WEBVTT 1 00:00.000 --> 00:06.200 So, now that we know how exactly Spanning Tree operates, we start to see that 2 00:06.200 --> 00:06.720 there's 3 00:06.720 --> 00:10.280 a problem with how Spanning Tree works. 4 00:10.280 --> 00:12.720 And to be clear, it does its job really well. 5 00:12.720 --> 00:14.920 It blocks traffic to prevent loops. 6 00:14.920 --> 00:18.040 It makes it so that we don't take our networks down by forming one. 7 00:18.040 --> 00:21.400 It allows us to create redundant connections like what I have drawn here, 8 00:21.400 --> 00:21.920 whereas otherwise 9 00:21.920 --> 00:26.000 we'd have to do something like this just to physically prevent a loop. 10 00:26.000 --> 00:29.480 But we can fully loop it, take advantage of redundancy. 11 00:29.480 --> 00:31.320 However, here's the problem. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 11 00:29.480 --> 00:31.320 However, here's the problem. 12 00:31.320 --> 00:35.990 The original Spanning Tree was not designed with a high level of efficiency and 13 00:35.990 --> 00:36.320 a high 14 00:36.320 --> 00:37.960 level of speed in mind. 15 00:37.960 --> 00:39.880 So, we've got a few issues here. 16 00:39.880 --> 00:42.890 For example, the fact that we said that we've got to wait 30 seconds to bring 17 00:42.890 --> 00:43.400 something 18 00:43.400 --> 00:47.840 online, fine for a new network switch, that doesn't happen very often. 19 00:47.840 --> 00:51.260 Not so good for people plugging their laptops in because they're going to 20 00:51.260 --> 00:51.960 assume that the 21 00:51.960 --> 00:56.830 network is down and unplugged before those 30 seconds are up, especially with 22 00:56.830 --> 00:57.240 modern 23 00:57.240 --> 00:58.240 expectations. 24 00:58.240 --> 01:01.800 But here's the other big problem is with efficiency. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 24 00:58.240 --> 01:01.800 But here's the other big problem is with efficiency. 25 01:01.800 --> 01:02.800 Take a look at this. 26 01:02.800 --> 01:06.880 So, let's say that we've got our switches and we've got the root switch here. 27 01:06.880 --> 01:11.160 And we've decided that in between these two other switches that this switch 28 01:11.160 --> 01:12.040 here is going 29 01:12.040 --> 01:15.680 to place that port into a blocking state. 30 01:15.680 --> 01:19.000 So we know that we've got our designated ports here. 31 01:19.000 --> 01:21.880 We've got our root ports here. 32 01:21.880 --> 01:26.110 And then we've got another designated port over here as well as a non-design 33 01:26.110 --> 01:26.760 ated port 34 01:26.760 --> 01:28.480 there that is blocking. 35 01:28.480 --> 01:30.080 So those are our roles. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 35 01:28.480 --> 01:30.080 So those are our roles. 36 01:30.080 --> 01:31.200 Those are our states as well. 37 01:31.200 --> 01:32.160 We've got one in blocking. 38 01:32.160 --> 01:33.520 The rest are in forwarding. 39 01:33.520 --> 01:34.640 Life is good. 40 01:34.640 --> 01:39.840 And as we've sketched out a few times, we do see that we have built out our 41 01:39.840 --> 01:40.780 logical loop 42 01:40.780 --> 01:42.480 free topology. 43 01:42.480 --> 01:46.440 So where is traffic not flowing right here? 44 01:46.440 --> 01:48.000 That's a good thing. 45 01:48.000 --> 01:49.840 We have prevented a loop. 46 01:49.840 --> 01:52.680 There are no loops in this topology. 47 01:52.680 --> 01:54.480 It's also a bad thing. 48 01:54.480 --> 01:58.360 We have created an idle link. 49 01:58.360 --> 02:02.720 And link bandwidth is a limited resource on networks. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 49 01:58.360 --> 02:02.720 And link bandwidth is a limited resource on networks. 50 02:02.720 --> 02:07.080 And so let's say this is a situation where all of these links are 10 gigabit 51 02:07.080 --> 02:07.800 per second 52 02:07.800 --> 02:12.760 links and we're even maybe using single mode fiber between the switches. 53 02:12.760 --> 02:21.930 Well, at that point, 10 gig fiber optics and cables, these are expensive to 54 02:21.930 --> 02:23.240 deploy. 55 02:23.240 --> 02:27.870 And yet we've just said that with spanning tree, these are we are creating an 56 02:27.870 --> 02:28.840 idle link, 57 02:28.840 --> 02:33.770 meaning that we paid a bunch of money and it's just sitting there not being WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 57 02:28.840 --> 02:33.770 meaning that we paid a bunch of money and it's just sitting there not being 58 02:33.770 --> 02:34.440 used. 59 02:34.440 --> 02:39.160 Now the reality is that back when 802.1d spanning tree came out, we weren't 60 02:39.160 --> 02:39.880 really dealing 61 02:39.880 --> 02:45.230 with 10 gigabit per second links, but we were dealing with an era where 100 meg 62 02:45.230 --> 02:45.720 abits per 63 02:45.720 --> 02:48.160 second was actually pretty expensive. 64 02:48.160 --> 02:53.260 And so we had these 100 meg connections and maybe even gig fiber and at that 65 02:53.260 --> 02:53.920 point, we 66 02:53.920 --> 02:55.760 saw the same issue. 67 02:55.760 --> 02:59.920 They were spending a bunch of money on whether the time were very high 68 02:59.920 --> 03:01.280 bandwidth links and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 68 02:59.920 --> 03:01.280 bandwidth links and 69 03:01.280 --> 03:05.160 nobody was happy with the fact that they were sitting there idle. 70 03:05.160 --> 03:10.350 And so just like with some of the other protocols we've talked about, Cisco 71 03:10.350 --> 03:11.440 decided that they 72 03:11.440 --> 03:14.550 didn't want to wait for an industry solution, which is good because it took the 73 03:14.550 --> 03:15.040 industry 74 03:15.040 --> 03:18.880 a very long time to come up with a solution. 75 03:18.880 --> 03:21.240 Cisco came up with their own solution. 76 03:21.240 --> 03:28.080 And this is where we introduced the concept of Cisco's per VLAN spanning tree. 77 03:28.080 --> 03:38.480 So per VLAN spanning tree often gets abbreviated as PVST. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 77 03:28.080 --> 03:38.480 So per VLAN spanning tree often gets abbreviated as PVST. 78 03:38.480 --> 03:44.710 Now eventually they came out with PVST plus and this is the version that we use 79 03:44.710 --> 03:45.520 today. 80 03:45.520 --> 03:52.160 In fact, eventually eventually Cisco developed rapid PVST plus. 81 03:52.160 --> 03:58.360 And so this is the evolution it went from PVST to PVST plus to rapid PVST plus. 82 03:58.360 --> 04:05.190 And what we're going to focus on here is traditional PVST plus and rapid PVST WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 82 03:58.360 --> 04:05.190 And what we're going to focus on here is traditional PVST plus and rapid PVST 83 04:05.190 --> 04:05.400 plus. 84 04:05.400 --> 04:09.040 Because these are the versions that are supported on modern Cisco hardware. 85 04:09.040 --> 04:13.600 And yes, iOSXE does indeed support both of these. 86 04:13.600 --> 04:19.250 In fact, somehow still to this day, PVST plus not the rapid version is the 87 04:19.250 --> 04:20.240 version that 88 04:20.240 --> 04:22.000 is the default. 89 04:22.000 --> 04:25.640 So what exactly is per VLAN spanning tree protocol? 90 04:25.640 --> 04:29.080 Well, it's right there in the name per VLAN. 91 04:29.080 --> 04:36.030 The idea here is to equate a spanning tree topology within each individual VLAN WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 91 04:29.080 --> 04:36.030 The idea here is to equate a spanning tree topology within each individual VLAN 92 04:36.030 --> 04:36.440 . 93 04:36.440 --> 04:38.000 Now keep in mind what a VLAN is. 94 04:38.000 --> 04:42.160 As we defined before, it is a broadcast domain. 95 04:42.160 --> 04:44.320 And what's a key facet of broadcast domains? 96 04:44.320 --> 04:47.040 Well, the fact that broadcasts are contained within them. 97 04:47.040 --> 04:48.320 So that's interesting. 98 04:48.320 --> 04:52.720 That means that every single VLAN is its own broadcast domain. 99 04:52.720 --> 04:59.150 And therefore, each one could have its own topology without impacting the other 100 04:59.150 --> 05:00.000 VLANs. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 100 04:59.150 --> 05:00.000 VLANs. 101 05:00.000 --> 05:01.320 Now what's a topology? 102 05:01.320 --> 05:02.880 Well, this is a topology. 103 05:02.880 --> 05:05.720 And we see that we have exactly one topology here. 104 05:05.720 --> 05:09.260 We have a bunch of designated ports and root ports and non-designated port 105 05:09.260 --> 05:09.760 right there 106 05:09.760 --> 05:11.720 in the middle is what we're blocking. 107 05:11.720 --> 05:17.000 But that is a single topology that is established by spanning tree protocol. 108 05:17.000 --> 05:21.270 And before Cisco came up with per VLAN spanning tree, this topology is what 109 05:21.270 --> 05:23.120 resulted in creating 110 05:23.120 --> 05:25.240 the idle link. 111 05:25.240 --> 05:29.130 Well Cisco said, you know what we want to do is we want to actually create 112 05:29.130 --> 05:31.200 multiple topologies. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 112 05:29.130 --> 05:31.200 multiple topologies. 113 05:31.200 --> 05:35.040 So we're going to create, let's say a topology here. 114 05:35.040 --> 05:42.260 And let's just say that we have VLAN 10 and VLAN 20 configured in this 115 05:42.260 --> 05:43.960 environment. 116 05:43.960 --> 05:47.960 So VLAN 10 represented by pink is going to get its own topology. 117 05:47.960 --> 05:50.240 It's going to get its own root. 118 05:50.240 --> 05:54.880 It's going to set its own designated ports and blocking ports. 119 05:54.880 --> 05:58.890 Let's say this port over here is blocking, which means it's going to have its 120 05:58.890 --> 05:59.400 own set 121 05:59.400 --> 06:01.240 of active connections. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 121 05:59.400 --> 06:01.240 of active connections. 122 06:01.240 --> 06:04.080 Meanwhile VLAN 20 is going to do the same thing. 123 06:04.080 --> 06:07.160 It's going to establish this switch as the root. 124 06:07.160 --> 06:11.110 It's going to ignore the fact that it's blocking on VLAN 10 because VLAN 10 125 06:11.110 --> 06:11.640 doesn't 126 06:11.640 --> 06:13.560 impact VLAN 20. 127 06:13.560 --> 06:17.880 VLAN 20 will block one of these connections down here. 128 06:17.880 --> 06:24.440 And therefore we now see that VLAN 20 sort of has this topology and VLAN 10 has 129 06:24.440 --> 06:25.320 a topology 130 06:25.320 --> 06:27.080 that looks like this. 131 06:27.080 --> 06:32.130 So we see that we have two different overlapping topologies within the same WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 131 06:27.080 --> 06:32.130 So we see that we have two different overlapping topologies within the same 132 06:32.130 --> 06:33.560 physical domain. 133 06:33.560 --> 06:38.200 So it's not just one topology, it is many. 134 06:38.200 --> 06:43.440 In fact, it's a one-to-one relationship with VLANs. 135 06:43.440 --> 06:47.160 So if I have 50 VLANs, I have 50 spanning tree topologies. 136 06:47.160 --> 06:52.170 Now some switch hardware will support fewer spanning tree topologies than VLANs 137 06:52.170 --> 06:52.400 and that 138 06:52.400 --> 06:55.340 can get dangerous because we don't want to create a VLAN without a spanning 139 06:55.340 --> 06:56.200 tree topology. 140 06:56.200 --> 07:00.880 But for the most part, it's just going to be this one-to-one relationship. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 140 06:56.200 --> 07:00.880 But for the most part, it's just going to be this one-to-one relationship. 141 07:00.880 --> 07:08.270 Now Cisco liked PVST so much that they created it as the only option on a Cisco 142 07:08.270 --> 07:09.480 switch. 143 07:09.480 --> 07:15.440 And so if you have a Cisco switch, you can only run PVST plus or like we said, 144 07:15.440 --> 07:16.040 rapid 145 07:16.040 --> 07:22.100 PVST plus or a industry standard that came later known as MST, which we'll talk 146 07:22.100 --> 07:22.680 about 147 07:22.680 --> 07:23.920 later. 148 07:23.920 --> 07:31.530 But this version of spanning tree up here became known as common spanning tree WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 148 07:23.920 --> 07:31.530 But this version of spanning tree up here became known as common spanning tree 149 07:31.530 --> 07:32.720 or sometimes 150 07:32.720 --> 07:37.000 abbreviated down to, whoops, CST. 151 07:37.000 --> 07:41.080 So you had common spanning tree and per VLAN spanning tree. 152 07:41.080 --> 07:44.850 Now there is compatibility between these, but generally speaking, we do try to 153 07:44.850 --> 07:45.200 avoid 154 07:45.200 --> 07:48.230 connecting them together because it can result in a lot of issues, at least in 155 07:48.230 --> 07:49.760 the real world. 156 07:49.760 --> 07:53.040 But again, Cisco does not support CST. 157 07:53.040 --> 07:55.960 It's not an option on a Cisco switch. 158 07:55.960 --> 07:59.500 And it's worth noting that some switches like Nexus switches don't even support 159 07:59.500 --> 08:00.160 traditional WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 159 07:59.500 --> 08:00.160 traditional 160 08:00.160 --> 08:06.010 PAVST plus anymore, they require a rapid version, which is between rapid PVST 161 08:06.010 --> 08:06.880 plus and the 162 08:06.880 --> 08:10.360 industry MST, which runs rapid under the hood. 163 08:10.360 --> 08:13.480 Now as a brief aside, why the plus? 164 08:13.480 --> 08:15.600 Where did the plus come from? 165 08:15.600 --> 08:21.720 Well the original PVST, this was actually only for Cisco devices, meaning that 166 08:21.720 --> 08:22.240 there 167 08:22.240 --> 08:28.480 was no compatibility whatsoever with CST. 168 08:28.480 --> 08:34.660 So if I wanted to connect a Cisco PVST switch to a common spanning tree switch, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 168 08:28.480 --> 08:34.660 So if I wanted to connect a Cisco PVST switch to a common spanning tree switch, 169 08:34.660 --> 08:35.600 this would 170 08:35.600 --> 08:36.600 completely break. 171 08:36.600 --> 08:41.390 It was not doable, whereas PVST plus, like we said, does allow for some amount 172 08:41.390 --> 08:42.600 of compatibility 173 08:42.600 --> 08:45.400 between them, even though we generally don't like to do it. 174 08:45.400 --> 08:48.880 The other thing with PVST was it was ISL only. 175 08:48.880 --> 08:54.590 So we were called during the trunking conversation that the industry landed on 176 08:54.590 --> 08:56.320 802.1q. 177 08:56.320 --> 09:00.840 And so there is no .1q support with the original PVST. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 177 08:56.320 --> 09:00.840 And so there is no .1q support with the original PVST. 178 09:00.840 --> 09:06.130 So PVST plus fixed both of these, it introduced compatibility and it introduced 179 09:06.130 --> 09:07.720 802.1q, just 180 09:07.720 --> 09:12.120 in case you were wondering whatever happened to the original PVST. 181 09:12.120 --> 09:18.470 So now as we wrap up this conversation, we come back to the concept of our 182 09:18.470 --> 09:19.640 designs. 183 09:19.640 --> 09:22.480 Because in the real world, we're going to absolutely take advantage of this. 184 09:22.480 --> 09:27.920 And we're going to see that by creating topologies like this, there is no idle 185 09:27.920 --> 09:29.720 link in the network. 186 09:29.720 --> 09:33.360 Now there's an imbalance for sure, because this link in the middle is going to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 186 09:29.720 --> 09:33.360 Now there's an imbalance for sure, because this link in the middle is going to 187 09:33.360 --> 09:33.800 have the 188 09:33.800 --> 09:37.680 most traffic, it supports traffic from both VLANs, whereas these links down 189 09:37.680 --> 09:38.240 here each 190 09:38.240 --> 09:40.480 only support a single VLAN. 191 09:40.480 --> 09:44.070 And so there's maybe an imbalance, but at least there aren't any idle links, 192 09:44.070 --> 09:44.760 which helps 193 09:44.760 --> 09:49.300 us cost justify redundancy, especially if it's going to involve expensive links 194 09:49.300 --> 09:49.640 like 195 09:49.640 --> 09:53.400 10 gig or 40 gig, something along those lines. 196 09:53.400 --> 09:57.910 And so we can achieve better efficiency, which is exactly why we wanted to take 197 09:57.910 --> 09:58.640 advantage 198 09:58.640 --> 10:01.320 of multiple topologies in a spanning tree environment. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 198 09:58.640 --> 10:01.320 of multiple topologies in a spanning tree environment.