1 1 00:00:00,380 --> 00:00:02,610 Spanning Tree Protocol, or STP, 2 2 00:00:02,610 --> 00:00:05,680 is an additional ethernet feature that is really important, 3 3 00:00:05,680 --> 00:00:08,450 and so we've broken it out into this separate lesson. 4 4 00:00:08,450 --> 00:00:09,920 Now, when you look at the number for it, 5 5 00:00:09,920 --> 00:00:13,930 it is known as 802.1d, and so I want you to write that down 6 6 00:00:13,930 --> 00:00:14,763 in your note sheet as well. 7 7 00:00:14,763 --> 00:00:19,480 802.1d is the Spanning Tree Protocol or STP. 8 8 00:00:19,480 --> 00:00:22,160 Now what does spanning tree protocol do? 9 9 00:00:22,160 --> 00:00:24,210 Well, it allows us to have redundant links 10 10 00:00:24,210 --> 00:00:25,830 between different switches, 11 11 00:00:25,830 --> 00:00:28,720 and it will prevent loops in our network traffic. 12 12 00:00:28,720 --> 00:00:31,240 Now, why is it important to prevent these loops? 13 13 00:00:31,240 --> 00:00:32,150 Well, you may remember, 14 14 00:00:32,150 --> 00:00:34,330 that we talked about the availability of networks 15 15 00:00:34,330 --> 00:00:35,990 is measured in nines. 16 16 00:00:35,990 --> 00:00:40,990 We want to have five nines of availability, 99.999% uptime, 17 17 00:00:41,220 --> 00:00:42,740 which means that we're only going to get 18 18 00:00:42,740 --> 00:00:45,410 five minutes of downtime each and every year. 19 19 00:00:45,410 --> 00:00:47,600 Now, if I want to have redundant network, 20 20 00:00:47,600 --> 00:00:50,180 I have to be able to have multiple links to create that 21 21 00:00:50,180 --> 00:00:52,890 and give me that five nines of availability. 22 22 00:00:52,890 --> 00:00:55,510 Now, there's this thing out there called SPB, 23 23 00:00:55,510 --> 00:00:57,520 or the shortest path bridging, 24 24 00:00:57,520 --> 00:00:59,560 and this is used instead of STP 25 25 00:00:59,560 --> 00:01:01,720 for really large network environments. 26 26 00:01:01,720 --> 00:01:03,930 Now, we're not going to go in depth into SPB, 27 27 00:01:03,930 --> 00:01:06,980 because your exam isn't going to dig into it in depth. 28 28 00:01:06,980 --> 00:01:09,240 If you go on to do the CCNP, 29 29 00:01:09,240 --> 00:01:10,960 and you start working in higher level networks, 30 30 00:01:10,960 --> 00:01:12,840 you may dig into SPB there, 31 31 00:01:12,840 --> 00:01:15,810 but for the Network Plus exam, you really don't need to. 32 32 00:01:15,810 --> 00:01:20,180 Instead, we are going to dig deep into STP, because STP, 33 33 00:01:20,180 --> 00:01:22,200 which is our spanning tree protocol, 34 34 00:01:22,200 --> 00:01:25,050 is what we're going to be using for our smaller networks 35 35 00:01:25,050 --> 00:01:27,610 that are covered from the Network Plus exam. 36 36 00:01:27,610 --> 00:01:29,210 Now, let's take a look at a network 37 37 00:01:29,210 --> 00:01:32,240 without spanning tree protocol and see how it works. 38 38 00:01:32,240 --> 00:01:34,810 Now, you can see that the Mac address table here 39 39 00:01:34,810 --> 00:01:36,570 can have corruption that occurs. 40 40 00:01:36,570 --> 00:01:40,640 Let's say, that I have PC2 trying to send a message to PC1. 41 41 00:01:40,640 --> 00:01:42,940 You can see that there's a redundant network here. 42 42 00:01:42,940 --> 00:01:45,760 It can take a path going from switch four, to switch two, 43 43 00:01:45,760 --> 00:01:47,970 to switch one, over to PC1, 44 44 00:01:47,970 --> 00:01:51,030 or it can take a path from switch four, to switch three, 45 45 00:01:51,030 --> 00:01:52,910 to switch one, to PC1. 46 46 00:01:52,910 --> 00:01:55,590 And that looks great, because we have two different ways, 47 47 00:01:55,590 --> 00:01:57,930 but, if you remember how Mac address tables work 48 48 00:01:57,930 --> 00:01:59,470 inside our switching tables, 49 49 00:01:59,470 --> 00:02:02,330 you're going to notice that there's going to be a problem here. 50 50 00:02:02,330 --> 00:02:05,070 When PC2 reaches out to talk to PC1, 51 51 00:02:05,070 --> 00:02:08,090 switch four is going to learn that the CC Mac address 52 52 00:02:08,090 --> 00:02:11,000 for PC2 is coming in from that side. 53 53 00:02:11,000 --> 00:02:12,530 Now it's going to broadcast that out 54 54 00:02:12,530 --> 00:02:15,840 to switch three and to switch two, who both learn of that, 55 55 00:02:15,840 --> 00:02:17,590 and they put that in their Mac address table 56 56 00:02:17,590 --> 00:02:18,633 for port 0/2. 57 57 00:02:19,690 --> 00:02:21,320 Then, they go and tell switch one, 58 58 00:02:21,320 --> 00:02:23,180 and it's coming from both sides, 59 59 00:02:23,180 --> 00:02:24,760 so switch one now thinks 60 60 00:02:24,760 --> 00:02:27,350 that it can rebroadcast that out both sides, 61 61 00:02:27,350 --> 00:02:30,140 which then feeds back to switch two and switch three, 62 62 00:02:30,140 --> 00:02:33,390 and this creates a loop known as a switching loop. 63 63 00:02:33,390 --> 00:02:35,080 Now you can see it here in red, 64 64 00:02:35,080 --> 00:02:37,230 because as the data starts going back, 65 65 00:02:37,230 --> 00:02:39,350 these interfaces start figuring out, 66 66 00:02:39,350 --> 00:02:42,210 hey, how do I get to network CC? 67 67 00:02:42,210 --> 00:02:45,330 Well, the way I get to network device CC, that Mac address, 68 68 00:02:45,330 --> 00:02:47,230 is that it comes through both interfaces, 69 69 00:02:47,230 --> 00:02:49,250 and so both of those switches now tell me 70 70 00:02:49,250 --> 00:02:51,100 that I can go there for CC, 71 71 00:02:51,100 --> 00:02:53,470 and that means I really don't know which way to go. 72 72 00:02:53,470 --> 00:02:56,630 Because as a device on a network, I can only go one way, 73 73 00:02:56,630 --> 00:02:58,640 and I need to choose which way that is. 74 74 00:02:58,640 --> 00:03:00,460 Now, this switching loop can happen, 75 75 00:03:00,460 --> 00:03:02,940 and we get what's called a broadcast storm. 76 76 00:03:02,940 --> 00:03:03,980 This is what's going to happen, 77 77 00:03:03,980 --> 00:03:06,480 if you don't have spanning tree protocol in your network. 78 78 00:03:06,480 --> 00:03:08,640 But if you do have spanning tree protocol, 79 79 00:03:08,640 --> 00:03:10,700 you can actually solve this problem. 80 80 00:03:10,700 --> 00:03:13,110 So let's talk about how we can get through this. 81 81 00:03:13,110 --> 00:03:15,370 Now we see this broadcast storm that's happening, 82 82 00:03:15,370 --> 00:03:18,360 and if this broadcast frame is received by both switches, 83 83 00:03:18,360 --> 00:03:20,200 they'll start to forward it to each other, 84 84 00:03:20,200 --> 00:03:22,750 and so one tells it, and the other one tells it back, 85 85 00:03:22,750 --> 00:03:24,410 and they keep going back and forth. 86 86 00:03:24,410 --> 00:03:25,580 Think of it like this. 87 87 00:03:25,580 --> 00:03:28,530 I tell you a secret, and then you tell me that same secret, 88 88 00:03:28,530 --> 00:03:30,200 and then I tell it to you again, 89 89 00:03:30,200 --> 00:03:31,480 and you tell it to me again. 90 90 00:03:31,480 --> 00:03:34,210 And we keep doing this over and over, and each time, 91 91 00:03:34,210 --> 00:03:37,200 more copies of that secret, in this case, a frame, 92 92 00:03:37,200 --> 00:03:40,080 are being forwarded back and forth between each other. 93 93 00:03:40,080 --> 00:03:41,480 It can actually start replicating 94 94 00:03:41,480 --> 00:03:43,680 and then being forwarded again, and again, and again, 95 95 00:03:43,680 --> 00:03:46,110 until your entire network is just consumed up 96 96 00:03:46,110 --> 00:03:48,490 by all of these copies of this art packet, 97 97 00:03:48,490 --> 00:03:49,650 that's being sent out, 98 98 00:03:49,650 --> 00:03:52,610 trying to tell everybody where that device should be. 99 99 00:03:52,610 --> 00:03:54,800 Now, it just takes this to happen over time 100 100 00:03:54,800 --> 00:03:56,840 through your entire network, and eventually, 101 101 00:03:56,840 --> 00:03:59,540 your network will just crash under the weight of this. 102 102 00:03:59,540 --> 00:04:01,750 So, if your switch starts having this problem, 103 103 00:04:01,750 --> 00:04:02,900 the only way to fix it, 104 104 00:04:02,900 --> 00:04:05,190 if you don't have spanning tree protocol involved, 105 105 00:04:05,190 --> 00:04:07,180 is to actually unplug the switch, 106 106 00:04:07,180 --> 00:04:09,350 wait about 30 seconds for all that data 107 107 00:04:09,350 --> 00:04:12,210 to be forgotten and lost, and then plug it back in. 108 108 00:04:12,210 --> 00:04:14,270 Now, that's not a great way to run a network, 109 109 00:04:14,270 --> 00:04:17,470 so someone decided we're going to create something electronic 110 110 00:04:17,470 --> 00:04:20,010 to fix this problem, and that's where STP, 111 111 00:04:20,010 --> 00:04:22,580 or the spanning tree protocol, gets involved. 112 112 00:04:22,580 --> 00:04:25,200 Now, the way STP works is that it uses a thing, 113 113 00:04:25,200 --> 00:04:27,790 called a root and a non-root bridge. 114 114 00:04:27,790 --> 00:04:29,910 Now a root bridge is where a switch is elected 115 115 00:04:29,910 --> 00:04:33,630 to act as a reference point for the entire spanning tree, 116 116 00:04:33,630 --> 00:04:35,880 the switch is then going to select the lowest bridge ID, 117 117 00:04:35,880 --> 00:04:39,920 or BID, and that's going to be elected as our root bridge. 118 118 00:04:39,920 --> 00:04:41,510 Now, the bridge ID is made up 119 119 00:04:41,510 --> 00:04:43,780 of a priority value and a Mac address, 120 120 00:04:43,780 --> 00:04:46,550 with the lowest value being considered the root bridge 121 121 00:04:46,550 --> 00:04:48,100 inside our network. 122 122 00:04:48,100 --> 00:04:49,950 Now, if everything is considered equal, 123 123 00:04:49,950 --> 00:04:52,470 we're just going to go with the manufacturer's Mac address, 124 124 00:04:52,470 --> 00:04:55,210 being the lowest, and that one will be our root bridge, 125 125 00:04:55,210 --> 00:04:57,710 whichever has the lowest assigned Mac address. 126 126 00:04:57,710 --> 00:05:01,540 A non-root bridge is every other switch on the topology, 127 127 00:05:01,540 --> 00:05:05,090 so one root, everybody else becomes non-root. 128 128 00:05:05,090 --> 00:05:06,780 Now let's assume here we have switches 129 129 00:05:06,780 --> 00:05:08,980 one, two, three, and four, again. 130 130 00:05:08,980 --> 00:05:10,200 How's it going to end up looking 131 131 00:05:10,200 --> 00:05:12,320 when we start implementing STP? 132 132 00:05:12,320 --> 00:05:14,640 Well, if I look at switch two and switch three, 133 133 00:05:14,640 --> 00:05:18,010 their Mac addresses are all twos and all threes accordingly, 134 134 00:05:18,010 --> 00:05:19,550 both have the same priority, 135 135 00:05:19,550 --> 00:05:21,840 because they're all using the exact same cabling, 136 136 00:05:21,840 --> 00:05:23,070 because priority is based 137 137 00:05:23,070 --> 00:05:25,210 on the category of cable you're using. 138 138 00:05:25,210 --> 00:05:27,780 Now, who is going to end up being my root bridge? 139 139 00:05:27,780 --> 00:05:29,630 Well, if all the priorities are the same, 140 140 00:05:29,630 --> 00:05:32,150 we're going to go with the one that has the lowest Mac address. 141 141 00:05:32,150 --> 00:05:34,510 So, in this case, it's going to be switch number two, 142 142 00:05:34,510 --> 00:05:37,080 because it had all twos as its Mac address. 143 143 00:05:37,080 --> 00:05:40,050 That makes switch number three, one, and four, 144 144 00:05:40,050 --> 00:05:42,320 all non-root bridges. 145 145 00:05:42,320 --> 00:05:43,850 Now, when we look at the root bridge, 146 146 00:05:43,850 --> 00:05:45,200 switch two, in our case, 147 147 00:05:45,200 --> 00:05:47,650 we also have to look at the concept of a root port, 148 148 00:05:47,650 --> 00:05:50,400 a designated port, and a non-designated port. 149 149 00:05:50,400 --> 00:05:52,370 Now, when I talk about a root port, 150 150 00:05:52,370 --> 00:05:55,060 this has to be assigned on every non-root bridge. 151 151 00:05:55,060 --> 00:05:57,590 So I talked about switches one, three, four, 152 152 00:05:57,590 --> 00:05:59,630 were all considered non-root bridges, 153 153 00:05:59,630 --> 00:06:02,300 so each one of those has to have one port assigned 154 154 00:06:02,300 --> 00:06:04,110 as its root port. 155 155 00:06:04,110 --> 00:06:06,560 Now, the port that is closest to the root bridge, 156 156 00:06:06,560 --> 00:06:08,480 in terms of cost and its number, 157 157 00:06:08,480 --> 00:06:10,340 is going to be the root port. 158 158 00:06:10,340 --> 00:06:11,650 If the cost is equal, 159 159 00:06:11,650 --> 00:06:13,020 and all the cost is determined 160 160 00:06:13,020 --> 00:06:15,120 based on those cable types we talked about, 161 161 00:06:15,120 --> 00:06:18,480 then the lowest port number on the switch will be chosen. 162 162 00:06:18,480 --> 00:06:20,460 The way we determine what the cost is, 163 163 00:06:20,460 --> 00:06:22,750 is faster cables have a lower cost, 164 164 00:06:22,750 --> 00:06:24,990 and slower cables have a higher cost, 165 165 00:06:24,990 --> 00:06:26,010 because we want to put things 166 166 00:06:26,010 --> 00:06:27,730 on the fastest cable as possible. 167 167 00:06:27,730 --> 00:06:30,440 So, if I have a cat three cable, a cat five cable, 168 168 00:06:30,440 --> 00:06:33,020 and a cat seven cable plugged into the switch, 169 169 00:06:33,020 --> 00:06:34,690 then the port with the cat seven cable 170 170 00:06:34,690 --> 00:06:37,070 is going to be considered the fastest port, 171 171 00:06:37,070 --> 00:06:39,300 because it has the fastest type of cable on it, 172 172 00:06:39,300 --> 00:06:41,360 and therefore, it will be the root port 173 173 00:06:41,360 --> 00:06:43,370 on this non-root bridge. 174 174 00:06:43,370 --> 00:06:45,200 Now, if you have all the same type of cable, 175 175 00:06:45,200 --> 00:06:47,870 all cat five, or all cat six, or all cat seven, 176 176 00:06:47,870 --> 00:06:49,810 then we're going to choose the lowest port number, 177 177 00:06:49,810 --> 00:06:52,850 in this case, port number one on the switch. 178 178 00:06:52,850 --> 00:06:54,470 Now, the designated port is, 179 179 00:06:54,470 --> 00:06:56,210 every network segment is going to have 180 180 00:06:56,210 --> 00:06:58,450 at least one designated port on it. 181 181 00:06:58,450 --> 00:07:01,020 The port closest to the root bridge, in terms of cost, 182 182 00:07:01,020 --> 00:07:03,700 will be considered its designated port. 183 183 00:07:03,700 --> 00:07:05,260 All of the ports on the root bridge 184 184 00:07:05,260 --> 00:07:07,240 are considered designated ports, 185 185 00:07:07,240 --> 00:07:09,070 because they all are on the root bridge, 186 186 00:07:09,070 --> 00:07:11,180 and therefore, they're really, really fast. 187 187 00:07:11,180 --> 00:07:12,610 Now I'll show you this in a diagram, 188 188 00:07:12,610 --> 00:07:14,820 so it'll make a little bit more sense here. 189 189 00:07:14,820 --> 00:07:15,653 You can see here, 190 190 00:07:15,653 --> 00:07:17,500 the non-designated ports are the ports 191 191 00:07:17,500 --> 00:07:19,780 that are going to block our traffic for us. 192 192 00:07:19,780 --> 00:07:22,090 This is the benefit of STP. 193 193 00:07:22,090 --> 00:07:23,930 This is where your loop free topology 194 194 00:07:23,930 --> 00:07:25,520 is going to come into play. 195 195 00:07:25,520 --> 00:07:27,640 So, as we look at this diagram, you can see, 196 196 00:07:27,640 --> 00:07:30,480 I have a single root port on a non-root bridge. 197 197 00:07:30,480 --> 00:07:33,000 The non-root bridge was switch number three. 198 198 00:07:33,000 --> 00:07:34,660 I designated it as purple, 199 199 00:07:34,660 --> 00:07:36,800 because this is the lowest number port. 200 200 00:07:36,800 --> 00:07:40,090 Now, it's port 0/1 versus port 0/2, 201 201 00:07:40,090 --> 00:07:41,680 and it also has the lowest cost, 202 202 00:07:41,680 --> 00:07:44,050 because the cost of 19 is assigned to anything 203 203 00:07:44,050 --> 00:07:47,110 that's using fast ethernet or a cat five cable. 204 204 00:07:47,110 --> 00:07:50,530 Remember, the faster the cable, the lower the cost. 205 205 00:07:50,530 --> 00:07:53,110 Now, all the other ports on this non-root bridge, 206 206 00:07:53,110 --> 00:07:54,870 in our case, switch number three, 207 207 00:07:54,870 --> 00:07:57,320 are going to be considered non-designated. 208 208 00:07:57,320 --> 00:07:59,100 This means that we're going to make them red, 209 209 00:07:59,100 --> 00:08:01,090 and if you think of it red, think of it like a stop. 210 210 00:08:01,090 --> 00:08:03,920 There's no traffic coming through those ports. 211 211 00:08:03,920 --> 00:08:05,410 Now, when I go to the root bridge, 212 212 00:08:05,410 --> 00:08:07,010 which was switch number two, 213 213 00:08:07,010 --> 00:08:09,810 all of those ports are considered designated. 214 214 00:08:09,810 --> 00:08:11,750 These are all going to be considered blue in color, 215 215 00:08:11,750 --> 00:08:13,290 as shown in my diagram here, 216 216 00:08:13,290 --> 00:08:16,500 and when traffic comes in from PC2 to go to PC1, 217 217 00:08:16,500 --> 00:08:17,810 what is going to happen? 218 218 00:08:17,810 --> 00:08:21,700 Well, port number 0/2 on switch three is red, 219 219 00:08:21,700 --> 00:08:23,570 and it's not going to let traffic go through it. 220 220 00:08:23,570 --> 00:08:25,330 It acts as a stop sign. 221 221 00:08:25,330 --> 00:08:27,320 Traffic going from switch four, to switch two, 222 222 00:08:27,320 --> 00:08:30,810 to switch one, and over to PC1, will be able to go through, 223 223 00:08:30,810 --> 00:08:33,150 based on the way the diagram shows it here. 224 224 00:08:33,150 --> 00:08:35,970 If it comes all the way around, and it gets to switch four, 225 225 00:08:35,970 --> 00:08:38,580 to switch two, to switch one, to switch three, 226 226 00:08:38,580 --> 00:08:41,600 it's going to get stopped at the root port, because again, 227 227 00:08:41,600 --> 00:08:44,140 the non-designated port here is not going to allow it 228 228 00:08:44,140 --> 00:08:46,120 to broadcast back through. 229 229 00:08:46,120 --> 00:08:47,920 This is what prevents our loop, 230 230 00:08:47,920 --> 00:08:50,550 and this is what's going to make a C for us in the diagram 231 231 00:08:50,550 --> 00:08:52,160 instead of a circle. 232 232 00:08:52,160 --> 00:08:53,320 That's the whole benefit here 233 233 00:08:53,320 --> 00:08:55,400 of using root and non-root bridges 234 234 00:08:55,400 --> 00:08:57,160 is that we put blocks in place, 235 235 00:08:57,160 --> 00:08:59,200 so that we don't have a circle that completes 236 236 00:08:59,200 --> 00:09:01,970 and allows things to create a broadcast storm. 237 237 00:09:01,970 --> 00:09:04,330 Now, each port can go through a couple of states 238 238 00:09:04,330 --> 00:09:06,160 as they do this process. 239 239 00:09:06,160 --> 00:09:08,470 Non-designated ports are not forwarding traffic 240 240 00:09:08,470 --> 00:09:10,050 during normal operations, 241 241 00:09:10,050 --> 00:09:12,170 that's 'cause they're a red stop sign, right. 242 242 00:09:12,170 --> 00:09:15,020 They receive information as a bridge protocol data unit, 243 243 00:09:15,020 --> 00:09:18,540 or BPDU, and once they get that information, 244 244 00:09:18,540 --> 00:09:19,900 they're not going to do anything with it, 245 245 00:09:19,900 --> 00:09:21,110 and they're not forwarding it, 246 246 00:09:21,110 --> 00:09:23,440 because again, those are non designated ports. 247 247 00:09:23,440 --> 00:09:25,730 They're red, they stop information. 248 248 00:09:25,730 --> 00:09:28,110 Now, if a link in the topology goes down though, 249 249 00:09:28,110 --> 00:09:31,050 then the non-designated port will detect that failure, 250 250 00:09:31,050 --> 00:09:32,690 and it can determine whether or not 251 251 00:09:32,690 --> 00:09:35,590 it needs to transition itself into a forwarding state 252 252 00:09:35,590 --> 00:09:38,740 and become a designated port or a root port. 253 253 00:09:38,740 --> 00:09:40,650 As it goes through that forwarding state, 254 254 00:09:40,650 --> 00:09:43,380 it's going to transition through four different states. 255 255 00:09:43,380 --> 00:09:44,440 These four states are 256 256 00:09:44,440 --> 00:09:48,200 blocking, listening, learning, and forwarding. 257 257 00:09:48,200 --> 00:09:50,910 Now first, it's blocking, and when it's blocking, 258 258 00:09:50,910 --> 00:09:53,320 this is when it has that big red X on it. 259 259 00:09:53,320 --> 00:09:55,010 And it's a non-designated port, 260 260 00:09:55,010 --> 00:09:55,960 and it's going to take any 261 261 00:09:55,960 --> 00:09:57,690 of those bridge protocol data units, 262 262 00:09:57,690 --> 00:10:00,700 and it's going to stop them and not forward them through. 263 263 00:10:00,700 --> 00:10:02,990 They're being used at the beginning and on redundant links, 264 264 00:10:02,990 --> 00:10:06,130 as shown on the display we had just a few slides ago. 265 265 00:10:06,130 --> 00:10:08,120 Then we're going to switch to listening, 266 266 00:10:08,120 --> 00:10:10,780 and it'll do this by populating the Mac address table 267 267 00:10:10,780 --> 00:10:12,030 and starting to learn, 268 268 00:10:12,030 --> 00:10:14,370 but it's not forwarding those frames yet. 269 269 00:10:14,370 --> 00:10:17,230 Again, here we're creating that C not a circle, 270 270 00:10:17,230 --> 00:10:19,430 and so we don't have a loop that's happening. 271 271 00:10:19,430 --> 00:10:22,320 Next, we move from listening to learning. 272 272 00:10:22,320 --> 00:10:23,730 Now, it's going to start processing 273 273 00:10:23,730 --> 00:10:26,340 those bridge protocol data units, and when it does that, 274 274 00:10:26,340 --> 00:10:28,050 the switch is going to determine its role 275 275 00:10:28,050 --> 00:10:29,710 inside the spanning tree. 276 276 00:10:29,710 --> 00:10:32,210 It's thinking, do I need to become a root port? 277 277 00:10:32,210 --> 00:10:33,930 Do I need to become a designated port, 278 278 00:10:33,930 --> 00:10:36,530 or should I just stay as non designated? 279 279 00:10:36,530 --> 00:10:37,363 Then it's going to decide 280 280 00:10:37,363 --> 00:10:38,870 if it needs to go into one of those states, 281 281 00:10:38,870 --> 00:10:41,220 as either a designated port or a root port. 282 282 00:10:41,220 --> 00:10:42,760 If it decides it needs to do that, 283 283 00:10:42,760 --> 00:10:44,820 then it's going to start forwarding those frames 284 284 00:10:44,820 --> 00:10:46,740 and those protocol data units. 285 285 00:10:46,740 --> 00:10:48,140 Now, this is called forwarding, 286 286 00:10:48,140 --> 00:10:49,580 and it starts forwarding those frames 287 287 00:10:49,580 --> 00:10:50,930 over and over and over again, 288 288 00:10:50,930 --> 00:10:52,970 and it starts taking over the process 289 289 00:10:52,970 --> 00:10:54,600 of being the root port. 290 290 00:10:54,600 --> 00:10:55,740 Now, in our example, 291 291 00:10:55,740 --> 00:10:58,390 we either have a root port or a non-designated port 292 292 00:10:58,390 --> 00:10:59,530 that are blocking. 293 293 00:10:59,530 --> 00:11:00,780 We have our designated ports, 294 294 00:11:00,780 --> 00:11:01,990 which are forwarding things, 295 295 00:11:01,990 --> 00:11:05,040 so switch three is not sending traffic through. 296 296 00:11:05,040 --> 00:11:07,810 Now, everything is going to go from switch four, to switch two, 297 297 00:11:07,810 --> 00:11:10,350 to switch one, to PC1, in our example. 298 298 00:11:10,350 --> 00:11:13,930 Now, if switch two goes down, what will end up happening is, 299 299 00:11:13,930 --> 00:11:16,070 switch three will go through those four states, 300 300 00:11:16,070 --> 00:11:18,580 and it will then be able to start forwarding that traffic. 301 301 00:11:18,580 --> 00:11:20,630 It goes from blocking, to listening, 302 302 00:11:20,630 --> 00:11:22,290 to learning, to forwarding, 303 303 00:11:22,290 --> 00:11:24,250 and it'll take over as the root bridge, 304 304 00:11:24,250 --> 00:11:26,150 and its ports will become root ports, 305 305 00:11:26,150 --> 00:11:28,520 and they'll have designated ports on them, 306 306 00:11:28,520 --> 00:11:31,260 then there'll be able to keep forwarding on. 307 307 00:11:31,260 --> 00:11:34,050 Now, all this talk about link cost is really important, 308 308 00:11:34,050 --> 00:11:36,120 and I kind of glossed over it earlier in the video, 309 309 00:11:36,120 --> 00:11:38,460 so I want to go a little bit more in depth right now. 310 310 00:11:38,460 --> 00:11:40,310 The link cost is an association 311 311 00:11:40,310 --> 00:11:43,290 with the speed of a given link, as I said before, 312 312 00:11:43,290 --> 00:11:44,770 the lower the link speed, 313 313 00:11:44,770 --> 00:11:47,020 the higher the cost associated with it. 314 314 00:11:47,020 --> 00:11:48,370 And so, as you can see, 315 315 00:11:48,370 --> 00:11:50,310 you might have something like a cat three cable, 316 316 00:11:50,310 --> 00:11:53,230 which is ethernet, and it's only 10 megabits per second 317 317 00:11:53,230 --> 00:11:55,240 Now, because that is a very slow cable, 318 318 00:11:55,240 --> 00:11:56,860 it's going to have a very high cost, 319 319 00:11:56,860 --> 00:11:59,140 so we'll give it a cost of 100. 320 320 00:11:59,140 --> 00:12:01,680 Now, when I go to fast ethernet, which is cat five, 321 321 00:12:01,680 --> 00:12:04,580 or 100 megabits per second, that's a faster connection, 322 322 00:12:04,580 --> 00:12:06,540 so my cost goes down. 323 323 00:12:06,540 --> 00:12:09,440 It goes from 100 down to 19. 324 324 00:12:09,440 --> 00:12:10,510 Now, you don't necessarily have 325 325 00:12:10,510 --> 00:12:13,350 to memorize these numbers for cost for your exam, 326 326 00:12:13,350 --> 00:12:15,880 but you should realize that if you have a lower speed, 327 327 00:12:15,880 --> 00:12:17,430 you're going to have a higher number. 328 328 00:12:17,430 --> 00:12:18,630 If you have a higher speed, 329 329 00:12:18,630 --> 00:12:20,300 you're going to have a lower number. 330 330 00:12:20,300 --> 00:12:22,880 In fact, there's this thing called Long STP, 331 331 00:12:22,880 --> 00:12:24,340 that's been adopted recently, 332 332 00:12:24,340 --> 00:12:26,470 because higher link speeds kept being created, 333 333 00:12:26,470 --> 00:12:27,800 and we didn't have much room 334 334 00:12:27,800 --> 00:12:30,110 to make those numbers smaller and smaller. 335 335 00:12:30,110 --> 00:12:31,060 So, as you can see here, 336 336 00:12:31,060 --> 00:12:33,630 with a fiber connection or a cat seven connection, 337 337 00:12:33,630 --> 00:12:35,580 which might be 10 gigabits per second, 338 338 00:12:35,580 --> 00:12:37,240 we have a cost of two. 339 339 00:12:37,240 --> 00:12:39,270 If I went to 100 gigabits per second, 340 340 00:12:39,270 --> 00:12:40,600 I really can't go much less than two, 341 341 00:12:40,600 --> 00:12:42,000 I might be able to go to one. 342 342 00:12:42,000 --> 00:12:44,660 And what they ended up doing with this Long STP 343 343 00:12:44,660 --> 00:12:45,660 was adopting values 344 344 00:12:45,660 --> 00:12:48,880 that actually go for 100 from a cat three 345 345 00:12:48,880 --> 00:12:51,420 to something like 2 million for a cat three, 346 346 00:12:51,420 --> 00:12:53,920 and then we might have more room here at the bottom 347 347 00:12:53,920 --> 00:12:56,550 for something like a 10 terabit per second connection. 348 348 00:12:56,550 --> 00:13:00,010 So again, don't worry too much about the STP cost itself 349 349 00:13:00,010 --> 00:13:01,890 and the numbers associated with it. 350 350 00:13:01,890 --> 00:13:03,700 If you're dealing with designing a network, 351 351 00:13:03,700 --> 00:13:05,450 you can always Google the cost table, 352 352 00:13:05,450 --> 00:13:06,530 and you can have it in your hand 353 353 00:13:06,530 --> 00:13:07,710 as you're designing the thing. 354 354 00:13:07,710 --> 00:13:10,320 So, you don't need to memorize these for the exam. 355 355 00:13:10,320 --> 00:13:12,230 So, for the exam, I want you to remember, 356 356 00:13:12,230 --> 00:13:14,520 that a lower speed is going to have a higher cost 357 357 00:13:14,520 --> 00:13:16,860 and a higher speed is going to have a lower cost. 358 358 00:13:16,860 --> 00:13:17,693 If you remember 359 359 00:13:17,693 --> 00:13:18,940 that there's that inverse relationship 360 360 00:13:18,940 --> 00:13:20,470 between speed and cost, 361 361 00:13:20,470 --> 00:13:22,410 you're going to do good on those questions 362 362 00:13:22,410 --> 00:13:25,110 that come up on the exam when you're dealing with STP.