WEBVTT 1 00:00.000 --> 00:04.040 All right, we have a topology challenge. 2 00:04.040 --> 00:07.280 That means we're starting with this topology and it's our job to figure out 3 00:07.280 --> 00:07.760 what spanning 4 00:07.760 --> 00:12.320 trees going to look like once everything reaches steady state operation. 5 00:12.320 --> 00:14.880 And from there we'll make things a little bit more interesting as we explore 6 00:14.880 --> 00:15.280 different 7 00:15.280 --> 00:16.840 failure scenarios. 8 00:16.840 --> 00:20.560 So first of all, we want to identify all the port roles. 9 00:20.560 --> 00:25.800 So we have to make sure we remember the distinction between a role and a state. 10 00:25.800 --> 00:31.260 A role is one of those three options, we have the root port, the designated WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 10 00:25.800 --> 00:31.260 A role is one of those three options, we have the root port, the designated 11 00:31.260 --> 00:32.000 port, and then 12 00:32.000 --> 00:35.880 we'll either call it the non-designated port or the blocking port. 13 00:35.880 --> 00:38.800 But either way we can tell it's the port that's going to block. 14 00:38.800 --> 00:41.400 Remember our root port is the upstream port. 15 00:41.400 --> 00:43.720 It's going to be our best path to the root. 16 00:43.720 --> 00:46.560 Our designated ports are all downstream ports. 17 00:46.560 --> 00:48.160 So that's the roles. 18 00:48.160 --> 00:52.470 The states are going to be either forwarding or blocking or disabled or 19 00:52.470 --> 00:54.120 listening and learning. 20 00:54.120 --> 00:57.960 There's these five states that they can possibly be in. 21 00:57.960 --> 01:00.940 So in our example here, when we're talking about steady state of operation, we WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 21 00:57.960 --> 01:00.940 So in our example here, when we're talking about steady state of operation, we 22 01:00.940 --> 01:01.400 're really 23 01:01.400 --> 01:04.630 just talking about either forwarding or blocking, since everything will have 24 01:04.630 --> 01:05.720 gone through listening 25 01:05.720 --> 01:09.640 and learning and in theory none of these ports are in a disabled state. 26 01:09.640 --> 01:11.200 So let's go ahead and go through this. 27 01:11.200 --> 01:15.160 The first step in any spanning tree situation or scenario like this is to 28 01:15.160 --> 01:17.160 identify the root. 29 01:17.160 --> 01:21.020 And this scenario makes things a little bit easy for us because the bridge 30 01:21.020 --> 01:22.000 priorities here 31 01:22.000 --> 01:26.720 labeled in this blue with all the Ks there tells us that there's only one 32 01:26.720 --> 01:27.560 switch that 33 01:27.560 --> 01:29.040 can be the root. 34 01:29.040 --> 01:32.650 And that would be switch one right in the middle and go figure it's the switch WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 34 01:29.040 --> 01:32.650 And that would be switch one right in the middle and go figure it's the switch 35 01:32.650 --> 01:32.960 in the 36 01:32.960 --> 01:33.960 middle. 37 01:33.960 --> 01:38.560 But we would want to compare the bridge priorities. 38 01:38.560 --> 01:42.120 And if we have a tie, at that point we have to look at the MAC addresses. 39 01:42.120 --> 01:44.000 Now we are given the MAC addresses here. 40 01:44.000 --> 01:46.200 Those are these purple colors. 41 01:46.200 --> 01:49.680 They all start with a zero and then they have another number after them. 42 01:49.680 --> 01:52.760 And so that will allow us to compare MAC addresses when needed. 43 01:52.760 --> 01:56.660 And if we had another switch that had a 4K priority, we'd have to compare MAC 44 01:56.660 --> 01:57.400 addresses. 45 01:57.400 --> 01:59.680 But since it's the only one, we're already good to go. 46 01:59.680 --> 02:01.560 We've identified the root. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 46 01:59.680 --> 02:01.560 We've identified the root. 47 02:01.560 --> 02:06.960 So from here we can also identify the roles of the root bridge. 48 02:06.960 --> 02:09.880 They're all going to be designated ports. 49 02:09.880 --> 02:15.700 All of the ports on the root are designated because again designated means 50 02:15.700 --> 02:16.880 downstream. 51 02:16.880 --> 02:19.200 Whereas root port means upstream. 52 02:19.200 --> 02:24.000 So they're can't, they're they can, cannot be an upstream port on the root. 53 02:24.000 --> 02:26.240 They all must be downstream. 54 02:26.240 --> 02:31.120 And so we already have a bunch of port roles that we can figure out here. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 54 02:26.240 --> 02:31.120 And so we already have a bunch of port roles that we can figure out here. 55 02:31.120 --> 02:36.550 Now it's easy to assume that on the other side of a root designated port that 56 02:36.550 --> 02:37.040 these 57 02:37.040 --> 02:38.480 are all going to be root ports. 58 02:38.480 --> 02:44.790 But we have to be careful here because switch five has a root path cost of four 59 02:44.790 --> 02:45.080 going to 60 02:45.080 --> 02:46.120 the root. 61 02:46.120 --> 02:48.480 And so yeah, this is going to be the root port. 62 02:48.480 --> 02:53.870 Because going anywhere else is going to be a whole lot more than a cost of four 63 02:53.870 --> 02:53.920 . 64 02:53.920 --> 02:54.920 But that's just it. 65 02:54.920 --> 02:56.720 We have to compare costs. 66 02:56.720 --> 03:00.440 And so looking at switch two for example, cost of eight. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 66 02:56.720 --> 03:00.440 And so looking at switch two for example, cost of eight. 67 03:00.440 --> 03:02.520 Okay, what are my other options? 68 03:02.520 --> 03:05.400 Thirty two, yeah, that's not going to do it. 69 03:05.400 --> 03:10.400 Hey, this path is four, but then it's another 10 and another four. 70 03:10.400 --> 03:12.280 And so we add that up it's 18. 71 03:12.280 --> 03:15.400 So my best path to the root is the directly connected link. 72 03:15.400 --> 03:19.040 And again, in a lot of situations, that's going to be the case. 73 03:19.040 --> 03:20.760 But now let's look at switch three. 74 03:20.760 --> 03:25.000 Switch three has a cost of 12 directly connected to switch one. 75 03:25.000 --> 03:29.190 Meanwhile, we have cost of four cost of four, which means we actually have a 76 03:29.190 --> 03:30.000 path here. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 76 03:29.190 --> 03:30.000 path here. 77 03:30.000 --> 03:32.200 That's a cost of eight. 78 03:32.200 --> 03:35.960 That's the better path, even though I'm directly connected to the root. 79 03:35.960 --> 03:42.280 So believe it or not, this connection is going to be my root port. 80 03:42.280 --> 03:46.460 And yeah, we're going to have to start to figure things out as far as what that 81 03:46.460 --> 03:46.880 means 82 03:46.880 --> 03:48.600 for some of these other links. 83 03:48.600 --> 03:52.690 But at least we can identify which port is best for getting to the root from 84 03:52.690 --> 03:53.180 switch 85 03:53.180 --> 03:54.180 three's perspective. 86 03:54.180 --> 03:56.360 And that would be that link to switch five. 87 03:56.360 --> 04:00.050 And by the way, if that's my best path and there's a root port here, that means WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 87 03:56.360 --> 04:00.050 And by the way, if that's my best path and there's a root port here, that means 88 04:00.050 --> 04:00.400 it's 89 04:00.400 --> 04:03.320 going to be a designated port on the other side. 90 04:03.320 --> 04:07.180 If there was any situation where this is a non-designated ports in a blocking 91 04:07.180 --> 04:07.680 state, 92 04:07.680 --> 04:09.280 that wouldn't work so well. 93 04:09.280 --> 04:13.680 So naturally, if we have a root port, it has to be a designated port on the 94 04:13.680 --> 04:14.760 other side. 95 04:14.760 --> 04:15.760 Make sense. 96 04:15.760 --> 04:17.320 I'm going to send BPDUs this way. 97 04:17.320 --> 04:21.010 It's a downstream connection from switch five's perspective and it's an 98 04:21.010 --> 04:22.120 upstream connection 99 04:22.120 --> 04:24.920 from switch three's perspective. 100 04:24.920 --> 04:30.120 Now anytime we're in a scenario where we're doing this type of exploration of a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 100 04:24.920 --> 04:30.120 Now anytime we're in a scenario where we're doing this type of exploration of a 101 04:30.120 --> 04:30.800 spanning 102 04:30.800 --> 04:33.880 tree topology, we say, okay, well, where do we go next? 103 04:33.880 --> 04:37.150 What we can start to do is identify where there are loops because if there's a 104 04:37.150 --> 04:37.640 loop, 105 04:37.640 --> 04:39.680 then we know there must be a blocked port. 106 04:39.680 --> 04:42.720 And so we can start to take a look at things from that angle. 107 04:42.720 --> 04:46.290 And so when I'm looking at switches one, three, and five, I see that there's a 108 04:46.290 --> 04:46.560 loop 109 04:46.560 --> 04:47.400 here. 110 04:47.400 --> 04:52.540 And so far I've identified a bunch of port roles and there's one port where I 111 04:52.540 --> 04:53.200 haven't 112 04:53.200 --> 04:56.760 explored and there's probably means that there's going to be a blocked port 113 04:56.760 --> 04:58.080 right there. 114 04:58.080 --> 05:01.540 Now if there's additional ports, like let's say here, there's a loop, there's a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 114 04:58.080 --> 05:01.540 Now if there's additional ports, like let's say here, there's a loop, there's a 115 05:01.540 --> 05:01.840 lot of 116 05:01.840 --> 05:04.720 other ports to explore at this point. 117 05:04.720 --> 05:09.210 But here on this side, we can just immediately say, well, there has to be at 118 05:09.210 --> 05:10.160 least one, not 119 05:10.160 --> 05:13.440 well, ideally just one non designated port within a loop. 120 05:13.440 --> 05:16.760 And so that means that this is going to be the case here. 121 05:16.760 --> 05:19.040 Now how would we figure that out? 122 05:19.040 --> 05:24.060 Well the way we figure it out is the fact that switch one, and when I say 123 05:24.060 --> 05:24.840 figure it 124 05:24.840 --> 05:30.230 out, I mean using actual spanning tree metrics, switch one is advertising a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 124 05:24.840 --> 05:30.230 out, I mean using actual spanning tree metrics, switch one is advertising a 125 05:30.230 --> 05:31.400 cost of zero. 126 05:31.400 --> 05:33.640 The cost of zero can't be beat. 127 05:33.640 --> 05:39.050 And so we know that this is going to have to be an upstream port, meaning that 128 05:39.050 --> 05:39.760 the BPD 129 05:39.760 --> 05:42.960 use are going to flow into that interface. 130 05:42.960 --> 05:46.470 Well if it's an upstream port, I already have my root port figured out and 131 05:46.470 --> 05:46.960 therefore an 132 05:46.960 --> 05:50.560 upstream port is going to be non designated and blocking. 133 05:50.560 --> 05:55.690 So this is where we can use our spanning tree knowledge on top of a little bit 134 05:55.690 --> 05:56.360 of logic 135 05:56.360 --> 06:00.620 here to say that, well, there's only one port left, it must be non-blot or non WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 135 05:56.360 --> 06:00.620 here to say that, well, there's only one port left, it must be non-blot or non 136 06:00.620 --> 06:01.320 designated 137 06:01.320 --> 06:02.440 and it must be blocking. 138 06:02.440 --> 06:05.800 So either one of those paths of logic would work. 139 06:05.800 --> 06:08.950 Naturally, we always want to make sure we understand the spanning tree 140 06:08.950 --> 06:09.920 mechanisms that 141 06:09.920 --> 06:12.560 ultimately lead to those decisions. 142 06:12.560 --> 06:18.620 So next we see a loop here and we have a link that doesn't have port roles 143 06:18.620 --> 06:19.560 identified 144 06:19.560 --> 06:20.560 on either side. 145 06:20.560 --> 06:22.840 So what are we going to do here? 146 06:22.840 --> 06:26.630 The good news is we've identified the roles on this link and the roles on this 147 06:26.630 --> 06:27.120 link so 148 06:27.120 --> 06:32.040 we just have to figure out which side of this link is actually blocking. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 148 06:27.120 --> 06:32.040 we just have to figure out which side of this link is actually blocking. 149 06:32.040 --> 06:33.760 So how does this work? 150 06:33.760 --> 06:38.410 Well we're going to exchange BPDUs and one of these will be inferior and one of 151 06:38.410 --> 06:38.800 them 152 06:38.800 --> 06:40.360 will be superior. 153 06:40.360 --> 06:44.100 So we have to look at the information that's in the BPDU that would make it 154 06:44.100 --> 06:44.960 superior or 155 06:44.960 --> 06:46.600 inferior. 156 06:46.600 --> 06:48.520 So what does make it superior and inferior? 157 06:48.520 --> 06:52.650 Well this is where we need to think through the order of operations and we 158 06:52.650 --> 06:53.360 always start 159 06:53.360 --> 06:54.680 with cost. 160 06:54.680 --> 06:57.240 So what is my cost to the root? 161 06:57.240 --> 07:02.070 So switch two, well I should say any switch that has a root port identified, we WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 161 06:57.240 --> 07:02.070 So switch two, well I should say any switch that has a root port identified, we 162 07:02.070 --> 07:02.520 can say 163 07:02.520 --> 07:04.640 what the cost to the root is. 164 07:04.640 --> 07:09.850 I know that the cost for switch three to get to the root is four plus four and 165 07:09.850 --> 07:11.080 that's eight. 166 07:11.080 --> 07:15.310 And I also know that switch two is just going to go straight to the root and 167 07:15.310 --> 07:16.080 that would 168 07:16.080 --> 07:18.360 be eight as well which is kind of interesting. 169 07:18.360 --> 07:24.400 So we actually are advertising a cost of eight in both directions. 170 07:24.400 --> 07:27.920 So that's not going to help us because we have a tie. 171 07:27.920 --> 07:32.190 And once we have a tie at the cost level this is where we need to start to look WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 171 07:27.920 --> 07:32.190 And once we have a tie at the cost level this is where we need to start to look 172 07:32.190 --> 07:32.800 at the other 173 07:32.800 --> 07:34.440 options. 174 07:34.440 --> 07:36.360 So what are the other options? 175 07:36.360 --> 07:42.230 Well we are going to first of all start with the bridge, ultimately the bridge 176 07:42.230 --> 07:42.760 ID. 177 07:42.760 --> 07:46.750 So yes it starts with a priority comparison and then we can look at our MAC 178 07:46.750 --> 07:47.640 addresses. 179 07:47.640 --> 07:53.720 So for example if we look at the MAC addresses, zero C is lower than zero E. 180 07:53.720 --> 08:00.340 C is hex for twelve E is hex for fourteen and therefore C is better or a better WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 180 07:53.720 --> 08:00.340 C is hex for twelve E is hex for fourteen and therefore C is better or a better 181 08:00.340 --> 08:01.000 shape 182 08:01.000 --> 08:03.240 than E. 183 08:03.240 --> 08:07.210 But if we are looking at the MAC address we're skipping the priority comparison 184 08:07.210 --> 08:07.920 and we have 185 08:07.920 --> 08:11.760 priorities that we're going to start by or start to compare. 186 08:11.760 --> 08:16.170 Switch two's priority is better than switch three's and therefore it actually 187 08:16.170 --> 08:16.720 wins the 188 08:16.720 --> 08:18.800 bridge ID comparison. 189 08:18.800 --> 08:23.530 So even though it has a higher MAC address the priority it takes up the more 190 08:23.530 --> 08:24.420 significant 191 08:24.420 --> 08:30.900 values of the bridge ID as a whole and therefore switch two wins this BPDU WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 191 08:24.420 --> 08:30.900 values of the bridge ID as a whole and therefore switch two wins this BPDU 192 08:30.900 --> 08:32.440 comparison. 193 08:32.440 --> 08:37.540 So switch two is sending the superior BPDU which means switch three is going to 194 08:37.540 --> 08:37.960 stop 195 08:37.960 --> 08:42.160 sending BPDUs and place this port into a blocking state. 196 08:42.160 --> 08:46.480 So this will be our non-designated port and this would be a designated port. 197 08:46.480 --> 08:53.380 Okay well we are coming along we're about halfway done with the topology 198 08:53.380 --> 08:54.120 exploration. 199 08:54.120 --> 08:55.600 Where do we want to go next? 200 08:55.600 --> 08:59.070 Well when we look at the fact that there's two links here we might just want to 201 08:59.070 --> 08:59.560 go ahead 202 08:59.560 --> 09:02.880 and identify what's happening on this side. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 202 08:59.560 --> 09:02.880 and identify what's happening on this side. 203 09:02.880 --> 09:04.080 It's certainly an option. 204 09:04.080 --> 09:09.280 It will also help us to determine switch four's cost to the root potentially 205 09:09.280 --> 09:10.440 because probably 206 09:10.440 --> 09:13.800 switch four and switch five are going to have to compare here as well. 207 09:13.800 --> 09:15.480 So what's going on here? 208 09:15.480 --> 09:20.400 Well when we're looking at switch four we know that either way it's cost to the 209 09:20.400 --> 09:20.800 root 210 09:20.800 --> 09:25.910 is either going to be fourteen or twelve and switch two's cost to the root we 211 09:25.910 --> 09:26.840 've already 212 09:26.840 --> 09:33.100 said is eight which means that it's going to win any kind of comparison here WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 212 09:26.840 --> 09:33.100 said is eight which means that it's going to win any kind of comparison here 213 09:33.100 --> 09:33.800 just on 214 09:33.800 --> 09:35.440 cost alone. 215 09:35.440 --> 09:41.840 So switch two is sending a cost of eight and that means that both of these 216 09:41.840 --> 09:42.600 ports are 217 09:42.600 --> 09:47.080 going to be designated because it's going to win that battle with switch four. 218 09:47.080 --> 09:50.470 Switch four now needs to decide which of these ports and potentially both of 219 09:50.470 --> 09:51.040 them by the 220 09:51.040 --> 09:54.930 way we'll get to that because it might go this way but since they're all 221 09:54.930 --> 09:55.760 upstream or 222 09:55.760 --> 10:00.220 both of these are upstream we're going to have to put one of them into the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 222 09:55.760 --> 10:00.220 both of these are upstream we're going to have to put one of them into the 223 10:00.220 --> 10:00.920 blocking 224 10:00.920 --> 10:01.920 state. 225 10:01.920 --> 10:04.200 So which one do we put? 226 10:04.200 --> 10:08.690 Well this is where we need to go through our conversations so first and 227 10:08.690 --> 10:09.560 foremost we 228 10:09.560 --> 10:11.360 always start with cost. 229 10:11.360 --> 10:15.920 It's really easy to jump into port IDs and things like that but if one of these 230 10:15.920 --> 10:16.280 was a 231 10:16.280 --> 10:20.040 cost of eight we would always take the better cost. 232 10:20.040 --> 10:24.400 Doesn't matter what the port IDs are but in this case they're both the same and 233 10:24.400 --> 10:26.080 therefore 234 10:26.080 --> 10:27.240 I can't use cost. 235 10:27.240 --> 10:31.020 And I also can't use bridge ID by the way because switch two is sending both of WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 235 10:27.240 --> 10:31.020 And I also can't use bridge ID by the way because switch two is sending both of 236 10:31.020 --> 10:31.360 these 237 10:31.360 --> 10:37.080 BP to use down and therefore the bridge ID is the same coming from switch two. 238 10:37.080 --> 10:41.000 So that leads us to the next conversation which is the port ID. 239 10:41.000 --> 10:46.840 The port ID is combining our port priority with the port number. 240 10:46.840 --> 10:51.910 Now we didn't give port priorities here so they're all default to 128 and so we 241 10:51.910 --> 10:52.360 can't 242 10:52.360 --> 10:58.830 use the priority part but we can dig into the deeper into the port ID and look 243 10:58.830 --> 10:59.440 at the 244 10:59.440 --> 11:01.640 actual port number. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 244 10:59.440 --> 11:01.640 actual port number. 245 11:01.640 --> 11:06.320 So the question is this switch four has clearly a port two and a port three and 246 11:06.320 --> 11:06.800 this is the 247 11:06.800 --> 11:08.160 lower number. 248 11:08.160 --> 11:15.300 Meanwhile switch two has zero and one with one or zero rather being the lower 249 11:15.300 --> 11:16.200 number. 250 11:16.200 --> 11:19.960 So which one of these gets placed into a blocking state? 251 11:19.960 --> 11:24.080 We might look at switch four and say well two wins and therefore three gets 252 11:24.080 --> 11:24.760 placed into 253 11:24.760 --> 11:30.760 a blocking but always remember that port IDs are an outbound concept. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 253 11:24.760 --> 11:30.760 a blocking but always remember that port IDs are an outbound concept. 254 11:30.760 --> 11:35.840 So when switch two is pushing these BP to use down it's going to let it know on 255 11:35.840 --> 11:36.200 the other 256 11:36.200 --> 11:40.210 side switch four here that hey this is coming from a port ID of zero and this 257 11:40.210 --> 11:41.000 is coming from 258 11:41.000 --> 11:47.560 a port ID of really port numbers at this point but port number of one. 259 11:47.560 --> 11:53.820 So switch four is going to see zero as actually winning out and therefore 260 11:53.820 --> 11:55.560 because zero wins 261 11:55.560 --> 12:01.600 I'm going to block the higher port from switch two's perspective port one and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 261 11:55.560 --> 12:01.600 I'm going to block the higher port from switch two's perspective port one and 262 12:01.600 --> 12:02.720 at this point 263 12:02.720 --> 12:08.440 now switch four has to decide is this my root port or is this my root port. 264 12:08.440 --> 12:12.520 So now that we're down to just this last conversation let's take a look at this 265 12:12.520 --> 12:12.600 . 266 12:12.600 --> 12:15.400 So switch four and switch five. 267 12:15.400 --> 12:19.760 Switch five is advertising a cost of four. 268 12:19.760 --> 12:25.900 Switch four we know is saying hey I don't actually I mean I've got a cost of 269 12:25.900 --> 12:26.760 twelve it 270 12:26.760 --> 12:30.160 looks like wrong one there we go has a cost of twelve. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 270 12:26.760 --> 12:30.160 looks like wrong one there we go has a cost of twelve. 271 12:30.160 --> 12:36.120 And so that is going to lose to the cost of four. 272 12:36.120 --> 12:41.080 And therefore switch four is going to place this port into blocking as a non 273 12:41.080 --> 12:41.960 designated 274 12:41.960 --> 12:48.400 port and switch five will maintain that side as a designated port. 275 12:48.400 --> 12:52.820 But here's the thing we kind of jumped into that but technically switch four 276 12:52.820 --> 12:53.560 has a choice 277 12:53.560 --> 12:59.030 it can go this way I call this my root port we just know it's an upstream port 278 12:59.030 --> 12:59.600 or I can 279 12:59.600 --> 13:01.640 go this way. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 279 12:59.600 --> 13:01.640 go this way. 280 13:01.640 --> 13:06.290 But we see that this path is again a cost of twelve this path is a cost of 281 13:06.290 --> 13:07.320 fourteen which 282 13:07.320 --> 13:14.280 is why there we go this port here is going to become the root port. 283 13:14.280 --> 13:20.180 So now we have our full spanning tree topology there are no loops in this 284 13:20.180 --> 13:22.000 entire environment 285 13:22.000 --> 13:26.960 and we could highlight all of the links that are actually still online just to 286 13:26.960 --> 13:27.760 prove that 287 13:27.760 --> 13:35.520 there are no loops and so as I whoops yeah no I highlighted the wrong one there WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 287 13:27.760 --> 13:35.520 there are no loops and so as I whoops yeah no I highlighted the wrong one there 288 13:35.520 --> 13:35.960 . 289 13:35.960 --> 13:40.710 Okay so this one this one shouldn't highlight any of them that have a blocked 290 13:40.710 --> 13:41.480 link there 291 13:41.480 --> 13:42.480 we go. 292 13:42.480 --> 13:48.300 So that forms a letter N look at that up down and up and all of the other links 293 13:48.300 --> 13:48.800 have 294 13:48.800 --> 13:54.320 a blocked port on them and so we do indeed have a loop free topology. 295 13:54.320 --> 13:59.300 So that was quite an endeavor but it is absolutely worth it for us to go 296 13:59.300 --> 14:00.880 through this process WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 296 13:59.300 --> 14:00.880 through this process 297 14:00.880 --> 14:05.260 and to make sure we understand how to identify what's happening on each link it 298 14:05.260 --> 14:05.920 starts by 299 14:05.920 --> 14:09.930 identifying the root we then go into identifying as many designated ports and 300 14:09.930 --> 14:10.680 root ports as 301 14:10.680 --> 14:14.820 we can and then we just sort of tackle every single loop one at a time and 302 14:14.820 --> 14:15.720 eventually it 303 14:15.720 --> 14:20.430 does become clear which of these ports are going to go into or as a going to 304 14:20.430 --> 14:21.560 non designated 305 14:21.560 --> 14:26.100 and start blocking and which of these ports are going to be designated ports 306 14:26.100 --> 14:26.600 and so on 307 14:26.600 --> 14:27.160 and so forth. 308 14:27.160 --> 14:37.160 [BLANK_AUDIO]