1 00:00:00,360 --> 00:00:02,610 Spanning Tree Protocol, or STP, 2 00:00:02,610 --> 00:00:04,590 is an additional ethernet feature 3 00:00:04,590 --> 00:00:06,150 that is really important. 4 00:00:06,150 --> 00:00:07,620 Now, when you look at the number for it, 5 00:00:07,620 --> 00:00:10,620 it is known as 802.1d, 6 00:00:10,620 --> 00:00:11,640 and so I want you to write that down 7 00:00:11,640 --> 00:00:12,720 in your note sheet as well. 8 00:00:12,720 --> 00:00:17,190 802.1d is the Spanning Tree Protocol, or STP. 9 00:00:17,190 --> 00:00:19,890 Now, what does Spanning Tree Protocol do? 10 00:00:19,890 --> 00:00:21,900 Well, it allows us to have redundant links 11 00:00:21,900 --> 00:00:23,550 between different switches 12 00:00:23,550 --> 00:00:26,430 and it will prevent loops in our network traffic. 13 00:00:26,430 --> 00:00:28,950 Now, why is it important to prevent these loops? 14 00:00:28,950 --> 00:00:30,390 Well, you may remember that we talked 15 00:00:30,390 --> 00:00:33,720 about the availability of networks is measured in nines. 16 00:00:33,720 --> 00:00:36,120 We want to have five nines of availability. 17 00:00:36,120 --> 00:00:38,940 99.999% uptime, 18 00:00:38,940 --> 00:00:41,070 which means that we're only going to get five minutes 19 00:00:41,070 --> 00:00:43,140 of downtime each and every year. 20 00:00:43,140 --> 00:00:45,300 Now, if I want to have a redundant network, 21 00:00:45,300 --> 00:00:47,880 I have to be able to have multiple links to create that 22 00:00:47,880 --> 00:00:50,550 and give me that five nines of availability. 23 00:00:50,550 --> 00:00:52,140 Now let's take a look at a network 24 00:00:52,140 --> 00:00:55,170 without Spanning Tree Protocol and see how it works. 25 00:00:55,170 --> 00:00:57,750 Now you can see that the MAC address table here 26 00:00:57,750 --> 00:00:59,520 can have corruption that occurs. 27 00:00:59,520 --> 00:01:01,710 Let's say that I have PC2 trying 28 00:01:01,710 --> 00:01:03,600 to send a message to PC1. 29 00:01:03,600 --> 00:01:05,910 You can see that there's a redundant network here. 30 00:01:05,910 --> 00:01:08,730 It can take a path going from Switch 4, to Switch 2, 31 00:01:08,730 --> 00:01:10,920 to Switch 1, over to PC1. 32 00:01:10,920 --> 00:01:14,010 Or it can take a path from Switch 4, to Switch 3, 33 00:01:14,010 --> 00:01:15,870 to Switch 1, to PC1. 34 00:01:15,870 --> 00:01:18,540 And that looks great because we have two different ways, 35 00:01:18,540 --> 00:01:20,910 but if you remember how MAC address tables work 36 00:01:20,910 --> 00:01:23,280 inside our switching tables, you're going to notice 37 00:01:23,280 --> 00:01:25,290 that there's going to be a problem here. 38 00:01:25,290 --> 00:01:28,050 When PC2 reaches out to talk to PC1, 39 00:01:28,050 --> 00:01:31,050 Switch 4 is going to learn that the CC MAC address 40 00:01:31,050 --> 00:01:33,960 for PC2 is coming in from that side. 41 00:01:33,960 --> 00:01:35,460 Now it's going to broadcast that out 42 00:01:35,460 --> 00:01:38,790 to Switch 3 and to Switch 2, who both learn that 43 00:01:38,790 --> 00:01:40,530 and they put that in their MAC address table 44 00:01:40,530 --> 00:01:42,630 for Port 0/2. 45 00:01:42,630 --> 00:01:44,280 Then they go and tell Switch 1 46 00:01:44,280 --> 00:01:46,140 and it's coming from both sides. 47 00:01:46,140 --> 00:01:47,430 So Switch 1 now, 48 00:01:47,430 --> 00:01:50,310 thinks that it can re-broadcast that out both sides, 49 00:01:50,310 --> 00:01:53,100 which then feeds back to Switch 2 and Switch 3, 50 00:01:53,100 --> 00:01:56,340 and this creates a loop known as a switching loop. 51 00:01:56,340 --> 00:01:58,050 Now you can see it here in red 52 00:01:58,050 --> 00:02:00,180 because as the data starts going back, 53 00:02:00,180 --> 00:02:02,257 these interfaces start figuring out, 54 00:02:02,257 --> 00:02:05,160 "Hey, how do I get to network CC? 55 00:02:05,160 --> 00:02:07,170 Well, when I get to network device CC, 56 00:02:07,170 --> 00:02:10,169 that MAC address, is that it comes through both interfaces. 57 00:02:10,169 --> 00:02:12,180 And so both of those switches now tell me 58 00:02:12,180 --> 00:02:14,100 that I can go there for CC 59 00:02:14,100 --> 00:02:16,440 and that means I really don't know which way to go 60 00:02:16,440 --> 00:02:19,590 because as a device on a network, I can only go one way 61 00:02:19,590 --> 00:02:21,570 and I need to choose which way that is." 62 00:02:21,570 --> 00:02:23,460 Now this switching loop can happen 63 00:02:23,460 --> 00:02:25,890 and we get what's called a broadcast storm. 64 00:02:25,890 --> 00:02:26,910 This is what's going to happen 65 00:02:26,910 --> 00:02:28,590 if you don't have Spanning Tree Protocol 66 00:02:28,590 --> 00:02:31,590 in your network, but if you do have Spanning Tree Protocol, 67 00:02:31,590 --> 00:02:33,660 you can actually solve this problem. 68 00:02:33,660 --> 00:02:36,060 So let's talk about how we can get through this. 69 00:02:36,060 --> 00:02:38,370 Now we see this broadcast storm that's happening, 70 00:02:38,370 --> 00:02:41,310 and if this broadcast frame is received by both switches, 71 00:02:41,310 --> 00:02:43,140 they'll start to forward it to each other. 72 00:02:43,140 --> 00:02:45,750 And so, one tells it and the other one tells it back, 73 00:02:45,750 --> 00:02:47,370 and they keep going back and forth. 74 00:02:47,370 --> 00:02:48,540 Think of it like this. 75 00:02:48,540 --> 00:02:49,620 I tell you a secret, 76 00:02:49,620 --> 00:02:51,480 and then you tell me that same secret, 77 00:02:51,480 --> 00:02:53,160 and then I tell it to you again, 78 00:02:53,160 --> 00:02:54,540 and you tell it to me again, 79 00:02:54,540 --> 00:02:56,040 and we keep doing this over and over, 80 00:02:56,040 --> 00:02:58,800 and each time more copies of that secret, 81 00:02:58,800 --> 00:03:00,150 in this case, a frame, 82 00:03:00,150 --> 00:03:03,060 are being forwarded back and forth between each other. 83 00:03:03,060 --> 00:03:04,410 It can actually start replicating 84 00:03:04,410 --> 00:03:06,630 and then being forward again, and again, and again 85 00:03:06,630 --> 00:03:09,060 until your entire network is just consumed up 86 00:03:09,060 --> 00:03:11,430 by all of these copies of this ARC packet 87 00:03:11,430 --> 00:03:13,590 that's being sent out trying to tell everybody 88 00:03:13,590 --> 00:03:15,570 where that device should be. 89 00:03:15,570 --> 00:03:17,760 Now it does takes this to happen over time 90 00:03:17,760 --> 00:03:18,900 through your entire network, 91 00:03:18,900 --> 00:03:20,670 and eventually your network will just 92 00:03:20,670 --> 00:03:22,470 crash under the weight of this. 93 00:03:22,470 --> 00:03:24,720 So if your switch starts having this problem, 94 00:03:24,720 --> 00:03:25,830 the only way to fix it 95 00:03:25,830 --> 00:03:28,170 if you don't have Spanning Tree Protocol involved 96 00:03:28,170 --> 00:03:30,120 is to actually unplug the switch, 97 00:03:30,120 --> 00:03:32,280 wait about 30 seconds for all that data 98 00:03:32,280 --> 00:03:35,160 to be forgotten and lost, and then plug it back in. 99 00:03:35,160 --> 00:03:37,230 Now, that's not a great way to run a network. 100 00:03:37,230 --> 00:03:38,820 So someone decided we're going 101 00:03:38,820 --> 00:03:41,640 to create something electronic to fix this problem, 102 00:03:41,640 --> 00:03:42,990 and that's where STP 103 00:03:42,990 --> 00:03:45,540 or the Spanning Tree Protocol gets involved. 104 00:03:45,540 --> 00:03:48,150 Now, the way STP works is that it uses a thing 105 00:03:48,150 --> 00:03:50,730 called a root and a non-root bridge. 106 00:03:50,730 --> 00:03:53,250 Now, a root bridge is where a switch is elected to act 107 00:03:53,250 --> 00:03:56,580 as a reference point for the entire spanning tree. 108 00:03:56,580 --> 00:03:58,830 The switch is then going to select the lowest bridge ID, 109 00:03:58,830 --> 00:04:02,880 or BID, and that's going to be elected as our root bridge. 110 00:04:02,880 --> 00:04:05,520 Now the bridge ID is made up of a priority value 111 00:04:05,520 --> 00:04:07,770 and a MAC address with the lowest value 112 00:04:07,770 --> 00:04:11,040 being considered the root bridge inside our network. 113 00:04:11,040 --> 00:04:12,900 Now, if everything is considered equal, 114 00:04:12,900 --> 00:04:15,420 we're just going to go with the manufacturer's MAC address 115 00:04:15,420 --> 00:04:18,180 being the lowest and that one will be our root bridge, 116 00:04:18,180 --> 00:04:20,670 whichever has the lowest assigned MAC address. 117 00:04:20,670 --> 00:04:24,510 A non-root bridge is every other switch on the topology. 118 00:04:24,510 --> 00:04:28,020 So one root, everybody else becomes non-root. 119 00:04:28,020 --> 00:04:31,920 Now let's assume here we have Switches 1, 2, 3, and 4 again. 120 00:04:31,920 --> 00:04:33,120 How's it going to end up looking 121 00:04:33,120 --> 00:04:35,280 when we start implementing STP? 122 00:04:35,280 --> 00:04:37,590 Well, if I look at Switch 2 and Switch 3, 123 00:04:37,590 --> 00:04:40,950 their MAC addresses are all twos and all threes accordingly. 124 00:04:40,950 --> 00:04:42,510 Both have the same priority 125 00:04:42,510 --> 00:04:44,820 because they're all using the exact same cabling, 126 00:04:44,820 --> 00:04:46,650 because priority is based on the category 127 00:04:46,650 --> 00:04:48,150 of cable you're using. 128 00:04:48,150 --> 00:04:50,730 Now, who is going to end up being my root bridge? 129 00:04:50,730 --> 00:04:52,560 Well, if all the priorities are the same, 130 00:04:52,560 --> 00:04:53,520 we're going to go with the one 131 00:04:53,520 --> 00:04:55,110 that has the lowest MAC address. 132 00:04:55,110 --> 00:04:57,480 So in this case, it's going to be Switch number two 133 00:04:57,480 --> 00:05:00,030 because it had all twos as its MAC address. 134 00:05:00,030 --> 00:05:02,070 That makes Switch number three, one, 135 00:05:02,070 --> 00:05:05,250 and four all non-root bridges. 136 00:05:05,250 --> 00:05:08,160 Now, when we look at the root bridge, Switch 2 in our case, 137 00:05:08,160 --> 00:05:09,450 we also have to look at the concept 138 00:05:09,450 --> 00:05:13,230 of a root port, designated port, and a non-designated port. 139 00:05:13,230 --> 00:05:15,210 Now, when I talk about a root port, 140 00:05:15,210 --> 00:05:17,880 this has to be assigned on every non-root bridge. 141 00:05:17,880 --> 00:05:19,770 So I talked about Switches 1, 3, 142 00:05:19,770 --> 00:05:22,470 and 4 were all considered non-root bridges, 143 00:05:22,470 --> 00:05:25,140 so each one of those has to have one port assigned 144 00:05:25,140 --> 00:05:26,940 as its root port. 145 00:05:26,940 --> 00:05:29,790 Now, the port that is closest to the root bridge in terms 146 00:05:29,790 --> 00:05:33,180 of cost and its number is going to be the root port. 147 00:05:33,180 --> 00:05:35,850 If the cost is equal and all the cost is determined 148 00:05:35,850 --> 00:05:37,950 based on those cable types we talked about, 149 00:05:37,950 --> 00:05:41,310 then the lowest port number on the switch will be chosen. 150 00:05:41,310 --> 00:05:43,320 The way we determine what the cost is, 151 00:05:43,320 --> 00:05:45,720 is faster cables have a lower cost 152 00:05:45,720 --> 00:05:47,850 and slower cables have a higher cost 153 00:05:47,850 --> 00:05:48,840 because we want to put things 154 00:05:48,840 --> 00:05:50,550 on the fastest cables possible. 155 00:05:50,550 --> 00:05:53,280 So if I have a Cat 3 cable, a Cat 5 cable, 156 00:05:53,280 --> 00:05:55,860 and a Cat 7 cable plugged into the switch, 157 00:05:55,860 --> 00:05:57,840 then the port with the Cat 7 cable is going 158 00:05:57,840 --> 00:05:59,910 to be considered the fastest port 159 00:05:59,910 --> 00:06:02,130 because it has the fastest type of cable on it, 160 00:06:02,130 --> 00:06:04,170 and therefore, it will be the root port 161 00:06:04,170 --> 00:06:06,210 on this non-root bridge. 162 00:06:06,210 --> 00:06:08,040 Now, if you have all the same type of cable, 163 00:06:08,040 --> 00:06:10,710 all Cat 5, or all Cat 6, or all Cat 7, 164 00:06:10,710 --> 00:06:12,630 then we're going to choose the lowest port number, 165 00:06:12,630 --> 00:06:15,660 in this case, port number one on the switch. 166 00:06:15,660 --> 00:06:18,840 Now the designated port is every network segment is going 167 00:06:18,840 --> 00:06:21,270 to have at least one designated port on it. 168 00:06:21,270 --> 00:06:23,160 The port closest to the root bridge in terms 169 00:06:23,160 --> 00:06:26,520 of cost will be considered its designated port. 170 00:06:26,520 --> 00:06:28,080 All of the ports on the root bridge 171 00:06:28,080 --> 00:06:30,060 are considered designated ports 172 00:06:30,060 --> 00:06:31,890 because they all are on the root bridge 173 00:06:31,890 --> 00:06:34,020 and therefore, they're really, really fast. 174 00:06:34,020 --> 00:06:35,430 Now, I'll show you this in a diagram 175 00:06:35,430 --> 00:06:37,650 so it'll make a little bit more sense here. 176 00:06:37,650 --> 00:06:39,933 You can see here the non-designated ports are the ports 177 00:06:39,933 --> 00:06:42,600 that are going to block our traffic for us. 178 00:06:42,600 --> 00:06:44,910 This is the benefit of STP. 179 00:06:44,910 --> 00:06:46,740 This is where your loop-free topology 180 00:06:46,740 --> 00:06:48,360 is going to come into play. 181 00:06:48,360 --> 00:06:50,010 So as we look at this diagram, 182 00:06:50,010 --> 00:06:51,660 you can see I have a single root port 183 00:06:51,660 --> 00:06:53,310 on a non-root bridge. 184 00:06:53,310 --> 00:06:55,830 The non-root bridge was Switch number three. 185 00:06:55,830 --> 00:06:57,510 I designated it as purple 186 00:06:57,510 --> 00:06:59,640 because this is the lowest number port. 187 00:06:59,640 --> 00:07:02,940 Now it's Port 0/1 versus Port 0/2, 188 00:07:02,940 --> 00:07:04,530 and it also has the lowest cost 189 00:07:04,530 --> 00:07:06,870 because the cost of 19 is assigned to anything 190 00:07:06,870 --> 00:07:09,960 that's using fast ethernet or a Cat 5 cable. 191 00:07:09,960 --> 00:07:13,380 Remember, the faster the cable, the lower the cost. 192 00:07:13,380 --> 00:07:15,900 Now all the other ports on this non-root bridge, 193 00:07:15,900 --> 00:07:17,700 in our case, Switch number three, 194 00:07:17,700 --> 00:07:20,160 are going to be considered non-designated. 195 00:07:20,160 --> 00:07:21,930 This means that we're going to make them red. 196 00:07:21,930 --> 00:07:23,910 If you think of it, red, think of it like a stop. 197 00:07:23,910 --> 00:07:26,760 There's no traffic coming through those ports. 198 00:07:26,760 --> 00:07:28,230 Now, when I go to the root bridge, 199 00:07:28,230 --> 00:07:29,850 which was Switch number two, 200 00:07:29,850 --> 00:07:32,640 all of those ports are considered designated. 201 00:07:32,640 --> 00:07:34,590 These are all going to be considered blue in color 202 00:07:34,590 --> 00:07:36,150 as shown in my diagram here, 203 00:07:36,150 --> 00:07:39,330 and when traffic comes in from PC2 to go to PC1, 204 00:07:39,330 --> 00:07:40,620 what is going to happen? 205 00:07:40,620 --> 00:07:44,580 Well, port number 0/2 on Switch 3 is red 206 00:07:44,580 --> 00:07:46,380 and it's not going to let traffic go through it, 207 00:07:46,380 --> 00:07:48,150 it acts as a stop sign. 208 00:07:48,150 --> 00:07:50,880 Traffic going from Switch 4, to Switch 2, to Switch 1, 209 00:07:50,880 --> 00:07:53,640 and over to PC1 will be able to go through 210 00:07:53,640 --> 00:07:56,010 based on the way the diagram shows it here. 211 00:07:56,010 --> 00:07:57,600 If it comes all the way around 212 00:07:57,600 --> 00:08:00,420 and it gets to Switch 4, to Switch 2, to Switch 1, 213 00:08:00,420 --> 00:08:03,570 to Switch 3, it's going to get stopped at the root port. 214 00:08:03,570 --> 00:08:06,630 Because again, the non-designated port here is not going 215 00:08:06,630 --> 00:08:08,940 to allow it to broadcast back through. 216 00:08:08,940 --> 00:08:10,770 This is what prevents our loop, 217 00:08:10,770 --> 00:08:12,660 and this is what's going to make a C for us 218 00:08:12,660 --> 00:08:14,970 in the diagram instead of a circle. 219 00:08:14,970 --> 00:08:16,890 That's the whole benefit here of using root 220 00:08:16,890 --> 00:08:20,010 and non-root bridges is that we put blocks in place, 221 00:08:20,010 --> 00:08:22,020 so that we don't have a circle that completes 222 00:08:22,020 --> 00:08:24,810 and allows things to create a broadcast storm. 223 00:08:24,810 --> 00:08:27,150 Now, each port can go through a couple of states 224 00:08:27,150 --> 00:08:28,980 as they do this process. 225 00:08:28,980 --> 00:08:31,290 Non-designated ports are not forwarding traffic 226 00:08:31,290 --> 00:08:32,880 during normal operations. 227 00:08:32,880 --> 00:08:34,980 That's 'cause they're a red stop sign, right? 228 00:08:34,980 --> 00:08:37,860 They receive information as a bridge protocol data unit, 229 00:08:37,860 --> 00:08:41,340 or BPDU, and once they get that information, 230 00:08:41,340 --> 00:08:42,720 they're not going to do anything with it 231 00:08:42,720 --> 00:08:43,950 and they're not forwarding it. 232 00:08:43,950 --> 00:08:46,260 Because again, those are non-designated ports, 233 00:08:46,260 --> 00:08:48,540 they're red, they stop information. 234 00:08:48,540 --> 00:08:50,940 Now, if a link in the topology goes down though, 235 00:08:50,940 --> 00:08:53,850 then the non-designated port will detect that failure 236 00:08:53,850 --> 00:08:55,830 and it can determine or not it needs 237 00:08:55,830 --> 00:08:58,410 to transition itself into a forwarding state 238 00:08:58,410 --> 00:09:01,590 and become a designated port or a root port. 239 00:09:01,590 --> 00:09:03,480 As it goes through that forwarding state, 240 00:09:03,480 --> 00:09:06,210 it's going to transition through four different states. 241 00:09:06,210 --> 00:09:08,940 These four states are blocking, listening, 242 00:09:08,940 --> 00:09:11,040 learning, and forwarding. 243 00:09:11,040 --> 00:09:13,740 Now first it's blocking, and when it's blocking, 244 00:09:13,740 --> 00:09:16,170 this is when it has that big red X on it. 245 00:09:16,170 --> 00:09:17,880 And it's a non-designated port, 246 00:09:17,880 --> 00:09:18,780 and it's going to take any 247 00:09:18,780 --> 00:09:20,520 of those bridge protocol data units 248 00:09:20,520 --> 00:09:23,550 and it's going to stop them and not forward them through. 249 00:09:23,550 --> 00:09:26,040 They're being used at the beginning and on redundant links, 250 00:09:26,040 --> 00:09:28,110 then we're going to switch to listening. 251 00:09:28,110 --> 00:09:30,690 And it'll do this by populating the MAC address table 252 00:09:30,690 --> 00:09:31,950 and starting to learn, 253 00:09:31,950 --> 00:09:34,320 but it's not forwarding those frames yet. 254 00:09:34,320 --> 00:09:37,170 Again, here we're creating that C, not a circle, 255 00:09:37,170 --> 00:09:39,360 and so we don't have a loop that's happening. 256 00:09:39,360 --> 00:09:42,240 Next, we move from listening to learning. 257 00:09:42,240 --> 00:09:43,650 Now it's going to start processing 258 00:09:43,650 --> 00:09:45,390 those bridge protocol data units. 259 00:09:45,390 --> 00:09:46,890 And when it does that, the switch is going 260 00:09:46,890 --> 00:09:49,650 to determine its role inside the spanning tree. 261 00:09:49,650 --> 00:09:52,110 It's thinking, "Do I need to become a root port? 262 00:09:52,110 --> 00:09:53,850 Do I need to become a designated port? 263 00:09:53,850 --> 00:09:56,460 Or should I just stay as non-designated?" 264 00:09:56,460 --> 00:09:58,230 Then it's going to decide if it needs to go into one 265 00:09:58,230 --> 00:10:01,140 of those states as either a designated port or a root port. 266 00:10:01,140 --> 00:10:02,640 If it decides it needs to do that, 267 00:10:02,640 --> 00:10:04,740 then it's going to start forwarding those frames 268 00:10:04,740 --> 00:10:06,690 and those protocol data units. 269 00:10:06,690 --> 00:10:08,040 Now this is called forwarding 270 00:10:08,040 --> 00:10:10,200 and it starts forwarding those frames over, and over, 271 00:10:10,200 --> 00:10:12,900 and over again, and it starts taking over the process 272 00:10:12,900 --> 00:10:14,520 of being the root port. 273 00:10:14,520 --> 00:10:16,830 Now in our example, we either have a root port 274 00:10:16,830 --> 00:10:19,440 or a non-designated port that are blocking. 275 00:10:19,440 --> 00:10:21,900 We have our designated ports which are forwarding things, 276 00:10:21,900 --> 00:10:24,960 so Switch 3 is not sending traffic through. 277 00:10:24,960 --> 00:10:27,750 Now everything is going to go from Switch 4, to Switch 2, 278 00:10:27,750 --> 00:10:30,270 to Switch 1, to PC1 in our example. 279 00:10:30,270 --> 00:10:32,340 Now if Switch 2 goes down, 280 00:10:32,340 --> 00:10:34,650 what will end up happening is Switch 3 will go 281 00:10:34,650 --> 00:10:36,810 through those four states and it will then be able 282 00:10:36,810 --> 00:10:38,520 to start forwarding that traffic. 283 00:10:38,520 --> 00:10:40,560 It goes from blocking, to listening, 284 00:10:40,560 --> 00:10:42,210 to learning, to forwarding, 285 00:10:42,210 --> 00:10:44,220 and it'll take over as the root bridge 286 00:10:44,220 --> 00:10:46,080 and its ports will become root ports, 287 00:10:46,080 --> 00:10:48,450 and they'll have designated ports on them. 288 00:10:48,450 --> 00:10:51,180 Then they'll be able to keep forwarding on. 289 00:10:51,180 --> 00:10:53,940 Now, all this talk about link cost is really important, 290 00:10:53,940 --> 00:10:56,040 and I kind of glossed over it earlier in the video, 291 00:10:56,040 --> 00:10:58,380 so I want to go a little bit more in depth right now. 292 00:10:58,380 --> 00:11:00,240 The link cost is an association 293 00:11:00,240 --> 00:11:02,130 with the speed of a given link. 294 00:11:02,130 --> 00:11:04,680 As I said before, the lower the link speed, 295 00:11:04,680 --> 00:11:06,990 the higher the cost associated with it. 296 00:11:06,990 --> 00:11:08,280 And so as you can see, 297 00:11:08,280 --> 00:11:10,230 you might have something like a Cat 3 cable, 298 00:11:10,230 --> 00:11:13,140 which is ethernet, and it's only 10 megabits per second. 299 00:11:13,140 --> 00:11:15,150 Now, because that is a very slow cable, 300 00:11:15,150 --> 00:11:16,800 it's going to have a very high cost, 301 00:11:16,800 --> 00:11:19,050 so we'll give it a cost of 100. 302 00:11:19,050 --> 00:11:20,640 Now, when I go to fast ethernet, 303 00:11:20,640 --> 00:11:23,130 which is Cat 5 or 100 megabits per second, 304 00:11:23,130 --> 00:11:24,510 that's a faster connection. 305 00:11:24,510 --> 00:11:26,460 So my cost goes down. 306 00:11:26,460 --> 00:11:29,370 It goes from 100 down to 19. 307 00:11:29,370 --> 00:11:31,740 Now, you don't necessarily have to memorize these numbers 308 00:11:31,740 --> 00:11:33,300 for cost for your exam, 309 00:11:33,300 --> 00:11:35,820 but you should realize that if you have a lower speed, 310 00:11:35,820 --> 00:11:37,350 you're going to have a higher number. 311 00:11:37,350 --> 00:11:38,550 If you have a higher speed, 312 00:11:38,550 --> 00:11:40,230 you're going to have a lower number. 313 00:11:40,230 --> 00:11:42,810 In fact, there's this thing called long STP 314 00:11:42,810 --> 00:11:44,310 that's been adopted recently 315 00:11:44,310 --> 00:11:46,380 because higher link speeds kept being created 316 00:11:46,380 --> 00:11:47,730 and we didn't have much room 317 00:11:47,730 --> 00:11:50,040 to make those numbers smaller and smaller. 318 00:11:50,040 --> 00:11:50,970 So as you can see here 319 00:11:50,970 --> 00:11:53,550 with a fiber connection or a Cat 7 connection, 320 00:11:53,550 --> 00:11:55,500 which might be 10 gigabits per second, 321 00:11:55,500 --> 00:11:57,150 we have a cost of two. 322 00:11:57,150 --> 00:11:59,190 If I went to 100 gigabits per second, 323 00:11:59,190 --> 00:12:00,510 I really can't go much less than two, 324 00:12:00,510 --> 00:12:01,950 I might be able to go to one. 325 00:12:01,950 --> 00:12:04,560 And what they ended up doing with this long STP 326 00:12:04,560 --> 00:12:08,850 was adopting values that actually go for 100 from a Cat 3, 327 00:12:08,850 --> 00:12:11,400 to something like 2 million for a CAT 3, 328 00:12:11,400 --> 00:12:13,860 and then we might have more room here at the bottom 329 00:12:13,860 --> 00:12:16,470 for something like a 10 terabit per second connection. 330 00:12:16,470 --> 00:12:20,040 So again, don't worry too much about the STP cost itself 331 00:12:20,040 --> 00:12:21,810 and the numbers associated with it, 332 00:12:21,810 --> 00:12:23,610 if you're dealing with designing a network, 333 00:12:23,610 --> 00:12:25,350 you can always Google the cost table 334 00:12:25,350 --> 00:12:27,660 and you can have it in your hand as you're designing things. 335 00:12:27,660 --> 00:12:30,240 So you don't need to memorize these for the exam. 336 00:12:30,240 --> 00:12:32,130 So for the exam, I want you to remember 337 00:12:32,130 --> 00:12:34,440 that a lower speed is going to have a higher cost, 338 00:12:34,440 --> 00:12:36,810 and a higher speed is going to have a lower cost. 339 00:12:36,810 --> 00:12:38,850 If you remember that there's that inverse relationship 340 00:12:38,850 --> 00:12:40,410 between speed and cost, 341 00:12:40,410 --> 00:12:42,330 you're going to do good on those questions 342 00:12:42,330 --> 00:12:45,030 that come up on the exam when you're dealing with STP.