WEBVTT 1 00:00.000 --> 00:05.040 We know that spanning true protocols going to start by establishing a root 2 00:05.040 --> 00:06.280 bridge. 3 00:06.280 --> 00:09.040 And we might say, okay, well, cool, these links are probably going to stay 4 00:09.040 --> 00:09.520 online. 5 00:09.520 --> 00:11.420 But what do we do with this link? 6 00:11.420 --> 00:13.800 We know we need to block a port, but is it on this side? 7 00:13.800 --> 00:15.240 Is it on this side? 8 00:15.240 --> 00:19.230 And so in order to start having that conversation, we need to understand how 9 00:19.230 --> 00:20.520 exactly the different 10 00:20.520 --> 00:22.520 ports can operate. 11 00:22.520 --> 00:26.000 And it starts with the concept of port roles. 12 00:26.000 --> 00:31.360 For now, we're going to assume, in this case, that this link on switch three is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 12 00:26.000 --> 00:31.360 For now, we're going to assume, in this case, that this link on switch three is 13 00:31.360 --> 00:31.920 going to 14 00:31.920 --> 00:32.920 be blocked. 15 00:32.920 --> 00:35.080 It gets placed into the blocking state. 16 00:35.080 --> 00:38.680 We'll be talking about the states in more detail in the next video. 17 00:38.680 --> 00:42.510 But at this point, if it's blocking, we can understand that any traffic that's 18 00:42.510 --> 00:43.040 received 19 00:43.040 --> 00:45.080 is going to get dropped. 20 00:45.080 --> 00:48.950 And so we do prevent the loop in this case, making it so this link and this 21 00:48.950 --> 00:49.840 link are truly 22 00:49.840 --> 00:52.480 the only ones that are active in the environment. 23 00:52.480 --> 00:55.440 And we can see how that's loop free. 24 00:55.440 --> 01:01.040 And each one of these ports is operating in a particular role. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 24 00:55.440 --> 01:01.040 And each one of these ports is operating in a particular role. 25 01:01.040 --> 01:04.450 And this is what's going to lead us to determining the topology that we're kind 26 01:04.450 --> 01:05.720 of working our 27 01:05.720 --> 01:07.360 way backwards from. 28 01:07.360 --> 01:09.760 So let's go and start with the definition of port roles. 29 01:09.760 --> 01:11.840 There are three primary port roles. 30 01:11.840 --> 01:17.520 We have the root port, which can be called the RP. 31 01:17.520 --> 01:25.480 We have a designated port, which can be called a DP. 32 01:25.480 --> 01:31.600 And then we also have a non-designated port. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 32 01:25.480 --> 01:31.600 And then we also have a non-designated port. 33 01:31.600 --> 01:36.320 And sometimes these are called blocked ports. 34 01:36.320 --> 01:39.120 So we'll see it called that way by engineers. 35 01:39.120 --> 01:41.120 We'll see it this way in documentation. 36 01:41.120 --> 01:45.280 But sometimes we'll be referred to as NDPs for non-designated ports. 37 01:45.280 --> 01:49.120 And sometimes we might see it as BP or a blocked port. 38 01:49.120 --> 01:53.750 So we can look at this and probably assume that this port here is running as a 39 01:53.750 --> 01:54.320 blocked 40 01:54.320 --> 01:56.040 port. 41 01:56.040 --> 01:57.920 And that would be the case. 42 01:57.920 --> 01:59.440 But what about the other ports? 43 01:59.440 --> 02:01.840 I mean, they're all forwarding traffic. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 43 01:59.440 --> 02:01.840 I mean, they're all forwarding traffic. 44 02:01.840 --> 02:05.400 What's the difference between these other two port roles? 45 02:05.400 --> 02:07.560 Well, let's go ahead and take a look at that. 46 02:07.560 --> 02:09.560 So first of all, the root port. 47 02:09.560 --> 02:16.490 The root port is going to be my upstream port, which means that it's going 48 02:16.490 --> 02:19.800 towards the root. 49 02:19.800 --> 02:24.040 When I look at things from Switch3's perspective, we could look at Switch3 and 50 02:24.040 --> 02:24.920 say, OK, this 51 02:24.920 --> 02:27.200 is my downstream direction. 52 02:27.200 --> 02:30.770 But if I look at it from a tree perspective, I realize I've got two different WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 52 02:27.200 --> 02:30.770 But if I look at it from a tree perspective, I realize I've got two different 53 02:30.770 --> 02:31.360 upstream 54 02:31.360 --> 02:33.720 paths to get to the root. 55 02:33.720 --> 02:34.720 Because that's what I'm trying to do. 56 02:34.720 --> 02:38.640 I'm trying to figure out how do I get to the root in a loop-free way. 57 02:38.640 --> 02:42.520 So if I have two different upstreams, that's going to be the big red flag. 58 02:42.520 --> 02:47.720 I'm going to know that I'm going to identify the port that has the least cost. 59 02:47.720 --> 02:50.000 This is my best path to the root. 60 02:50.000 --> 02:52.960 And I'm going to call this my root port. 61 02:52.960 --> 02:54.440 Well, Switch3 is going to do the same thing. 62 02:54.440 --> 02:59.840 It's going to see that it has its best connection to the root this way. 63 02:59.840 --> 03:04.800 And so it identifies this port as the root port for itself. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 63 02:59.840 --> 03:04.800 And so it identifies this port as the root port for itself. 64 03:04.800 --> 03:10.000 We're only going to have a single root port on any given switch. 65 03:10.000 --> 03:12.480 It is the only upstream port we're going to use. 66 03:12.480 --> 03:16.200 We're going to block all the other upstream ports if any of them exist. 67 03:16.200 --> 03:20.370 Other upstream ports will become non-designated ports, which is exactly what 68 03:20.370 --> 03:21.400 happened here. 69 03:21.400 --> 03:26.760 My other upstream path to the root is what gets blocked. 70 03:26.760 --> 03:32.320 And this is how we form the loop-free trees. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 70 03:26.760 --> 03:32.320 And this is how we form the loop-free trees. 71 03:32.320 --> 03:33.320 So that's great. 72 03:33.320 --> 03:35.080 This is a designated port. 73 03:35.080 --> 03:39.640 Well, a designated port is a downstream port. 74 03:39.640 --> 03:45.320 So if my root port is upstream, my designated port is downstream. 75 03:45.320 --> 03:50.310 Now we see right away that the root port should have all downstream ports, 76 03:50.310 --> 03:50.720 which means it's 77 03:50.720 --> 03:53.840 going to have all designated ports kind of interesting. 78 03:53.840 --> 04:00.000 So I can immediately assign these ports to be in the designated port role. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 78 03:53.840 --> 04:00.000 So I can immediately assign these ports to be in the designated port role. 79 04:00.000 --> 04:04.450 But down here with switch 2, we know that I'm going to see this as being a 80 04:04.450 --> 04:05.320 downstream 81 04:05.320 --> 04:06.640 path. 82 04:06.640 --> 04:09.060 So because this is considered to be downstream, I'm going to place this into a 83 04:09.060 --> 04:09.560 designated 84 04:09.560 --> 04:10.920 port status. 85 04:10.920 --> 04:13.890 And then on the other side, we're going to call this either a blocked port or 86 04:13.890 --> 04:14.240 we could 87 04:14.240 --> 04:15.960 call it a non-designated port. 88 04:15.960 --> 04:18.520 Again, either of those terms works. 89 04:18.520 --> 04:22.160 Switch 3 has to block that interface. 90 04:22.160 --> 04:25.900 Now there's a few takeaways from this conversation that we're going to want to 91 04:25.900 --> 04:26.960 keep in mind because 92 04:26.960 --> 04:30.260 they'll help us when we're trying to figure out which ports would be blocked in WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 92 04:26.960 --> 04:30.260 they'll help us when we're trying to figure out which ports would be blocked in 93 04:30.260 --> 04:30.720 a spanning 94 04:30.720 --> 04:32.280 tree topology. 95 04:32.280 --> 04:38.680 First and foremost, the root bridge is going to only have designated ports. 96 04:38.680 --> 04:42.240 We already said that, but it's worth pointing out here. 97 04:42.240 --> 04:48.460 Now additionally, when I look at a root bridge here, so the root bridge is 98 04:48.460 --> 04:49.560 going to have 99 04:49.560 --> 04:58.040 a DP and the DP is going to connect to another root port. 100 04:58.040 --> 04:59.480 This is in most cases. 101 04:59.480 --> 05:03.070 There will be some really oddball scenarios where this doesn't happen, but we WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 101 04:59.480 --> 05:03.070 There will be some really oddball scenarios where this doesn't happen, but we 102 05:03.070 --> 05:03.400 already 103 05:03.400 --> 05:04.400 see that. 104 05:04.400 --> 05:08.000 We see that my designated port here connects to a root port down here. 105 05:08.000 --> 05:11.520 My designated port here connects to a root port down here. 106 05:11.520 --> 05:14.400 This could change if this is a really bad link. 107 05:14.400 --> 05:19.200 Maybe this is 10 meg and these over here are 100 meg, something like that. 108 05:19.200 --> 05:22.780 At that point, we would probably prefer to go the other way and this wouldn't 109 05:22.780 --> 05:23.080 be my 110 05:23.080 --> 05:27.720 root port at that point, but we'll get into costs and figuring that all out. 111 05:27.720 --> 05:31.580 In this topology, we're just sort of assuming all of these links are created WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 111 05:27.720 --> 05:31.580 In this topology, we're just sort of assuming all of these links are created 112 05:31.580 --> 05:31.960 equal. 113 05:31.960 --> 05:35.740 So at that point, the root bridge's designated port is going to connect to 114 05:35.740 --> 05:36.640 other bridges 115 05:36.640 --> 05:38.840 root ports. 116 05:38.840 --> 05:46.210 Now another takeaway is that a DP, a designated port, cannot communicate with 117 05:46.210 --> 05:47.680 another designated 118 05:47.680 --> 05:48.680 port. 119 05:48.680 --> 05:52.880 Remember, designated port implies downstream and that's one of the biggest take 120 05:52.880 --> 05:53.360 aways from 121 05:53.360 --> 05:54.520 this conversation. 122 05:54.520 --> 05:58.360 A root port implies upstream, designated port implies downstream. 123 05:58.360 --> 06:01.920 We can't both be sending downstream to one another. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 123 05:58.360 --> 06:01.920 We can't both be sending downstream to one another. 124 06:01.920 --> 06:05.320 One of these interfaces has to be considered to be upstream. 125 06:05.320 --> 06:12.550 So a designated port will always communicate either downstream to a root port, 126 06:12.550 --> 06:13.160 which remember, 127 06:13.160 --> 06:17.770 root port is upstream, or a designated port will communicate to a non-design 128 06:17.770 --> 06:18.560 ated port, 129 06:18.560 --> 06:23.090 which is also upstream, which makes sense because again, these are going to be 130 06:23.090 --> 06:23.880 considered 131 06:23.880 --> 06:24.880 downstream. 132 06:24.880 --> 06:29.400 Now also notice in all of this that only one side of a link gets blocked. 133 06:29.400 --> 06:31.680 We're not just blocking this whole link. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 133 06:29.400 --> 06:31.680 We're not just blocking this whole link. 134 06:31.680 --> 06:36.560 We're only blocking this port, meaning that this switch right here, this 135 06:36.560 --> 06:37.880 designated port, 136 06:37.880 --> 06:39.640 is treated as normal. 137 06:39.640 --> 06:43.770 If a broadcast packet comes in to the switch to from a host, it's going to send 138 06:43.770 --> 06:44.160 it out 139 06:44.160 --> 06:45.280 this link. 140 06:45.280 --> 06:47.600 It's also going to send it out this link. 141 06:47.600 --> 06:52.060 It doesn't care or otherwise it doesn't impact it whether or not the downstream 142 06:52.060 --> 06:52.960 port is blocking 143 06:52.960 --> 06:54.040 or not. 144 06:54.040 --> 06:57.820 This harkens back to the days where we could have had a hub in here, which has 145 06:57.820 --> 06:58.480 additional 146 06:58.480 --> 06:59.680 hosts. 147 06:59.680 --> 07:02.760 If we have a hub that has hosts hanging off of it, then we still need to get WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 147 06:59.680 --> 07:02.760 If we have a hub that has hosts hanging off of it, then we still need to get 148 07:02.760 --> 07:03.600 those broadcast 149 07:03.600 --> 07:08.820 packets to those hosts, even though switch three is going to end up blocking it 150 07:08.820 --> 07:08.880 . 151 07:08.880 --> 07:13.760 So that behavior doesn't change even in a point-to-point switch-based world. 152 07:13.760 --> 07:18.200 Now one last conversation, recall that the root bridge creates BP DU's and 153 07:18.200 --> 07:18.880 sends them 154 07:18.880 --> 07:19.880 out. 155 07:19.880 --> 07:22.880 It's going to send them out by default every two seconds. 156 07:22.880 --> 07:24.920 Well these BP DU's flow downstream. 157 07:24.920 --> 07:30.200 So the BP DU flows this way and the BP DU flows this way. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 157 07:24.920 --> 07:30.200 So the BP DU flows this way and the BP DU flows this way. 158 07:30.200 --> 07:32.480 So what happens between switch two and three? 159 07:32.480 --> 07:34.840 Well again, upstream and downstream. 160 07:34.840 --> 07:39.380 If we're flowing downstream, that means that switch two is going to send a BP 161 07:39.380 --> 07:40.000 DU to switch 162 07:40.000 --> 07:41.000 three. 163 07:41.000 --> 07:45.720 If we send that broadcast message across, it's going to send the BP DU across. 164 07:45.720 --> 07:50.900 And the regular receiving of BP DU's is what keeps this port in a blocked state 165 07:50.900 --> 07:50.960 . 166 07:50.960 --> 07:54.460 If the BP DU stops showing up, well switch three is going to say, "Alright, 167 07:54.460 --> 07:55.160 well clearly 168 07:55.160 --> 07:58.560 you're not a path to the root, so I'm going to bring you out of being in a 169 07:58.560 --> 07:59.520 blocking state, 170 07:59.520 --> 08:01.880 which can be really, really bad." WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 170 07:59.520 --> 08:01.880 which can be really, really bad." 171 08:01.880 --> 08:07.120 But the flow of BP DU's, the constant flow of BP DU's, this is what maintains 172 08:07.120 --> 08:08.440 our topology. 173 08:08.440 --> 08:11.910 Even when they're received on blocked ports, they are processed and that again 174 08:11.910 --> 08:12.280 is what 175 08:12.280 --> 08:14.600 keeps that port in a blocked role. 176 08:14.600 --> 08:18.880 But because of that and because of the fact that BP DU's always flow downstream 177 08:18.880 --> 08:19.280 , that 178 08:19.280 --> 08:27.280 means that BP DU's are only ever going to be sent on designated ports. 179 08:27.280 --> 08:30.090 They're never sent on root ports because that's upstream and they're never sent WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 179 08:27.280 --> 08:30.090 They're never sent on root ports because that's upstream and they're never sent 180 08:30.090 --> 08:30.200 on 181 08:30.200 --> 08:31.920 blocked ports because that's upstream.