --> 00:01:30,280 So it is the least specific. 39 39 00:01:30,280 --> 00:01:34,750 So if I'm dealing with something like 10.1.1.0/24, 40 40 00:01:34,750 --> 00:01:36,850 that is really really specific because I have 41 41 00:01:36,850 --> 00:01:40,240 the first three octets specified, the 10.1.1. 42 42 00:01:40,240 --> 00:01:44,010 That means I only have 256 possible IPs left 43 43 00:01:44,010 --> 00:01:46,100 because of that .0 at the end. 44 44 00:01:46,100 --> 00:01:50,480 Now, instead if I use something like 10.0.0.0/8 45 45 00:01:50,480 --> 00:01:53,610 this means I only have one octet specified 46 46 00:01:53,610 --> 00:01:55,560 and the other three are really wild cards. 47 47 00:01:55,560 --> 00:01:58,390 So I can have up to 16 million IP addresses 48 48 00:01:58,390 --> 00:02:00,500 because of those three sets of zeros. 49 49 00:02:00,500 --> 00:02:02,710 So you can see here the longer the prefix we have 50 50 00:02:02,710 --> 00:02:04,520 or the higher the CIDR notation, 51 51 00:02:04,520 --> 00:02:06,950 the more specific that route becomes. 52 52 00:02:06,950 --> 00:02:09,213 Now, when we look at these routing tables 53 53 00:02:09,213 --> 00:02:10,310 and we look at all the routes in the table, 54 54 00:02:10,310 --> 00:02:11,720 those routes are going to tell us 55 55 00:02:11,720 --> 00:02:13,670 what the destination network is 56 56 00:02:13,670 --> 00:02:17,090 and which router it should go to to get to that network. 57 57 00:02:17,090 --> 00:02:19,730 It'll also tell us which port on the router is going to be used 58 58 00:02:19,730 --> 00:02:21,891 to send traffic out and the cost of that route, 59 59 00:02:21,891 --> 00:02:24,010 which is basically like switching, 60 60 00:02:24,010 --> 00:02:26,190 where it's dependent upon numbers of different factors, 61 61 00:02:26,190 --> 00:02:29,100 like the link speed and other factors like that. 62 62 00:02:29,100 --> 00:02:31,390 Now we're going to talk specifically about those different 63 63 00:02:31,390 --> 00:02:32,830 factors in another lecture 64 64 00:02:32,830 --> 00:02:34,840 when we start diving into routing protocols. 65 65 00:02:34,840 --> 00:02:37,780 But for now just realize that every link in a route 66 66 00:02:37,780 --> 00:02:39,960 does have a cost associated with it. 67 67 00:02:39,960 --> 00:02:42,050 So what are some of the sources of routing information 68 68 00:02:42,050 --> 00:02:43,060 that we can have? 69 69 00:02:43,060 --> 00:02:44,900 Well, there are really three different sources 70 70 00:02:44,900 --> 00:02:46,480 that routers are going to use. 71 71 00:02:46,480 --> 00:02:49,320 The first one is called a directly connected route. 72 72 00:02:49,320 --> 00:02:51,330 Now a directly connected route is learned by 73 73 00:02:51,330 --> 00:02:53,900 a physical connection between two routers. 74 74 00:02:53,900 --> 00:02:55,440 So if you look on the screen here, 75 75 00:02:55,440 --> 00:02:56,810 I have three different routers. 76 76 00:02:56,810 --> 00:02:59,340 Router one, router two, and router three. 77 77 00:02:59,340 --> 00:03:02,390 Now router two knows where router one is and router three 78 78 00:03:02,390 --> 00:03:05,120 are because it has direct cabling between those 79 79 00:03:05,120 --> 00:03:06,628 and this is shown by those zigzag lines 80 80 00:03:06,628 --> 00:03:08,590 or that lightning bolt looking line 81 81 00:03:08,590 --> 00:03:10,050 that's here on my diagram. 82 82 00:03:10,050 --> 00:03:12,467 This denotes that it is a serial connection. 83 83 00:03:12,467 --> 00:03:15,600 Next we have static routes and this is the second way 84 84 00:03:15,600 --> 00:03:18,160 that routers know how things are connected to them. 85 85 00:03:18,160 --> 00:03:20,320 Now a static route are routes that are configured 86 86 00:03:20,320 --> 00:03:22,000 by an administrator manually. 87 87 00:03:22,000 --> 00:03:23,150 So for example, 88 88 00:03:23,150 --> 00:03:25,310 router one knows how to get to router two 89 89 00:03:25,310 --> 00:03:26,920 because it's directly connected, 90 90 00:03:26,920 --> 00:03:29,313 but it doesn't know how to get to router three yet. 91 91 00:03:29,313 --> 00:03:32,070 So for me to tell it how to get through router three, 92 92 00:03:32,070 --> 00:03:34,092 I can put a route into my routing table that says 93 93 00:03:34,092 --> 00:03:37,040 every time you want to send something to router three, 94 94 00:03:37,040 --> 00:03:39,452 just go ahead and send it through router two. 95 95 00:03:39,452 --> 00:03:41,860 That would become a static route. 96 96 00:03:41,860 --> 00:03:44,250 Now, there is always one default static route 97 97 00:03:44,250 --> 00:03:45,911 in every router. 98 98 00:03:45,911 --> 00:03:46,744 And it's a special one. 99 99 00:03:46,744 --> 00:03:50,660 It's known as 0.0.0.0/0. 100 100 00:03:50,660 --> 00:03:52,972 Basically it says to the router, 101 101 00:03:52,972 --> 00:03:54,750 if you don't know where to go, just go here. 102 102 00:03:54,750 --> 00:03:56,710 It's kind of like your default gateway. 103 103 00:03:56,710 --> 00:03:58,180 And so for router one, 104 104 00:03:58,180 --> 00:04:02,000 it might have a default route of 0.0.0.0/0. 105 105 00:04:02,000 --> 00:04:04,410 That says go to router two anytime you don't know 106 106 00:04:04,410 --> 00:04:05,630 how to get someplace. 107 107 00:04:05,630 --> 00:04:07,530 And then it becomes router two's problem 108 108 00:04:07,530 --> 00:04:09,620 to figure out what to do with that traffic. 109 109 00:04:09,620 --> 00:04:11,560 Now the third way that routers can do this 110 110 00:04:11,560 --> 00:04:13,860 is by using what's known as dynamic routing. 111 111 00:04:13,860 --> 00:04:16,740 And this is done by using dynamic routing protocols. 112 112 00:04:16,740 --> 00:04:19,170 These protocols are learned by exchanging information 113 113 00:04:19,170 --> 00:04:22,387 between routers automatically based on the protocols. 114 114 00:04:22,387 --> 00:04:24,880 Now, instead of me having to go into router one 115 115 00:04:24,880 --> 00:04:26,880 and tell it how to get to router three, 116 116 00:04:26,880 --> 00:04:29,360 instead I can just let router two do it. 117 117 00:04:29,360 --> 00:04:31,150 Now, the idea here is that when router one 118 118 00:04:31,150 --> 00:04:33,510 and router two become directly connected, 119 119 00:04:33,510 --> 00:04:35,540 they start sharing their routing tables. 120 120 00:04:35,540 --> 00:04:37,667 So router one is going to say, hey router two, 121 121 00:04:37,667 --> 00:04:39,730 do you know how to talk to some people? 122 122 00:04:39,730 --> 00:04:41,550 And router two will say, 123 123 00:04:41,550 --> 00:04:42,740 oh yeah, I'm connected to router three. 124 124 00:04:42,740 --> 00:04:44,620 So anytime you've got something for router three, 125 125 00:04:44,620 --> 00:04:46,650 just send it to me and I'll pass it on for you. 126 126 00:04:46,650 --> 00:04:49,220 And they do this all by themselves automatically 127 127 00:04:49,220 --> 00:04:51,320 using these dynamic routing protocols. 128 128 00:04:51,320 --> 00:04:53,620 This is really great especially in large networks 129 129 00:04:53,620 --> 00:04:55,900 because we don't have to manually configure everything. 130 130 00:04:55,900 --> 00:04:57,140 So for example, 131 131 00:04:57,140 --> 00:04:59,010 think about if I took your router at home 132 132 00:04:59,010 --> 00:05:01,080 and I had to put down every single route for you 133 133 00:05:01,080 --> 00:05:02,440 to be able to find every website 134 134 00:05:02,440 --> 00:05:04,140 you wanted to get to on the internet. 135 135 00:05:04,140 --> 00:05:06,095 You would not be able to do it. 136 136 00:05:06,095 --> 00:05:07,390 It is just too many things out there, right? 137 137 00:05:07,390 --> 00:05:10,880 So instead we use dynamic routing to accomplish this. 138 138 00:05:10,880 --> 00:05:12,110 When you connect to the internet, 139 139 00:05:12,110 --> 00:05:14,460 you connect to your ISP and they know that they are 140 140 00:05:14,460 --> 00:05:15,710 now your default route. 141 141 00:05:15,710 --> 00:05:18,060 Anything you want to get to that's not inside your network 142 142 00:05:18,060 --> 00:05:19,370 you send it to your ISP 143 143 00:05:19,370 --> 00:05:21,740 and then your ISP will send it to the next router. 144 144 00:05:21,740 --> 00:05:23,740 And it keeps going up that chain that way. 145 145 00:05:23,740 --> 00:05:24,930 So that's a great overview of 146 146 00:05:24,930 --> 00:05:26,550 the three different types of routes. 147 147 00:05:26,550 --> 00:05:28,110 We have directly connected routes, 148 148 00:05:28,110 --> 00:05:30,350 static routes, and dynamic routes. 149 149 00:05:30,350 --> 00:05:32,620 So let's go ahead and dig a little bit deeper into each of 150 150 00:05:32,620 --> 00:05:34,380 these three types so you can understand them 151 151 00:05:34,380 --> 00:05:35,650 a little bit more. 152 152 00:05:35,650 --> 00:05:38,520 First, let's take a look at a directly connected route. 153 153 00:05:38,520 --> 00:05:40,150 When we look at a directly connected route, 154 154 00:05:40,150 --> 00:05:42,576 it's going to look something like what you see here 155 155 00:05:42,576 --> 00:05:43,409 on my screen. 156 156 00:05:43,409 --> 00:05:45,870 Now as you can see router one and router two are directly 157 157 00:05:45,870 --> 00:05:47,810 connected with that zigzag line. 158 158 00:05:47,810 --> 00:05:49,530 That's that serial connection. 159 159 00:05:49,530 --> 00:05:52,360 Now these routers both know how to get to the other switches 160 160 00:05:52,360 --> 00:05:54,690 because they're directly connected to them, right? 161 161 00:05:54,690 --> 00:05:56,260 So if I look at router one's table, 162 162 00:05:56,260 --> 00:06:00,590 it says I'm connected to 10.0.1.0/24, 163 163 00:06:00,590 --> 00:06:03,220 which is the network that it owns on switch one. 164 164 00:06:03,220 --> 00:06:06,392 It's also connected to 192.1.1.0, 165 165 00:06:06,392 --> 00:06:08,410 because this is the serial connection 166 166 00:06:08,410 --> 00:06:10,020 between the two routers. 167 167 00:06:10,020 --> 00:06:12,660 Now notice that router one does not know how to get to 168 168 00:06:12,660 --> 00:06:15,100 the 10.0.2.0 network, 169 169 00:06:15,100 --> 00:06:16,840 because this is on switch two 170 170 00:06:16,840 --> 00:06:18,740 and that's hanging off of router two. 171 171 00:06:18,740 --> 00:06:21,490 It has no information on this because it only knows things 172 172 00:06:21,490 --> 00:06:23,430 that are directly cabled to it. 173 173 00:06:23,430 --> 00:06:25,180 Now, if we go further into static routing, 174 174 00:06:25,180 --> 00:06:27,900 I can actually write that down as part of my default route 175 175 00:06:27,900 --> 00:06:31,436 or 0.0.0.0 for router one. 176 176 00:06:31,436 --> 00:06:34,700 Now this tells me that anytime I don't know an IP address, 177 177 00:06:34,700 --> 00:06:38,700 I'm simply going to send it out port serial 1/1, 178 178 00:06:38,700 --> 00:06:40,810 which is going to push it over to router two. 179 179 00:06:40,810 --> 00:06:44,150 And so in this case, if PC one tries to get to PC two, 180 180 00:06:44,150 --> 00:06:45,850 it would get as far as router one 181 181 00:06:45,850 --> 00:06:47,550 and then router one's going to say ugh, 182 182 00:06:47,550 --> 00:06:50,310 I don't know how to get to 10.0.2.2, 183 183 00:06:50,310 --> 00:06:52,140 but maybe router two does. 184 184 00:06:52,140 --> 00:06:54,316 And it's going to push that over to them 185 185 00:06:54,316 --> 00:06:55,450 over that wide area network connection, 186 186 00:06:55,450 --> 00:06:57,760 that serial connection that's directly connected. 187 187 00:06:57,760 --> 00:06:59,970 That's how this manual configuration works 188 188 00:06:59,970 --> 00:07:01,500 with a static route. 189 189 00:07:01,500 --> 00:07:03,557 Alright, so I think you understand how 190 190 00:07:03,557 --> 00:07:05,820 direct connections work and how static routing works. 191 191 00:07:05,820 --> 00:07:08,920 Let's take a look at the dynamic routing protocols instead. 192 192 00:07:08,920 --> 00:07:11,140 Now dynamic routing protocols can have more than one route 193 193 00:07:11,140 --> 00:07:13,210 for an existing network. 194 194 00:07:13,210 --> 00:07:15,480 If I have a more complex network diagram, 195 195 00:07:15,480 --> 00:07:17,550 like these five routers that you see here, 196 196 00:07:17,550 --> 00:07:19,610 and I wanted to get from one to five, 197 197 00:07:19,610 --> 00:07:21,170 I can go a couple of different ways. 198 198 00:07:21,170 --> 00:07:23,770 I can go one, two, three, four, five. 199 199 00:07:23,770 --> 00:07:25,680 And I really just start zigzagging all over the network. 200 200 00:07:25,680 --> 00:07:28,870 Or I might go one, two, four, five. 201 201 00:07:28,870 --> 00:07:31,170 Or I might go one, three, four, five. 202 202 00:07:31,170 --> 00:07:33,050 There's lots of different ways for me to go 203 203 00:07:33,050 --> 00:07:35,170 because all the different connections that are there. 204 204 00:07:35,170 --> 00:07:38,450 So how does my router know which one is best? 205 205 00:07:38,450 --> 00:07:40,918 Well, the dynamic routing is going to get negotiated for us 206 206 00:07:40,918 --> 00:07:43,650 based on the number of hops that are there, 207 207 00:07:43,650 --> 00:07:45,680 which is the number of times I have to 208 208 00:07:45,680 --> 00:07:46,513 go through a different router, 209 209 00:07:46,513 --> 00:07:48,540 the number of link bandwidth that's available. 210 210 00:07:48,540 --> 00:07:49,940 So how fast it is. 211 211 00:07:49,940 --> 00:07:52,000 I might go through the ones that are faster or slower 212 212 00:07:52,000 --> 00:07:54,010 and other criteria that are out there. 213 213 00:07:54,010 --> 00:07:56,870 All of these dynamic routing protocols can be able to be 214 214 00:07:56,870 --> 00:07:59,350 supported depending on what routers you're using 215 215 00:07:59,350 --> 00:08:01,400 and different criteria of how you want to set them up, 216 216 00:08:01,400 --> 00:08:03,160 depending on how we configure them. 217 217 00:08:03,160 --> 00:08:04,690 Now we're going to spend an entire lesson 218 218 00:08:04,690 --> 00:08:07,070 on the different types of dynamic routing protocols. 219 219 00:08:07,070 --> 00:08:08,500 So we can really dig into each of them. 220 220 00:08:08,500 --> 00:08:10,640 And you can really understand because you're going to get 221 221 00:08:10,640 --> 00:08:13,230 test questions on them come test day. 222 222 00:08:13,230 --> 00:08:15,480 Now, the last thing I want to talk about in this lesson 223 223 00:08:15,480 --> 00:08:17,540 is preventing routing loops. 224 224 00:08:17,540 --> 00:08:19,090 Just like we had an issue with switches 225 225 00:08:19,090 --> 00:08:21,150 where you can get loops and broadcast storms, 226 226 00:08:21,150 --> 00:08:22,700 you can get issues with routing 227 227 00:08:23,659 --> 00:08:24,870 if things start going into a circular manner. 228 228 00:08:24,870 --> 00:08:27,540 Things will just end up getting lost in cyberspace, right? 229 229 00:08:27,540 --> 00:08:30,100 So, to prevent that we have two different techniques 230 230 00:08:30,100 --> 00:08:31,130 we can use. 231 231 00:08:31,130 --> 00:08:34,760 These are known as the split horizon and the poison reverse. 232 232 00:08:34,760 --> 00:08:36,220 Now with the split horizon, 233 233 00:08:36,220 --> 00:08:37,850 this is going to prevent a route that's learned 234 234 00:08:37,850 --> 00:08:40,500 on one interface from being advertised back out 235 235 00:08:40,500 --> 00:08:42,150 that same interface. 236 236 00:08:42,150 --> 00:08:43,580 So in this example, 237 237 00:08:43,580 --> 00:08:46,000 you see router one knows how to get to router two 238 238 00:08:46,000 --> 00:08:49,110 because of that direct connection between those two routers. 239 239 00:08:49,110 --> 00:08:51,200 Now it's not going to go and tell router two 240 240 00:08:51,200 --> 00:08:53,930 how to get to router two based on that same connection, 241 241 00:08:53,930 --> 00:08:56,210 because guess what, it came in that connection. 242 242 00:08:56,210 --> 00:08:58,050 So it can't go out the same connection. 243 243 00:08:58,050 --> 00:09:00,030 This is essentially what our spanning tree protocol 244 244 00:09:00,030 --> 00:09:01,840 did for us in our switching networks. 245 245 00:09:01,840 --> 00:09:04,383 But now we can do this inside of routing. 246 246 00:09:04,383 --> 00:09:07,561 Now, the second way we do this is what's known as 247 247 00:09:07,561 --> 00:09:09,481 a poison reverse and a poison reverse is going to cause a route 248 248 00:09:09,481 --> 00:09:12,770 that's received on one interface to be advertised back out 249 249 00:09:12,770 --> 00:09:16,063 that same interface, but with a really really high cost 250 250 00:09:16,063 --> 00:09:18,480 so that nobody would ever want to use it. 251 251 00:09:18,480 --> 00:09:21,279 Now, these are just two different techniques 252 252 00:09:21,279 --> 00:09:22,112 for you to stop writing loops. 253 253 00:09:22,112 --> 00:09:23,080 You could use one or the other. 254 254 00:09:23,080 --> 00:09:24,490 It really doesn't make a difference. 255 255 00:09:24,490 --> 00:09:25,900 As long as you're using one, 256 256 00:09:25,900 --> 00:09:27,840 it's going to prevent that running loop for you. 257 257 00:09:27,840 --> 00:09:30,450 The first one is I'm just not going to repeat anything. 258 258 00:09:30,450 --> 00:09:32,170 The second one is, I'm going to repeat it, 259 259 00:09:32,170 --> 00:09:34,150 but I'm going to tell you it's so expensive 260 260 00:09:34,150 --> 00:09:36,350 that you'll never ever want to use it. 261 261 00:09:36,350 --> 00:09:38,420 So let's go ahead and take a look at an example 262 262 00:09:38,420 --> 00:09:39,430 of these routing loops 263 263 00:09:39,430 --> 00:09:41,090 and how we can make sure we prevent them. 264 264 00:09:41,090 --> 00:09:43,410 So here's a network with no issues at all. 265 265 00:09:43,410 --> 00:09:44,870 I have three routers here. 266 266 00:09:44,870 --> 00:09:47,690 I have router one, router two, and router three. 267 267 00:09:47,690 --> 00:09:50,100 You can see the different networks based on the IPs 268 268 00:09:50,100 --> 00:09:52,500 and their routing tables are shown here on the screen. 269 269 00:09:52,500 --> 00:09:54,840 Now we're going to look specifically at the routing tables 270 270 00:09:54,840 --> 00:09:57,870 for router two and router three in this example. 271 271 00:09:57,870 --> 00:10:00,020 Now, if I start having an issue, like for instance, 272 272 00:10:00,020 --> 00:10:01,860 the network connection from router three, 273 273 00:10:01,860 --> 00:10:04,740 for the 10.1.4.0 network went down 274 274 00:10:04,740 --> 00:10:07,220 because ethernet 0/1 went down, 275 275 00:10:07,220 --> 00:10:08,970 what do you thinks going to happen? 276 276 00:10:08,970 --> 00:10:11,460 Well, router two still thinks it can get to that network 277 277 00:10:11,460 --> 00:10:12,713 because it says, hey, 278 278 00:10:13,663 --> 00:10:15,800 I can get there through my serial port of 0/1, 279 279 00:10:15,800 --> 00:10:17,780 because I have a connection to router three. 280 280 00:10:17,780 --> 00:10:20,150 Now if router three went down, it's going to say, 281 281 00:10:20,150 --> 00:10:21,690 I don't know how to get there anymore. 282 282 00:10:21,690 --> 00:10:23,490 And so it's going to ask its friends 283 283 00:10:24,623 --> 00:10:25,456 using the dynamic router protocols. 284 284 00:10:25,456 --> 00:10:27,161 And so at this point it's going to say hey router two, 285 285 00:10:27,161 --> 00:10:28,799 do you know how to get to that network? 286 286 00:10:28,799 --> 00:10:31,540 And what happens is router two would say oh yeah, 287 287 00:10:31,540 --> 00:10:33,160 I know how to get to that network. 288 288 00:10:33,160 --> 00:10:34,850 I can do it in just one hop. 289 289 00:10:34,850 --> 00:10:36,280 So go ahead and send it to me. 290 290 00:10:36,280 --> 00:10:38,180 And then router three says, oh great. 291 291 00:10:38,180 --> 00:10:39,870 That means I know how to get there too, 292 292 00:10:39,870 --> 00:10:41,750 because I'm connected to you router two, 293 293 00:10:41,750 --> 00:10:42,700 and you know how to do it. 294 294 00:10:42,700 --> 00:10:45,170 So that one hop for you becomes two hops for me 295 295 00:10:45,170 --> 00:10:46,460 and it keeps going back and forth. 296 296 00:10:46,460 --> 00:10:48,660 And then router two goes, oh, I know how to get there. 297 297 00:10:48,660 --> 00:10:49,910 Router three knows how to get there. 298 298 00:10:49,910 --> 00:10:51,540 And he says, he can do it in two hops, 299 299 00:10:51,540 --> 00:10:52,860 so I can do it in three hops. 300 300 00:10:52,860 --> 00:10:55,190 And it keeps going back and forth until both of those 301 301 00:10:55,190 --> 00:10:57,920 get to be such a high number that neither of those routers 302 302 00:10:57,920 --> 00:11:00,200 can actually ever get to that route anymore. 303 303 00:11:00,200 --> 00:11:02,870 Now, this is the idea of what a poisoned reverse would do. 304 304 00:11:02,870 --> 00:11:05,241 But when you use a poison reverse 305 305 00:11:05,241 --> 00:11:06,980 instead of making it go one at a time 306 306 00:11:06,980 --> 00:11:08,650 and keep iterating up, it would immediately just say hey, 307 307 00:11:08,650 --> 00:11:10,970 I know how to get there and it will take me a thousand hops. 308 308 00:11:10,970 --> 00:11:12,430 And so it's got such a huge cost 309 309 00:11:12,430 --> 00:11:13,960 you won't want to send it to me. 310 310 00:11:13,960 --> 00:11:15,330 That's the way that router three can say, 311 311 00:11:15,330 --> 00:11:17,150 I don't have this route anymore. 312 312 00:11:17,150 --> 00:11:18,760 Now if you don't use poisoned reverse 313 313 00:11:18,760 --> 00:11:20,270 and you don't use split horizon, 314 314 00:11:20,270 --> 00:11:23,120 what ends up happening is you have this loop that happens. 315 315 00:11:23,120 --> 00:11:24,853 And two says I can do it in one, 316 316 00:11:25,797 --> 00:11:26,800 three says I can do it in two. 317 317 00:11:26,800 --> 00:11:29,040 Two says I can do it in three and they keep counting up. 318 318 00:11:29,040 --> 00:11:31,157 And that eats up a lot of your resources in your network 319 319 00:11:31,157 --> 00:11:32,560 by causing this loop 320 320 00:11:32,560 --> 00:11:35,080 and those two sending packets back and forth to each other 321 321 00:11:35,080 --> 00:11:38,020 continually increasing that cost until it gets so high 322 322 00:11:38,020 --> 00:11:39,770 that they stop sending traffic. 323 323 00:11:39,770 --> 00:11:41,833 Now if there's no other way to get to that network, 324 324 00:11:41,833 --> 00:11:43,970 it's just going to keep going in this infinite loop 325 325 00:11:43,970 --> 00:11:45,970 and that's going to cause a real problem for you. 326 326 00:11:45,970 --> 00:11:47,780 This is why it's really important to make sure you're 327 327 00:11:47,780 --> 00:11:50,833 setting up your poison reverse or your split horizons.