WEBVTT 1 00:00.000 --> 00:05.540 Cisco supports three different modes of spanning tree operation starting with 2 00:05.540 --> 00:07.280 this legacy PVST+ 3 00:07.280 --> 00:13.420 and now we have seen that we can support rapid PVST+ which takes all the 4 00:13.420 --> 00:16.260 enhancements of rapid spanning tree and applies them to that 5 00:16.260 --> 00:22.500 PVST+ architecture which creates a spanning tree topology per VLAN hence per V 6 00:22.500 --> 00:23.760 LAN spanning tree. 7 00:23.760 --> 00:28.790 But then there's this third option that we can support on Cisco switches which 8 00:28.790 --> 00:31.460 is known as MST multiple spanning tree. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 8 00:28.790 --> 00:31.460 is known as MST multiple spanning tree. 9 00:31.460 --> 00:36.400 It's time for us to talk about what exactly this is and start to understand 10 00:36.400 --> 00:38.760 when we might deploy it into our networks. 11 00:38.760 --> 00:43.920 And this has to do with the fact that Cisco started something that the industry 12 00:43.920 --> 00:44.760 wanted in on. 13 00:44.760 --> 00:49.540 If we think about this from an industry perspective, what the industry had was 14 00:49.540 --> 00:52.760 known as common spanning tree or CST 15 00:52.760 --> 00:58.970 back when Cisco had per VLAN spanning tree plus and CST just had a single top 16 00:58.970 --> 00:59.760 ology. 17 00:59.760 --> 01:04.760 We couldn't leverage all of the blocked links in a CST world. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 17 00:59.760 --> 01:04.760 We couldn't leverage all of the blocked links in a CST world. 18 01:04.760 --> 01:10.150 And so yeah, Cisco switches just had an advantage back in those days as one of 19 01:10.150 --> 01:12.760 the reasons why we'd purchase Cisco switches 20 01:12.760 --> 01:15.710 in addition to maybe some other features and benefits and Cisco did a lot to 21 01:15.710 --> 01:18.760 enhance spanning tree including the topology concept. 22 01:18.760 --> 01:25.260 So then when rapid spanning tree came out or RSTP, this was released as 802.1W 23 01:25.260 --> 01:30.760 as an industry standard and yet it still had a single topology. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 23 01:25.260 --> 01:30.760 as an industry standard and yet it still had a single topology. 24 01:30.760 --> 01:37.360 So it was a whole lot faster but rapid PVST+ still had an advantage over rapid 25 01:37.360 --> 01:39.760 spanning tree protocol. 26 01:39.760 --> 01:45.050 And so now MST comes around and says, "Okay, we like rapid spanning tree 27 01:45.050 --> 01:50.760 protocol but we want to have multiple topologies like Cisco has." 28 01:50.760 --> 01:58.050 And so the industry finally achieved this using MST and believe it or not, they 29 01:58.050 --> 02:02.750 actually were able to enhance or really make more efficient this multiple top WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 29 01:58.050 --> 02:02.750 actually were able to enhance or really make more efficient this multiple top 30 02:02.750 --> 02:03.760 ology concept. 31 02:03.760 --> 02:09.640 So to understand that, let's talk about the concept of per VLAN spanning tree 32 02:09.640 --> 02:12.760 or PVST+ and what exactly it's doing. 33 02:12.760 --> 02:19.750 So recall, again, Cisco had a good thing going with PVST+ and yes, this applies 34 02:19.750 --> 02:25.760 to rapid PVST+ as well because what we had here was multiple topologies. 35 02:25.760 --> 02:32.760 But keep in mind what exactly it actually was. It was a topology per VLAN. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 35 02:25.760 --> 02:32.760 But keep in mind what exactly it actually was. It was a topology per VLAN. 36 02:32.760 --> 02:38.630 And this was huge because if I had a topology like this, a triangle topology, 37 02:38.630 --> 02:43.380 maybe a couple of distribution layer switches up here, we'd have to block one 38 02:43.380 --> 02:43.760 of these. 39 02:43.760 --> 02:48.800 And again, if I only have a single topology, this is the only topology I have. 40 02:48.800 --> 02:52.760 I'm blocking this link which means it might as well not exist. 41 02:52.760 --> 02:57.610 Now, if I had expensive one gig or 10 gig connections, then I paid for a bunch 42 02:57.610 --> 03:01.800 of optics that are just sitting there not being used outside of a failure WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 42 02:57.610 --> 03:01.800 of optics that are just sitting there not being used outside of a failure 43 03:01.800 --> 03:02.760 scenario. 44 03:02.760 --> 03:06.460 Now, the failure scenario was important. We need our redundancy because if this 45 03:06.460 --> 03:10.470 link on the left goes down, then we fire up this link and I've got full 46 03:10.470 --> 03:11.760 bandwidth capabilities. 47 03:11.760 --> 03:16.210 But at the same time, we recognize that it's hard to cost justify maybe 10 gig 48 03:16.210 --> 03:20.500 optics, especially if they're expensive single mode fiber types of optics in 49 03:20.500 --> 03:23.760 this situation because it's just sitting there idle. 50 03:23.760 --> 03:30.570 And so Cisco created the multiple topology concept. So VLAN 10, for example, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 50 03:23.760 --> 03:30.570 And so Cisco created the multiple topology concept. So VLAN 10, for example, 51 03:30.570 --> 03:34.760 could use these two links and block this one over here. 52 03:34.760 --> 03:42.070 Meanwhile, VLAN 20 could use these two links and block this one over here. Now, 53 03:42.070 --> 03:46.290 we're no longer blocking entirely one link, rather we're blocking one link per 54 03:46.290 --> 03:52.720 VLAN, meaning that VLAN 10 has full access to all of the throughput on the left 55 03:52.720 --> 03:57.760 and VLAN 20 has full throughput on the link on the right there. 56 03:57.760 --> 04:02.620 Now, we can apply this to many other VLANs as well. So, for example, I could WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 56 03:57.760 --> 04:02.620 Now, we can apply this to many other VLANs as well. So, for example, I could 57 04:02.620 --> 04:07.920 take VLAN 30 and give it the same topology as VLAN 10, where it's writing on 58 04:07.920 --> 04:10.760 these two links and blocking this link over here. 59 04:10.760 --> 04:16.210 And likewise, then I could do that to VLAN 40 on the right and that will match 60 04:16.210 --> 04:18.760 the topology that VLAN 20 has. 61 04:18.760 --> 04:24.950 But herein lies the problem with per VLAN spanning tree. Each one of these many 62 04:24.950 --> 04:28.760 different VLANs is getting its own topology. 63 04:28.760 --> 04:34.050 In this architecture alone, we have four different topologies. And the reason WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 63 04:28.760 --> 04:34.050 In this architecture alone, we have four different topologies. And the reason 64 04:34.050 --> 04:38.760 for that is because we have four different VLANs. 65 04:38.760 --> 04:46.070 Every single VLAN gets its own topology, which naturally means if we have 100 V 66 04:46.070 --> 04:51.760 LANs, then we're going to have 100 topologies. 67 04:51.760 --> 04:57.360 Why is this a big deal? Well, we keep in mind what an STP topology is. STP top 68 04:57.360 --> 05:02.760 ology involves all of the aspects of spanning tree protocol. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 68 04:57.360 --> 05:02.760 ology involves all of the aspects of spanning tree protocol. 69 05:02.760 --> 05:07.600 So, we're sending BPDUs, for example, for every single topology. We're 70 05:07.600 --> 05:11.760 maintaining a list of blocked ports for every single topology. 71 05:11.760 --> 05:17.320 We have to manage TCNs for every single topology. This starts to get very 72 05:17.320 --> 05:22.580 overwhelming for switches to manage this many different spanning tree protocol 73 05:22.580 --> 05:23.760 topologies. 74 05:23.760 --> 05:27.060 So, here's a question that we should probably try to ask at some point. How 75 05:27.060 --> 05:31.760 many topologies are really needed in a given network? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 75 05:27.060 --> 05:31.760 many topologies are really needed in a given network? 76 05:31.760 --> 05:36.800 Now, as we've said a few times, back when spanning tree protocol was first 77 05:36.800 --> 05:41.760 developed, it was really developed with more dynamic architectures in mind. 78 05:41.760 --> 05:46.480 Meaning that we were just sort of, what's the word, organically? We were organ 79 05:46.480 --> 05:48.760 ically mapping new switches out here. 80 05:48.760 --> 05:54.360 We didn't really have an architecture in mind. But now we have network 81 05:54.360 --> 05:55.760 architectures. 82 05:55.760 --> 06:01.090 We have a pair of distribution switches in most campus environments. And when WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 82 05:55.760 --> 06:01.090 We have a pair of distribution switches in most campus environments. And when 83 06:01.090 --> 06:04.890 we talk about layer two architectures, we're usually either talking about 84 06:04.890 --> 06:05.760 campus or data center. 85 06:05.760 --> 06:10.170 Data centers these days either run a similar architecture or they run leaf and 86 06:10.170 --> 06:12.760 spine, which is a totally different architecture. 87 06:12.760 --> 06:19.490 But now in our campus architectures, we're still usually running distribution 88 06:19.490 --> 06:20.760 and access. 89 06:20.760 --> 06:24.400 So if these are our largest layer two domains, then this is where spanning tree 90 06:24.400 --> 06:26.760 protocol is going to need the most topologies. 91 06:26.760 --> 06:32.760 So here we have a pair of distribution switches with access switches. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 91 06:26.760 --> 06:32.760 So here we have a pair of distribution switches with access switches. 92 06:32.760 --> 06:36.690 Connect them all up to one another. We see that we've got triangles, a little 93 06:36.690 --> 06:40.760 bit of everywhere. And so these are all individual loops. 94 06:40.760 --> 06:45.120 So, let's just look at one individual loop. Let's look at this switch right 95 06:45.120 --> 06:45.760 here. 96 06:45.760 --> 06:48.470 What I'm going to find is we're going to split our VLANs between the two 97 06:48.470 --> 06:50.760 distribution switches. This is pretty common. 98 06:50.760 --> 06:55.760 So maybe 10, 20, and 30 are rooted here. 99 06:55.760 --> 07:05.500 On the left switch, meanwhile, VLANs, 15, 25, and 35 are rooted here on the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 99 06:55.760 --> 07:05.500 On the left switch, meanwhile, VLANs, 15, 25, and 35 are rooted here on the 100 07:05.500 --> 07:06.760 right switch. 101 07:06.760 --> 07:12.430 Which means that the right switch being the root and these three different VLAN 102 07:12.430 --> 07:17.040 s, we're going to see a blocked link maybe over here, whereas the root on the 103 07:17.040 --> 07:20.760 left is going to block it simply on that link. 104 07:20.760 --> 07:26.720 So we have two different topologies in mind. And if I scale this out to 50 VLAN 105 07:26.720 --> 07:31.670 s on the left and 50 VLANs on the right, I still have these exact same two top WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 105 07:26.720 --> 07:31.670 s on the left and 50 VLANs on the right, I still have these exact same two top 106 07:31.670 --> 07:32.760 ologies in mind. 107 07:32.760 --> 07:37.760 And I can apply this to all of the other access layer switches as well. 108 07:37.760 --> 07:42.720 So the topology doesn't change even if I add more VLANs because we have this 109 07:42.720 --> 07:46.760 distribution and access architecture already locked in. 110 07:46.760 --> 07:52.760 Meaning that usually in most scenarios, I need two topologies. 111 07:52.760 --> 07:56.760 Well, how many do I get? 112 07:56.760 --> 08:01.760 As many as I have VLANs, and that is the problem with PVST+ WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 112 07:56.760 --> 08:01.760 As many as I have VLANs, and that is the problem with PVST+ 113 08:01.760 --> 08:07.180 If we actually were to get on to a catalyst 9K, and this is going to vary 114 08:07.180 --> 08:13.760 depending on the platform, it only supports 128 spanning tree topologies. 115 08:13.760 --> 08:17.760 And the reason for that is, like we said, each spanning tree topology adds a 116 08:17.760 --> 08:19.760 whole lot of overhead to a switch. 117 08:19.760 --> 08:24.390 It has to process and send BP to use, it has to do all of the things involved 118 08:24.390 --> 08:25.760 with an STP topology. 119 08:25.760 --> 08:31.760 So once we get past 128 of these topologies, we start to run out of resources. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 119 08:25.760 --> 08:31.760 So once we get past 128 of these topologies, we start to run out of resources. 120 08:31.760 --> 08:36.760 And so if I enable 129 VLANs, what happens? 121 08:36.760 --> 08:42.760 Well, one of these VLANs has no topology. 122 08:42.760 --> 08:44.760 That is a major problem. 123 08:44.760 --> 08:49.130 If we have no STP topology, we have no blocked boards, which means that we have 124 08:49.130 --> 08:49.760 loops. 125 08:49.760 --> 08:55.890 Yes, Cisco switches will allow us to enable more VLANs than we have topologies, 126 08:55.890 --> 08:58.760 and it will allow us to form loops in that regard. 127 08:58.760 --> 09:05.790 So this is a major concern in our world, and we have to be very careful about WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 127 08:58.760 --> 09:05.790 So this is a major concern in our world, and we have to be very careful about 128 09:05.790 --> 09:11.330 deploying per VLAN spanning tree, or rapid PVST+ into our environments, where 129 09:11.330 --> 09:12.760 we have that many VLANs. 130 09:12.760 --> 09:16.910 Now, how often does this happen? Not terribly often, but in a very large 131 09:16.910 --> 09:22.440 extensive layer two network, we might find that we have VLANs sprawl, and this 132 09:22.440 --> 09:25.760 could become a concern in certain environments. 133 09:25.760 --> 09:30.670 So at last, we can talk about the value of MST and what they're trying to do WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 133 09:25.760 --> 09:30.670 So at last, we can talk about the value of MST and what they're trying to do 134 09:30.670 --> 09:35.760 with multiple topologies, again, adding in extra efficiency. 135 09:35.760 --> 09:43.480 What MST wants to do is to change the relationship. So it's not just a topology 136 09:43.480 --> 09:50.450 per VLAN. That's what Cisco does, but rather create a specific number of top 137 09:50.450 --> 09:55.410 ologies being very intentional, and then taking, let's say we create two top 138 09:55.410 --> 10:02.760 ologies here, and then we create our VLANs, and we add them into the topologies. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 138 09:55.410 --> 10:02.760 ologies here, and then we create our VLANs, and we add them into the topologies. 139 10:02.760 --> 10:06.840 We're changing the structure of this. We're no longer saying that every VLAN is 140 10:06.840 --> 10:10.950 going to have its own topologies and its own BPDs, etc. Rather, we're going to 141 10:10.950 --> 10:15.010 create individual topologies, and we can have more of these if we desire, but 142 10:15.010 --> 10:19.760 we've said that in most environments, we only need two topologies. 143 10:19.760 --> 10:24.840 So maybe we create two, maybe we create three, but either way, we can map our V 144 10:24.840 --> 10:27.760 LANs into the topologies that we create. 145 10:27.760 --> 10:33.970 These topologies have a name in the MST world, they are known as instances. We WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 145 10:27.760 --> 10:33.970 These topologies have a name in the MST world, they are known as instances. We 146 10:33.970 --> 10:38.930 're creating instances of MST, and again, we're applying VLANs into the 147 10:38.930 --> 10:40.760 different instances. 148 10:40.760 --> 10:46.120 So now as we might imagine, let's say in this topology here, we're going to 149 10:46.120 --> 10:50.810 need same concept here, we're going to need BPDUs and TCNs and blocked ports 150 10:50.810 --> 10:53.760 per topology, and yet at this point, we only have three. 151 10:53.760 --> 10:59.000 We have three BPDUs, we have to process, we have three sets of blocked ports 152 10:59.000 --> 11:04.240 that we need to manage, we have three sets of TCNs we have to manage, and yet WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 152 10:59.000 --> 11:04.240 that we need to manage, we have three sets of TCNs we have to manage, and yet 153 11:04.240 --> 11:09.760 we could have dozens, if not hundreds of VLANs, and continue to create new VLAN 154 11:09.760 --> 11:09.760 s 155 11:09.760 --> 11:14.760 and map them into the existing topologies, and this stays locked into place. 156 11:14.760 --> 11:20.620 So MST is going to attempt to create what Cisco has from a multiple topology 157 11:20.620 --> 11:26.030 perspective, while adding efficiency, and therefore making it potentially the 158 11:26.030 --> 11:30.580 right option, even in Cisco networks, where we don't necessarily have to rely WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000 158 11:26.030 --> 11:30.580 right option, even in Cisco networks, where we don't necessarily have to rely 159 11:30.580 --> 11:33.760 on industry standards, but in this case, we might want to do so. 160 11:33.760 --> 11:34.260 So.