WEBVTT 0:00:02.900000 --> 0:00:07.360000 Hello and welcome to this video titled Root Bridge Election. 0:00:07.360000 --> 0:00:11.260000 In this video we're going to cover how rapid spanning tree works. 0:00:11.260000 --> 0:00:14.580000 I'm going to give you an overview of the process or the logic. 0:00:14.580000 --> 0:00:16.740000 We're going to start by looking at the root bridge. 0:00:16.740000 --> 0:00:20.100000 Rapid spanning tree requires a root bridge and we're going to see of all 0:00:20.100000 --> 0:00:23.980000 the bridges or switches that are operating with rapid spanning tree, how 0:00:23.980000 --> 0:00:27.800000 do we figure out which one will take on this very important role? 0:00:27.800000 --> 0:00:30.780000 Then I'll also show you what a spanning tree BPDU looks like. 0:00:30.780000 --> 0:00:35.200000 We'll break down that acronym and look at the structure of it. 0:00:35.200000 --> 0:00:38.680000 Then we'll finish out by talking about things related to rapid spanning 0:00:38.680000 --> 0:00:42.440000 tree called port roles and port states. 0:00:42.440000 --> 0:00:50.900000 Let's start with a high level overview of how rapid spanning tree works. 0:00:50.900000 --> 0:00:56.480000 We know that the tree in rapid spanning tree means that you take a topology 0:00:56.480000 --> 0:00:58.020000 with switches or bridges. 0:00:58.020000 --> 0:01:01.240000 Now this was invented back in the days of bridges but it works with switches 0:01:01.240000 --> 0:01:06.500000 these days. Imagine a topology with two or more switches that's a highly 0:01:06.500000 --> 0:01:08.320000 redundant topology. 0:01:08.320000 --> 0:01:11.420000 There's lots of different ways that two points on the network could get 0:01:11.420000 --> 0:01:14.840000 to each other. They could go up this way, they could go down this way, 0:01:14.840000 --> 0:01:20.280000 straight across, but all that redundancy if left in place could cause 0:01:20.280000 --> 0:01:24.260000 bridging loops. So spanning tree says, nope, I don't like bridging loops, 0:01:24.260000 --> 0:01:29.480000 I'm going to take this highly redundant network, trim off the redundancy 0:01:29.480000 --> 0:01:34.060000 and create like a tree like structure where we have the root of the tree 0:01:34.060000 --> 0:01:38.160000 and everything branching off from there where there's only one path from 0:01:38.160000 --> 0:01:42.740000 any point A to point B on the network, not redundant paths. 0:01:42.740000 --> 0:01:46.800000 Well, in order for this to work and for this tree like structure to be 0:01:46.800000 --> 0:01:51.520000 created, the very first step is one of our switches has to be elected 0:01:51.520000 --> 0:01:54.740000 to the special role as the root bridge. 0:01:54.740000 --> 0:01:57.020000 We're going to talk about how that happens. 0:01:57.020000 --> 0:02:01.860000 The next step is all the other switches that are not the root bridge, 0:02:01.860000 --> 0:02:04.020000 they've got some more processing they have to do. 0:02:04.020000 --> 0:02:07.620000 They have to elect one of their interfaces or ports as what's called a 0:02:07.620000 --> 0:02:16.240000 root port and then on every cable connecting two switches together or 0:02:16.240000 --> 0:02:18.400000 even a cable is going to a host. 0:02:18.400000 --> 0:02:22.940000 An interface on that cable has to be elected as a designated port and 0:02:22.940000 --> 0:02:27.000000 then once that's done, everything else that's left over gets blocked. 0:02:27.000000 --> 0:02:31.180000 This is where we end up trimming off our redundancy by blocking non-designated 0:02:31.180000 --> 0:02:34.940000 ports. All right, so let's start with the most critical thing that has 0:02:34.940000 --> 0:02:40.200000 to take place here, which is the election of the root bridge. 0:02:40.200000 --> 0:02:46.140000 Now, in some types of protocols, if you've studied networking at all and 0:02:46.140000 --> 0:02:49.220000 you've studied other types of protocols where there are election processes 0:02:49.220000 --> 0:02:54.060000 like the OSPF designated router or the HSRP active router and there's 0:02:54.060000 --> 0:02:55.600000 many, many others. 0:02:55.600000 --> 0:03:01.340000 There's some protocols that once something gets elected to a certain role, 0:03:01.340000 --> 0:03:02.680000 it sticks with that role. 0:03:02.680000 --> 0:03:06.660000 In other words, it's not a preemptive protocol. 0:03:06.660000 --> 0:03:10.860000 Preemptive would mean that if something else enters that situation later 0:03:10.860000 --> 0:03:13.460000 on and says, hey, I should be that role. 0:03:13.460000 --> 0:03:14.740000 I'm better than you. 0:03:14.740000 --> 0:03:16.880000 Preemptive would mean he can do it. 0:03:16.880000 --> 0:03:20.500000 He can take over that role, even though he wasn't originally playing that 0:03:20.500000 --> 0:03:23.740000 role. Originally, he was not the best, but somebody can come in later 0:03:23.740000 --> 0:03:25.640000 and take it over. 0:03:25.640000 --> 0:03:29.580000 Well, OSPF and HSRP, they are not preemptive by default. 0:03:29.580000 --> 0:03:34.060000 Once someone gets a special role in those protocols, they stick with it. 0:03:34.060000 --> 0:03:36.800000 Spanning tree is preemptive. 0:03:36.800000 --> 0:03:40.940000 So at any point in time, a new bridge or switch could be introduced to 0:03:40.940000 --> 0:03:43.100000 a nice, stable network. 0:03:43.100000 --> 0:03:48.540000 And if that new switch is technically better than the existing root bridge, 0:03:48.540000 --> 0:03:52.260000 that new switch could take over that role, which could be very disruptive 0:03:52.260000 --> 0:03:53.760000 to spanning tree. 0:03:53.760000 --> 0:03:58.440000 Now, there are optional security methods that you can put in place to 0:03:58.440000 --> 0:04:01.060000 prevent that. But I just want to enforce here that what we're going to 0:04:01.060000 --> 0:04:05.440000 talk about, about how the root bridge is elected, this is not a one-shot 0:04:05.440000 --> 0:04:08.400000 deal. This is not a one and done type of thing. 0:04:08.400000 --> 0:04:13.980000 So what starts happening is when a switch detects that one or more of 0:04:13.980000 --> 0:04:17.940000 its interfaces have come up, it's not going to run spanning tree if its 0:04:17.940000 --> 0:04:19.640000 interfaces are all down. 0:04:19.640000 --> 0:04:23.400000 So if one interface comes up, then that switch is going to say, oh, I'm 0:04:23.400000 --> 0:04:24.520000 part of a network. 0:04:24.520000 --> 0:04:27.480000 And this network needs a root bridge. 0:04:27.480000 --> 0:04:31.800000 So it will start advertising itself as the root bridge. 0:04:31.800000 --> 0:04:35.540000 It'll start sending out these special spanning tree messages called BPdu 0:04:35.540000 --> 0:04:40.360000 saying, hey, here's my name and I am worthy of being the root bridge. 0:04:40.360000 --> 0:04:44.420000 Now, if there's another switch out there that is the root bridge or someone 0:04:44.420000 --> 0:04:47.480000 who disputes that, now there's going to be an election process that takes 0:04:47.480000 --> 0:04:50.900000 place. And only one of those guys will actually win. 0:04:50.900000 --> 0:04:54.780000 So as it says here, in the BPdu, we'll see the format of that in just 0:04:54.780000 --> 0:04:58.140000 one second. There's a field called a bridge ID. 0:04:58.140000 --> 0:05:01.240000 So when a switch or bridge starts running spanning tree, it comes up with 0:05:01.240000 --> 0:05:03.340000 a name for itself called a bridge ID. 0:05:03.340000 --> 0:05:08.660000 And the lowest bridge ID by default will be the winner and end up becoming 0:05:08.660000 --> 0:05:09.880000 the root bridge. 0:05:09.880000 --> 0:05:11.760000 So what is this bridge ID? 0:05:11.760000 --> 0:05:12.840000 Where does it come from? 0:05:12.840000 --> 0:05:16.740000 Well, the bridge ID contains basically three essential elements. 0:05:16.740000 --> 0:05:19.100000 Number one, it contains a priority. 0:05:19.100000 --> 0:05:23.220000 By default, and certainly if you're ever taking any kind of certification 0:05:23.220000 --> 0:05:34.120000 test, you'll want to know this, the default bridge priority is 32,768. 0:05:34.120000 --> 0:05:39.700000 Now, you might think, wow, that seems like such an arbitrary number. 0:05:39.700000 --> 0:05:41.440000 Why did they pick that of all things? 0:05:41.440000 --> 0:05:47.380000 Well, it actually makes sense because if I was to show you a BPdu, a bridge 0:05:47.380000 --> 0:05:50.040000 protocol data unit, and I would say, hey, see this right here? 0:05:50.040000 --> 0:05:53.900000 This is the field where the bridge priority fits. 0:05:53.900000 --> 0:05:57.160000 That field is a 16-bit field. 0:05:57.160000 --> 0:06:00.920000 One, two, three, four, five, six, seven, eight, one, two, three, four, 0:06:00.920000 --> 0:06:02.440000 five, six, seven, eight. 0:06:02.440000 --> 0:06:04.740000 16-bit field for priority. 0:06:04.740000 --> 0:06:09.080000 Well, if I was to set that to all zeros, that would be your lowest priority 0:06:09.080000 --> 0:06:13.900000 of zero. If I was to set that to all ones, that would be the absolute 0:06:13.900000 --> 0:06:15.440000 highest priority you could have. 0:06:15.440000 --> 0:06:19.540000 And right in the middle, I'm not going to do it like justice, but if I 0:06:19.540000 --> 0:06:26.560000 had 1,000,000, that bit right there is your 32,768 bit, right in the middle 0:06:26.560000 --> 0:06:29.420000 of the lowest value and the possible highest value. 0:06:29.420000 --> 0:06:31.700000 That's why it defaults to that. 0:06:31.700000 --> 0:06:36.040000 Now, that bridge priority value absolutely is configurable. 0:06:36.040000 --> 0:06:39.600000 So if you're in a Cisco switch or a Juniper switch or whatever switch 0:06:39.600000 --> 0:06:42.720000 you're in, you can tweak it. 0:06:42.720000 --> 0:06:44.800000 And lower is better. 0:06:44.800000 --> 0:06:49.340000 If you want a switch to become the root bridge, he needs to have the lowest 0:06:49.340000 --> 0:06:51.100000 priority possible. 0:06:51.100000 --> 0:06:57.680000 So as you can see right here, the bridge priority is going to be an increment 0:06:57.680000 --> 0:07:01.580000 of 4,096. Now, you might think, well, wait a second. 0:07:01.580000 --> 0:07:05.380000 If I got 16 bits available to me, why does my priority have to be in 4 0:07:05.380000 --> 0:07:10.680000 ,096? Why couldn't I set a bridge priority of 17 or 205? 0:07:10.680000 --> 0:07:13.160000 Why'd that to be zero of 4,096? 0:07:13.160000 --> 0:07:14.480000 Let's see, what's the next one? 0:07:14.480000 --> 0:07:18.840000 8192, 12,000. Why do I have that limitation? 0:07:18.840000 --> 0:07:24.260000 Well, that's because of the next thing, the system ID extension. 0:07:24.260000 --> 0:07:28.180000 You see, when Spanning True was originally developed back in 1998, you 0:07:28.180000 --> 0:07:30.260000 could set any value you wanted to. 0:07:30.260000 --> 0:07:32.900000 You had the full range of 16 bits available to you. 0:07:32.900000 --> 0:07:36.900000 But for the last several decades, that has not been the case, at least 0:07:36.900000 --> 0:07:38.100000 on Cisco switches. 0:07:38.100000 --> 0:07:41.980000 I can't speak for other switches because Cisco switches will take that 0:07:41.980000 --> 0:07:48.760000 16 bit priority and they'll put a system ID extension in there. 0:07:48.760000 --> 0:07:53.720000 So 1, 2, 3, 4, 5, 6, 7, 8, 1, 2, 3, 4, 5, 6, 7, 8. 0:07:53.720000 --> 0:07:58.920000 So this field is just one of the fields in my BPDU. 0:07:58.920000 --> 0:08:01.580000 And this is my bridge priority field. 0:08:01.580000 --> 0:08:08.420000 The way it works nowadays, at least for the last several decades, is that 0:08:08.420000 --> 0:08:14.320000 only these last 4 bits are actually used for priority. 0:08:14.320000 --> 0:08:18.940000 These other bits here, the remaining 12 bits on the right, those are what's 0:08:18.940000 --> 0:08:21.800000 called the system ID extension. 0:08:21.800000 --> 0:08:25.700000 Now, if you're doing per VLAN Spanning Tree, like Cisco switches do it, 0:08:25.700000 --> 0:08:29.720000 where every VLAN does its own unique Spanning Tree, what's put in there 0:08:29.720000 --> 0:08:31.780000 is your VLAN number. 0:08:31.780000 --> 0:08:37.280000 So, for example, if I had the default priority, which is 3,278, we know 0:08:37.280000 --> 0:08:41.480000 that this last bit on the left, that's my 3,278 bit. 0:08:41.480000 --> 0:08:47.020000 I didn't really write that very well, but you get the idea, 3,278, that 0:08:47.020000 --> 0:08:51.080000 would be set. And then the other bits would be zeroed out. 0:08:51.080000 --> 0:08:56.560000 Now, if this BPDU was working for Spanning Tree VLAN number three, we 0:08:56.560000 --> 0:08:59.420000 would encode three in here like that. 0:08:59.420000 --> 0:09:02.060000 That would be my system ID extension. 0:09:02.060000 --> 0:09:06.660000 And all these other bits would be zeroed out. 0:09:06.660000 --> 0:09:10.800000 So, we've got the priority as the last 4 bits, the system ID extension 0:09:10.800000 --> 0:09:16.180000 on the right. So, if I handed this BPDU to you, if you were another switch, 0:09:16.180000 --> 0:09:19.300000 you would not see my priority as 3,278. 0:09:19.300000 --> 0:09:27.780000 You would see it as 3,278 plus 3, which if I'm doing my math correctly, 0:09:27.780000 --> 0:09:30.680000 I believe that would be 3,271. 0:09:30.680000 --> 0:09:34.300000 Is how you would see my bridge priority. 0:09:34.300000 --> 0:09:40.860000 So, if you'll notice, the lowest value you can give it is zero. 0:09:40.860000 --> 0:09:45.060000 You can set a Spanning Tree priority of zero, which sets those four bits 0:09:45.060000 --> 0:09:49.400000 to zero. But then of those four bits here, the next combination you could 0:09:49.400000 --> 0:09:51.700000 do would be that. 0:09:51.700000 --> 0:09:57.140000 Well, this bit right here, in binary, if you start counting from the right, 0:09:57.140000 --> 0:10:05.300000 1, 2, 4, 8, 16, if you go all the way to there, that bit is bit 4,096. 0:10:05.300000 --> 0:10:09.100000 That's why no matter how you manipulate these four bits here, there'll 0:10:09.100000 --> 0:10:13.940000 always be some increment or multiple of 4,096. 0:10:13.940000 --> 0:10:20.180000 So, that's why these days, you can't set a priority of 200 or 3,007. 0:10:20.180000 --> 0:10:24.480000 It has to be some multiple of 4,096, because these bits here on the end 0:10:24.480000 --> 0:10:29.840000 are now used to encode your system extension, which is typically your 0:10:29.840000 --> 0:10:34.320000 VLAN number. But we're not done yet. 0:10:34.320000 --> 0:10:37.680000 So, that's basically the first part of your bridge ID. 0:10:37.680000 --> 0:10:41.380000 Your bridge ID is the combination of the priority and the system ID extension, 0:10:41.380000 --> 0:10:45.660000 and then the other part of your bridge ID is your MAC address. 0:10:45.660000 --> 0:10:51.120000 Now, as a human being, as a network engineer or a network administrator, 0:10:51.120000 --> 0:10:55.640000 the only thing that you have control over is the bridge priority. 0:10:55.640000 --> 0:10:59.980000 Okay? So, if you're configuring spanning tree, if you're trying to manipulate 0:10:59.980000 --> 0:11:05.800000 spanning tree on the switch right now for VLAN 10, you can't change the 0:11:05.800000 --> 0:11:07.180000 system ID extension. 0:11:07.180000 --> 0:11:09.060000 That's going to encode the number 10. 0:11:09.060000 --> 0:11:10.660000 And you can't change the MAC address. 0:11:10.660000 --> 0:11:13.360000 You can't influence what MAC address it will select. 0:11:13.360000 --> 0:11:16.780000 What you can influence is the bridge priority. 0:11:16.780000 --> 0:11:22.480000 So, just remember that in my bridge protocol data unit, which I'm going 0:11:22.480000 --> 0:11:26.660000 to show you on the next slide, there's a field in there called the bridge 0:11:26.660000 --> 0:11:35.680000 priority. And that bridge priority is a combination of all three of these 0:11:35.680000 --> 0:11:40.540000 things put together as one single number. 0:11:40.540000 --> 0:11:44.100000 And whoever's got the lowest number is the winner. 0:11:44.100000 --> 0:11:45.840000 They are the root bridge. 0:11:45.840000 --> 0:11:49.060000 So, let's take a look at that. 0:11:49.060000 --> 0:11:51.940000 Here is your bridge protocol data unit. 0:11:51.940000 --> 0:11:56.380000 So, this is the fundamental piece of information that switches running 0:11:56.380000 --> 0:12:00.300000 rapid spanning tree will exchange amongst each other to figure out how 0:12:00.300000 --> 0:12:02.180000 the topology is going to look. 0:12:02.180000 --> 0:12:07.160000 So, a lot of the stuff we're going to talk about in subsequent videos. 0:12:07.160000 --> 0:12:09.780000 So, I'm not going to go over the details of every single one of these 0:12:09.780000 --> 0:12:10.920000 fields right now. 0:12:10.920000 --> 0:12:13.600000 There's just a handful that I do want to call out to your attention that 0:12:13.600000 --> 0:12:15.460000 are relevant for this discussion. 0:12:15.460000 --> 0:12:20.040000 So, number one, you've got your protocol identifier. 0:12:20.040000 --> 0:12:25.140000 Well, that's just going to be the number zero indicating that this is 0:12:25.140000 --> 0:12:28.240000 spanning tree protocol version. 0:12:28.240000 --> 0:12:32.320000 Well, we're talking about rapid spanning tree, RSTP. 0:12:32.320000 --> 0:12:35.900000 So, that'll always have the number two encoded in there. 0:12:35.900000 --> 0:12:40.600000 So, rapid spanning tree is version two of spanning tree. 0:12:40.600000 --> 0:12:47.640000 The message type, well, notice this is only a version. 0:12:47.640000 --> 0:12:51.400000 So, this is a single byte right here. 0:12:51.400000 --> 0:12:54.720000 And this will indicate, I'm not sure what the number is. 0:12:54.720000 --> 0:12:56.540000 You'll never have to memorize the number. 0:12:56.540000 --> 0:13:03.020000 I'm pretty sure zero, zero is what's called a configuration BPDU. 0:13:03.020000 --> 0:13:06.080000 All right, that's the type of BPDU that we use to figure out the topology, 0:13:06.080000 --> 0:13:09.840000 to figure out who the root bridge is going to be, configuration BPDU. 0:13:09.840000 --> 0:13:11.840000 We have some flags. 0:13:11.840000 --> 0:13:14.040000 I'm not going to talk about those right now. 0:13:14.040000 --> 0:13:20.020000 But what's relevant to the discussion we've just been having, is you've 0:13:20.020000 --> 0:13:21.820000 got right there. 0:13:21.820000 --> 0:13:25.780000 So, that's the bridge ID of whoever the root is. 0:13:25.780000 --> 0:13:30.960000 And if I'm forwarding this BPDU to you, I put my own name in this other 0:13:30.960000 --> 0:13:36.480000 bridge ID. So, when I forward a BPDU to you, I put my own name. 0:13:36.480000 --> 0:13:39.460000 Sometimes this is called the sending bridge ID. 0:13:39.460000 --> 0:13:46.140000 So, I put my own name in there, my own bridge ID, which remember was my 0:13:46.140000 --> 0:13:50.860000 priority, my system extension, and my Mac, that all fits into sending 0:13:50.860000 --> 0:13:52.440000 bridge ID field. 0:13:52.440000 --> 0:13:56.940000 And then the root bridge, whoever he is, he put his own bridge ID into 0:13:56.940000 --> 0:13:58.400000 this field right here. 0:13:58.400000 --> 0:14:02.360000 Now, if I happen to be the root, these two fields will match. 0:14:02.360000 --> 0:14:04.540000 They'll both be exactly the same thing. 0:14:04.540000 --> 0:14:06.780000 Then we have a cost to the root. 0:14:06.780000 --> 0:14:08.720000 We'll talk about that later on. 0:14:08.720000 --> 0:14:14.600000 And then we'll talk about the rest of these things in subsequent videos. 0:14:14.600000 --> 0:14:18.700000 So, let's move on to the next point. 0:14:18.700000 --> 0:14:23.840000 So, now we know that in rapid spanning tree, we have to elect a root bridge. 0:14:23.840000 --> 0:14:27.640000 We know that the root bridge is whoever's got the lowest bridge ID in 0:14:27.640000 --> 0:14:31.700000 the entire topology for this VLAN will be the root bridge. 0:14:31.700000 --> 0:14:35.000000 Now, what do we do from there? 0:14:35.000000 --> 0:14:39.120000 We'll switch to our running rapid spanning tree as in order to create 0:14:39.120000 --> 0:14:44.900000 this non-redundant tree, their interfaces ultimately have to be put in 0:14:44.900000 --> 0:14:48.920000 special port roles and port states. 0:14:48.920000 --> 0:14:54.280000 So, in spanning tree, rapid spanning tree, a port role indicates, what's 0:14:54.280000 --> 0:14:58.780000 my job or responsibility with regards to rapid spanning tree? 0:14:58.780000 --> 0:15:01.100000 Is it my job to send you a BPDU? 0:15:01.100000 --> 0:15:03.020000 There's a certain port role that does that. 0:15:03.020000 --> 0:15:06.360000 Is it my job to receive a BPDU and then forward it on to people behind 0:15:06.360000 --> 0:15:08.600000 me? There's a port role for that. 0:15:08.600000 --> 0:15:11.380000 So, we're going to see here what the names of the port roles are. 0:15:11.380000 --> 0:15:17.020000 And then what the port state is, is if user data comes into me, like someone's 0:15:17.020000 --> 0:15:20.620000 trying to get to a web page or someone's trying to ping something, what 0:15:20.620000 --> 0:15:22.340000 am I going to do with that? 0:15:22.340000 --> 0:15:23.820000 And we'll see with that. 0:15:23.820000 --> 0:15:28.040000 So, let's start with the first port role of the designated port. 0:15:28.040000 --> 0:15:34.740000 Now, the designated port is the port that is closest to the root bridge. 0:15:34.740000 --> 0:15:38.500000 We'll get into more details of this in subsequent videos, but for now, 0:15:38.500000 --> 0:15:41.940000 just think of a design port as two things. 0:15:41.940000 --> 0:15:46.040000 Number one, it's a port that's the closest port to the root bridge. 0:15:46.040000 --> 0:15:49.920000 Well, if I am the root bridge, you can't get any closer than being on 0:15:49.920000 --> 0:15:54.260000 myself. So, the root bridge, his interfaces will always be designated 0:15:54.260000 --> 0:15:57.400000 ports. What's the job of this? 0:15:57.400000 --> 0:15:59.500000 Well, what's he designated to do? 0:15:59.500000 --> 0:16:03.220000 He is designated to deliver BPDUs. 0:16:03.220000 --> 0:16:07.200000 So, that's the job of a designated port to deliver BPDUs. 0:16:07.200000 --> 0:16:10.860000 If you can remember that, the designated port delivers, you'll know he 0:16:10.860000 --> 0:16:14.320000 delivers BPDUs, he puts BPDUs onto the wire. 0:16:14.320000 --> 0:16:18.040000 That's his job. And his state is forwarding. 0:16:18.040000 --> 0:16:22.660000 So, he is allowed to transmit or receive user data on these interfaces. 0:16:22.660000 --> 0:16:27.020000 Now, we're going to put another switch here on the right, and his port 0:16:27.020000 --> 0:16:30.160000 is going to be receiving these BPDUs. 0:16:30.160000 --> 0:16:31.820000 So, he's not going to be a designated port. 0:16:31.820000 --> 0:16:32.960000 He's not going to be delivering them. 0:16:32.960000 --> 0:16:36.380000 He's going to be receiving them, and his port is going to be what's called 0:16:36.380000 --> 0:16:44.020000 a root port. So, a designated port delivers a root port receives. 0:16:44.020000 --> 0:16:48.140000 So, we can see here on bridge B, that interface that's receiving the BPDUs 0:16:48.140000 --> 0:16:51.960000 is called a root port. 0:16:51.960000 --> 0:16:55.860000 And if we look at the cable, if you can identify a root port, the other 0:16:55.860000 --> 0:17:01.300000 side of a root port is always a designated port. 0:17:01.300000 --> 0:17:05.120000 So, then he's going to turn around in any interfaces he has that go further 0:17:05.120000 --> 0:17:09.400000 away from the root bridge that lead away, like that interface right there, 0:17:09.400000 --> 0:17:11.220000 that will become a designated port. 0:17:11.220000 --> 0:17:15.180000 He'll have to deliver BPDUs onto that cable. 0:17:15.180000 --> 0:17:17.360000 And then if we put another switch right there, guess what his interface 0:17:17.360000 --> 0:17:19.660000 would be? He's receiving those BPDUs. 0:17:19.660000 --> 0:17:21.840000 So, he would be a root port. 0:17:21.840000 --> 0:17:26.420000 Now, on rapid spanning tree, we also have another type of interface, which 0:17:26.420000 --> 0:17:28.080000 is called an edge port. 0:17:28.080000 --> 0:17:32.220000 This would be the port that is connected to a host device. 0:17:32.220000 --> 0:17:38.360000 Now, by default, a switch doesn't know when an interface comes up. 0:17:38.360000 --> 0:17:42.200000 If it's connected to another switch, if it's connected to a router, or 0:17:42.200000 --> 0:17:43.860000 if it's connected to a host. 0:17:43.860000 --> 0:17:47.960000 So, we actually have to supply some configuration command on that port 0:17:47.960000 --> 0:17:52.260000 to let rapid spanning tree know, hey, this is an edge port. 0:17:52.260000 --> 0:17:56.520000 It is at the edge of the broadcast domain, meaning, hey, if you flood 0:17:56.520000 --> 0:18:01.020000 something out this port, there's some device out there who will get it, 0:18:01.020000 --> 0:18:02.900000 but it will stop right there. 0:18:02.900000 --> 0:18:05.420000 You don't have to worry if you flood something out there about it circling 0:18:05.420000 --> 0:18:07.180000 around and becoming a broadcast arm. 0:18:07.180000 --> 0:18:11.700000 It is at the edge or the end of the broadcast domain. 0:18:11.700000 --> 0:18:15.000000 But we have to give him a special command to get him to realize that. 0:18:15.000000 --> 0:18:18.240000 Now, an edge port is also a designated port. 0:18:18.240000 --> 0:18:21.100000 He will still deliver BPDUs onto that interface. 0:18:21.100000 --> 0:18:24.140000 Now, that PC, that laptop, it could care less. 0:18:24.140000 --> 0:18:25.400000 PC is the laptops. 0:18:25.400000 --> 0:18:26.820000 They don't do spanning trees. 0:18:26.820000 --> 0:18:28.900000 So, it's just going to see that frame and say, what? 0:18:28.900000 --> 0:18:30.740000 What is that? And he'll throw it away. 0:18:30.740000 --> 0:18:33.300000 He'll discard it. 0:18:33.300000 --> 0:18:37.920000 And then lastly, in order to actually block our redundancy, we will have 0:18:37.920000 --> 0:18:41.360000 alternate ports and backup ports. 0:18:41.360000 --> 0:18:44.100000 And those two ports, we don't see any of those in this topology because 0:18:44.100000 --> 0:18:46.820000 this topology here doesn't have any redundancy. 0:18:46.820000 --> 0:18:49.220000 There's no need to block anything here because there's no way anything 0:18:49.220000 --> 0:18:50.740000 can loop around. 0:18:50.740000 --> 0:18:54.380000 But if we did add some additional links that create a loop, we would see 0:18:54.380000 --> 0:18:57.220000 that one or more of the interfaces would end up being either alternate 0:18:57.220000 --> 0:18:58.600000 or backup ports. 0:18:58.600000 --> 0:19:02.040000 And they would go into the discarding state. 0:19:02.040000 --> 0:19:06.740000 I'm going to use the term blocking and discarding interchangeably. 0:19:06.740000 --> 0:19:12.060000 When spanning tree first came out, we would say a port was in the blocking 0:19:12.060000 --> 0:19:16.300000 state. And actually, when you go onto a switch and you issue a command 0:19:16.300000 --> 0:19:20.920000 to see what spanning tree is doing, it'll actually say blocking. 0:19:20.920000 --> 0:19:22.060000 It won't say discarding. 0:19:22.060000 --> 0:19:25.960000 So even though rapid spanning tree technically says, uh-uh, we call that 0:19:25.960000 --> 0:19:30.780000 discarding, at least on Cisco switches, they'd still display as blocking 0:19:30.780000 --> 0:19:33.680000 if they're an alternate or a backup port. 0:19:33.680000 --> 0:19:35.800000 So I'm going to go back and forth between those two terms. 0:19:35.800000 --> 0:19:39.780000 So but just know that blocking discarding mean exactly the same thing. 0:19:39.780000 --> 0:19:45.480000 So that concludes this video on sort of a high level overview of rapid 0:19:45.480000 --> 0:19:48.060000 spanning tree and how we elect the root bridge. 0:19:48.060000 --> 0:19:51.880000 Continue watching because the other videos will go into even more detail 0:19:51.880000 --> 0:19:57.640000 about what do all the other switches do who did not become the root bridge. 0:19:57.640000 --> 0:19:58.400000 Thank you for watching.