WEBVTT 00:00:01.000 --> 00:00:17.000 So now that we've finished setting up our VLANs, setting up our trunks, taking a look at VTP as well as inter-VLAN routing, it's now time for us to dive into a discussion on spanning tree protocol. 00:00:17.000 --> 00:00:24.000 We're going to spend quite a bit of time in spanning tree protocol because there are a lot of different things we need to be aware of. 00:00:24.000 --> 00:00:35.000 One being the algorithm. Two being how we can manipulate that algorithm. So that way there, if on the exam we have to manipulate the algorithm in some way, shape, or form, we have the ability to do so. 00:00:35.000 --> 00:00:43.000 But the only way you're going to be able to achieve the desired result that is expected to be on the exam is to have a thorough understanding of the algorithm. 00:00:43.000 --> 00:00:47.000 So we're going to start off with an algorithm deep dive. This is something that we do in CCNA. 00:00:48.000 --> 00:00:55.000 So for many of you, it's going to be a review. For some of you, it might be new. Or for some of you, it might be something you haven't done in ages. 00:00:55.000 --> 00:01:05.000 So it's a learning process all over again for those of you who haven't looked at this algorithm in 20 years, let's say, as an example. 00:01:05.000 --> 00:01:10.000 Then we'll do a little bit of configuration and manipulation today. So that way you can learn how to manipulate it. 00:01:10.000 --> 00:01:13.000 We'll look at a few different features that we need to be aware of for the exam. 00:01:13.000 --> 00:01:18.000 We'll look at multiple Spanning Tree Protocol as well. So a lot of stuff here on Spanning Tree Protocol. 00:01:18.000 --> 00:01:23.000 All right. With that said, let's dive into Spanning Tree Protocol. 00:01:23.000 --> 00:01:29.000 So when you look at our environment so far that we have, we have it set up right now. 00:01:29.000 --> 00:01:35.000 So that way there, this is all Layer 3 and this is all Layer 2. 00:01:35.000 --> 00:01:40.000 So where is Spanning Tree Protocol going to be running for us? It's going to be running in this part of the environment here. 00:01:40.000 --> 00:01:44.000 That's where it's going to be running. It's not going to be running up here. 00:01:44.000 --> 00:01:51.000 So whenever you're dealing with Layer 3 connections, Layer 3 links, Layer 3 interfaces, Spanning Tree Protocol is not running on those interfaces. 00:01:51.000 --> 00:01:56.000 It's running on our Layer 2 interfaces that we have in our environment. 00:01:56.000 --> 00:02:06.000 So we are focused on this area. Now, when you look at this environment, we have quite a bit of redundancy. 00:02:06.000 --> 00:02:11.000 We have redundant distribution layer switches. We have redundant links between our distribution layer switches. 00:02:11.000 --> 00:02:16.000 We have redundant links between the distribution layer switches and the access layer switches. 00:02:16.000 --> 00:02:23.000 And we want redundancy because redundancy is going to eliminate single points of failure for us. 00:02:23.000 --> 00:02:27.000 So that way there, if one of our links go down, one of our switches go down, 00:02:27.000 --> 00:02:35.000 we'll have an alternative path that we can utilize through that Layer 2 network to get the traffic from our end users out to wherever they are trying to go. 00:02:35.000 --> 00:02:38.000 Maybe it's the data center. Maybe it's the Internet. 00:02:38.000 --> 00:02:46.000 But because of the way our switches behave, this redundancy is actually a problem. It's actually an issue. 00:02:46.000 --> 00:02:53.000 That's right. Because of the way our switches behave, the redundancy is actually a problem. It's actually an issue. 00:02:53.000 --> 00:03:02.000 Recall that switches flood unknown unicast traffic, meaning if the MAC address or the destination MAC address of the frame is not in the MAC address table, 00:03:02.000 --> 00:03:09.000 you flooded out all ports except the port it came in on. Broadcast frame, destination MAC address is FFFFFFF. 00:03:09.000 --> 00:03:13.000 What happens to that? Flooded out all ports except the port it came in on. 00:03:13.000 --> 00:03:21.000 Because of this behavior and the redundancy we have, this can cause what is known as a broadcast storm. 00:03:21.000 --> 00:03:30.000 This can cause MAC address table instability. This can lock up our network. Now, why is that? 00:03:30.000 --> 00:03:39.000 Well, let's say you have a PC who sends out a broadcast frame or an unknown unicast frame. 00:03:39.000 --> 00:03:43.000 Flooded out all ports except the port it came in on. 00:03:43.000 --> 00:03:51.000 So, what happens is that your switch multiplies that frame. That's normal behavior for both of these. Multiply it. 00:03:51.000 --> 00:04:00.000 So, in this case, 1 became how many? 6. 1, 2, 3, 4, 5, 6. So, 1 became 6. 00:04:00.000 --> 00:04:08.000 Now, obviously, when PC2 gets it, if it's not for PC2, it drops it. If PC3 gets it, it's not for PC3, it drops it. That's fine. 00:04:08.000 --> 00:04:12.000 That's behavior, normal behavior. That's perfectly fine. That's what we expect. 00:04:13.000 --> 00:04:27.000 So, let's focus on these four right here. So, these four right here are going to end up arriving at the other side of their links. 00:04:27.000 --> 00:04:33.000 Let's focus on this one right here. What happens when that one arrives? 00:04:33.000 --> 00:04:41.000 Well, if it's unknown unicast or broadcast, what has to happen? Flooded out all ports except the port it came in on. 00:04:41.000 --> 00:04:47.000 So, this one has to be flooded out, flooded out, flooded out, flooded out, flooded out, flooded out, flooded out. 00:04:47.000 --> 00:04:54.000 So, 1 becomes how many? 7. 1, 2, 3, 4, 5, 7 in this case. 00:04:54.000 --> 00:05:01.000 Now, focus on this one right here. 1 came in, but then it's got to get flooded out, flooded out, flooded out, flooded out. 00:05:01.000 --> 00:05:06.000 So, another 1 becomes 7. So, now we have a total of 14 on the network. 00:05:06.000 --> 00:05:09.000 So, what started out as 1 now is 14. 00:05:11.000 --> 00:05:21.000 And so, just imagine now as they start arriving inbound here and get multiplied out and arrive inbound here and get multiplied out. 00:05:21.000 --> 00:05:24.000 So, now you've got another 14 potentially plus another 14. 00:05:24.000 --> 00:05:27.000 Next thing you know, you've got 2830 on the network. 00:05:27.000 --> 00:05:33.000 So, that one frame has been duplicated how many times and multiplied how many times? 00:05:33.000 --> 00:05:39.000 And now, imagine just another PC sending out a broadcast framework, unknown, unicast, etc., etc., etc. 00:05:39.000 --> 00:05:44.000 So, you've got a massive now amount of traffic on the network that's your broadcast storm. 00:05:44.000 --> 00:05:54.000 And it's looping indefinitely. It's going around and around and around and around, around, oops, around and around and around and around, around and around and around and around. 00:05:54.000 --> 00:05:57.000 Right? So, that's what's happening. 00:05:57.000 --> 00:06:02.000 Why does this never end? Why does this never time out? 00:06:04.000 --> 00:06:10.000 Because in our Ethernet frame, there is no mechanism that times it out. 00:06:10.000 --> 00:06:22.000 Now, there isn't a packet. If you recall, the packet has a time to live field that gets decremented every time a router routes the packet. 00:06:22.000 --> 00:06:26.000 But are we routing here? No, we're switching. 00:06:27.000 --> 00:06:30.000 So, is anything decrementing the time to live? No. 00:06:30.000 --> 00:06:33.000 Is anything checking the time to live? No. 00:06:33.000 --> 00:06:40.000 So, what does that mean? This frame will flood indefinitely. 00:06:40.000 --> 00:06:50.000 Therefore, multiplication, multiplication, multiplication, eventually there's too much traffic on the network, saturates the network, network crashes on you. 00:06:50.000 --> 00:06:59.000 Also, MAC address or MAC table instability problems, what does that mean? 00:06:59.000 --> 00:07:08.000 Well, if this is the source MAC, let's call it just, our source MAC is 0011, 0011. 00:07:08.000 --> 00:07:15.000 When the frame comes in, ASW1 associates the MAC address with that port. 00:07:15.000 --> 00:07:20.000 And then what? We flood, we flood, we flood, we flood as an example. 00:07:20.000 --> 00:07:24.000 So, when the frame comes in this port, the ASW1 associates it with that port. 00:07:24.000 --> 00:07:27.000 But when the next frame comes in, then it's got to change it to that port. 00:07:27.000 --> 00:07:30.000 And let's not forget, when it comes around this way, then it's got to change it to that port. 00:07:30.000 --> 00:07:33.000 Oh, no, now that, nope, no, now that port, now that port. 00:07:33.000 --> 00:07:35.000 So, you see how it keeps having to change? 00:07:35.000 --> 00:07:39.000 And then when it gets flooded back down this way, now this MAC address is known on this port. 00:07:39.000 --> 00:07:43.000 Whoa, wait a second, that's not even where the PC lives. 00:07:43.000 --> 00:07:48.000 So, you see how the MAC address table is going to be changed every time that frame comes in a port. 00:07:48.000 --> 00:07:52.000 And that frame is going to be coming in all ports in this particular case. 00:07:52.000 --> 00:07:55.000 So, MAC address table instability, it's never stable. 00:07:55.000 --> 00:07:59.000 It's always changing, always adjusting for that source MAC address. 00:07:59.000 --> 00:08:04.000 So, broadcast storm, MAC table instability problems, your network's going to lock up on you. 00:08:04.000 --> 00:08:10.000 So, we need a way to stop all this from happening, and that's where spanning tree protocol comes into play. 00:08:10.000 --> 00:08:15.000 Spanning tree protocol is a layer two loop prevention mechanism. 00:08:15.000 --> 00:08:22.000 So, how do you stop the frame from living indefinitely on the network? 00:08:22.000 --> 00:08:26.000 Well, don't allow that frame to end up in any type of loop. 00:08:26.000 --> 00:08:36.000 Now, I mean, could it have been possible for Ethernet to have been designed in a way where there was a counter in the frame that would eventually time out, 00:08:36.000 --> 00:08:40.000 and the frame could be dropped, and maybe we wouldn't need spanning tree protocol? 00:08:40.000 --> 00:08:45.000 I mean, you know, they might have thought of it, and at the time just couldn't figure out how to do it. 00:08:45.000 --> 00:08:47.000 I mean, maybe it's something we could do today. 00:08:47.000 --> 00:08:51.000 I really haven't looked into it, because why? 00:08:51.000 --> 00:09:01.000 Spanning tree protocol says, well, we're just going to make sure that the network behaves a certain way so the frame doesn't do this stuff. 00:09:01.000 --> 00:09:02.000 That's all. 00:09:02.000 --> 00:09:06.000 So, let's change the behavior of the network, and then we won't have this problem. 00:09:06.000 --> 00:09:09.000 So, what does spanning tree protocol do? 00:09:09.000 --> 00:09:18.000 It's going to identify how to stop this looping from happening in the environment, 00:09:18.000 --> 00:09:25.000 and if that is prevented, then none of this will happen for this type of traffic. 00:09:25.000 --> 00:09:27.000 So, let's be clear here. 00:09:27.000 --> 00:09:35.000 Knowing unicast traffic, so traffic where we know the actual destination MAC address, is not going to cause loops, no matter what. 00:09:35.000 --> 00:09:37.000 Even if spanning tree protocol wasn't there. 00:09:37.000 --> 00:09:43.000 It's this type of traffic that is the reason why we end up in the problem we have. 00:09:43.000 --> 00:09:47.000 So, spanning tree protocol was designed way back in the day. 00:09:47.000 --> 00:09:48.000 We're talking about the 80s. 00:09:48.000 --> 00:09:55.000 The 80s, that's right, 1980s, by Radia Perlman, when she was working at Digital Equipment Corp. 00:09:55.000 --> 00:09:56.000 Not exam relevant. 00:09:56.000 --> 00:09:57.000 Don't worry about that. 00:09:57.000 --> 00:10:04.000 All right, but eventually it was standardized as IEEE 802.1D in 1990. 00:10:04.000 --> 00:10:17.000 So, with the multitude of Ethernet networks and Ethernet interfaces and Ethernet traffic and frames that were floating around a network, 00:10:17.000 --> 00:10:25.000 Radia Perlman saw a problem as the networks grew, and that was the bigger the networks grew, the slower they became, the more problems you had. 00:10:25.000 --> 00:10:26.000 Why was that? 00:10:26.000 --> 00:10:32.000 Well, traffic just kept exponentially multiplying, and as a result of that, we had what? 00:10:32.000 --> 00:10:37.000 We had broadcast storms, MAC address table instability. 00:10:37.000 --> 00:10:41.000 So, with spanning tree protocol, Radia came up with a way to stop it all. 00:10:41.000 --> 00:10:51.000 So, and the whole premise behind it is identify ports to block so that loops will be eliminated. 00:10:51.000 --> 00:10:52.000 It's as simple as that. 00:10:52.000 --> 00:10:58.000 Identify ports to block so that loops will be eliminated. 00:10:58.000 --> 00:11:04.000 So, in this case, if we block all these ports, then the loops will be eliminated. 00:11:04.000 --> 00:11:11.000 So, now, as you can see, if we send out a broadcast frame, flood, flood, flood, that's it. 00:11:11.000 --> 00:11:16.000 Now, it arrives up here, and yes, it'll flood, flood, flood, flood, flood, flood, flood. 00:11:16.000 --> 00:11:18.000 But look, can't get in. 00:11:18.000 --> 00:11:20.000 Can't get in. 00:11:20.000 --> 00:11:21.000 Can't get in. 00:11:21.000 --> 00:11:22.000 Can't get in. 00:11:22.000 --> 00:11:23.000 Can't get in. 00:11:23.000 --> 00:11:24.000 Can't get in. 00:11:24.000 --> 00:11:26.000 Therefore, this is the only one that survives. 00:11:26.000 --> 00:11:28.000 It's the only one that survives. 00:11:28.000 --> 00:11:30.000 Okay? 00:11:30.000 --> 00:11:33.000 And then, again, flood, flood, flood, flood. 00:11:33.000 --> 00:11:35.000 Oop, can't flood out the Xs, so can't go that way. 00:11:35.000 --> 00:11:38.000 So, now, again, can't get in. 00:11:38.000 --> 00:11:39.000 Can't get in. 00:11:39.000 --> 00:11:40.000 Can't get in. 00:11:40.000 --> 00:11:42.000 So, there's only one path the traffic can take. 00:11:42.000 --> 00:11:49.000 So, with spanning tree protocol, you identify ports that need to be blocked so that loops are prevented. 00:11:49.000 --> 00:11:51.000 Therefore, there ends up being what? 00:11:51.000 --> 00:11:55.000 A single, a single loop-free path in your network. 00:11:55.000 --> 00:11:57.000 A single loop-free path in your network. 00:11:57.000 --> 00:12:02.000 So, essentially, what it does is it eliminates all your redundancy, doesn't it? 00:12:02.000 --> 00:12:07.000 It eliminates your ability to use all your redundancy. 00:12:07.000 --> 00:12:16.000 So, it doesn't matter whether I have one link there, two links there, three links there, four links there, 10 links there, 100 links there. 00:12:16.000 --> 00:12:19.000 I can really only use one when it's all said and done. 00:12:19.000 --> 00:12:21.000 Same thing here. 00:12:21.000 --> 00:12:23.000 Doesn't matter how many links I have. 00:12:23.000 --> 00:12:26.000 In the end, I can really only use one potentially. 00:12:26.000 --> 00:12:29.000 And in this case, I'm using neither of them. 00:12:29.000 --> 00:12:37.000 So, that's why in a previous discussion, we said spanning tree protocol blocks like 50 plus percent of your links. 00:12:37.000 --> 00:12:38.000 That's right. 00:12:38.000 --> 00:12:41.000 It blocks 50 plus percent of your links. 00:12:41.000 --> 00:12:43.000 That's a lot. 00:12:43.000 --> 00:12:49.000 But the thing is, is if one of these links end up failing or having a problem, let's say this one here fails. 00:12:49.000 --> 00:12:50.000 It fails. 00:12:50.000 --> 00:12:54.000 Well, then, this one would become unblocked and we could use that one. 00:12:54.000 --> 00:12:56.000 So, the redundancy is there. 00:12:56.000 --> 00:13:02.000 It's just not usable unless there is a failure, okay? 00:13:02.000 --> 00:13:09.000 Now, I'm going to teach you later on today about something called Ether Channel. 00:13:09.000 --> 00:13:15.000 Ether Channel is a way that we can bundle links together, as Andrew has pointed out. 00:13:15.000 --> 00:13:21.000 So, if I can bundle those links together, what does that give me the opportunity to do? 00:13:21.000 --> 00:13:27.000 It gives me the opportunity for spanning tree protocol to think it's one link as opposed to two links. 00:13:27.000 --> 00:13:33.000 And we'll see how that changes this environment for us and how we'll be able to use these links. 00:13:33.000 --> 00:13:41.000 So, if you have multiple links between your switches and you're not Ether Channeling them together, then you're only using one link at a time. 00:13:41.000 --> 00:13:47.000 If you're Ether Channeling them together, you're going to be able to use more than one link at a time with spanning tree protocol. 00:13:47.000 --> 00:13:50.000 All right? 00:13:50.000 --> 00:13:52.000 So, spanning tree doesn't ignore it. 00:13:52.000 --> 00:13:58.000 Spanning tree is being tricked into thinking it's one link instead of individual links, okay? 00:13:58.000 --> 00:14:00.000 It's not ignoring it. 00:14:00.000 --> 00:14:07.000 It's essentially being tricked into thinking it's one link when it's all said and done. 00:14:07.000 --> 00:14:08.000 All right? 00:14:08.000 --> 00:14:16.000 So, what I want you to realize, too, now, is that spanning tree protocol is not just about loop prevention, okay? 00:14:16.000 --> 00:14:22.000 Most of us learn about it that way through other means, through other instructors, through whatever. 00:14:22.000 --> 00:14:27.000 But I hate stopping at that point because you're not getting a true picture, okay? 00:14:27.000 --> 00:14:30.000 Yes, it's a loop prevention mechanism. 00:14:30.000 --> 00:14:33.000 Yes, it prevents those broadcast storms. 00:14:33.000 --> 00:14:34.000 But what else does it do? 00:14:34.000 --> 00:14:40.000 It controls the flow of traffic in our layer 2 infrastructure. 00:14:40.000 --> 00:14:41.000 You see that now? 00:14:41.000 --> 00:14:50.000 I told you here that with the ports being blocked versus unblocked, doesn't spanning tree protocol control the flow of traffic? 00:14:50.000 --> 00:14:54.000 Traffic must flow this way. 00:14:54.000 --> 00:14:55.000 There is no exception. 00:14:55.000 --> 00:14:56.000 Why? 00:14:56.000 --> 00:14:58.000 Because the blocked ports versus unblocked ports. 00:14:59.000 --> 00:15:06.000 Meaning that if your ports that are blocked are not the optimal ports to block in your environment, 00:15:06.000 --> 00:15:12.000 that means you have suboptimal layer 2 forwarding in your environment, which may not be good. 00:15:12.000 --> 00:15:22.000 You may be getting less performance than you should truly be getting in your environment. 00:15:22.000 --> 00:15:26.000 Think about your layer 3 environment. 00:15:26.000 --> 00:15:37.000 Give me an example of a protocol you use in your layer 3 environment to control the flow of traffic. 00:15:37.000 --> 00:15:38.000 Give me an example. 00:15:47.000 --> 00:15:50.000 Okay, you could use static routes, right? 00:15:50.000 --> 00:15:54.000 That's not a protocol, but, you know, yeah, OSPF is a protocol. 00:15:54.000 --> 00:15:55.000 Absolutely. 00:15:55.000 --> 00:15:56.000 What else? 00:15:56.000 --> 00:16:00.000 EIGRP could use ISIS, right? 00:16:00.000 --> 00:16:02.000 So those are protocols. 00:16:02.000 --> 00:16:10.000 So think about your layer 3 environment using dynamic routing protocols to control the flow of traffic, right? 00:16:10.000 --> 00:16:18.000 Well, what protocol controls your layer 2 flow of traffic? 00:16:18.000 --> 00:16:20.000 Spanning tree protocol. 00:16:24.000 --> 00:16:25.000 Yeah. 00:16:25.000 --> 00:16:34.000 So if it's designed incorrectly or implemented incorrectly, is it possible that traffic could do this? 00:16:34.000 --> 00:16:35.000 Sure. 00:16:35.000 --> 00:16:36.000 Yeah. 00:16:36.000 --> 00:16:41.000 And I have had that issue once that I had to solve. 00:16:45.000 --> 00:16:46.000 That's not good, right? 00:16:46.000 --> 00:16:52.000 Because we don't want to traverse the layer 2 network multiple times just to come in and out. 00:16:52.000 --> 00:16:55.000 Remember, north-south traffic flow. 00:16:55.000 --> 00:16:57.000 Not north-south-north-south traffic flow. 00:16:57.000 --> 00:16:59.000 No, just north-south traffic flow. 00:16:59.000 --> 00:17:01.000 That's all we want to do, folks. 00:17:01.000 --> 00:17:02.000 All right? 00:17:02.000 --> 00:17:09.000 So just keep that in mind as you evolve your knowledge and level of expertise with various protocols. 00:17:09.000 --> 00:17:18.000 Start understanding them more about not just what you might read as their primary purpose, like loop prevention, 00:17:18.000 --> 00:17:23.000 but what else do they ultimately control and dictate in your environment? 00:17:23.000 --> 00:17:29.000 So now how does spanning tree protocol figure out what ports to block? 00:17:29.000 --> 00:17:37.000 Well, we use what are called bridge protocol data units, BPDUs, bridge protocol data units. 00:17:37.000 --> 00:17:43.000 These messages are sent every two seconds by default. 00:17:44.000 --> 00:17:49.000 And here's an example of what the BPDU might contain. 00:17:53.000 --> 00:17:57.000 So I send BPDUs at what? 00:17:57.000 --> 00:17:59.000 Every port. 00:17:59.000 --> 00:18:01.000 Every two seconds. 00:18:03.000 --> 00:18:04.000 Same thing. 00:18:04.000 --> 00:18:09.000 Whether their port is blocked or not, we need to be sending BPDUs. 00:18:09.000 --> 00:18:10.000 Why? 00:18:11.000 --> 00:18:21.000 So that way there we have the opportunity to build the network properly, meaning figure out what the block ports are. 00:18:21.000 --> 00:18:29.000 Figure out what the forwarding ports are. 00:18:29.000 --> 00:18:38.000 And if anything changes from the last BPDU to this new BPDU, we can make adjustments. 00:18:38.000 --> 00:18:43.000 But if nothing has changed and the information is still the same, then we leave it as it is. 00:18:43.000 --> 00:18:53.000 So the goal here is that every two seconds the BPDU we receive on an interface from another device is the same, is the same, is the same. 00:18:53.000 --> 00:18:58.000 We want spanning tree protocol to be boring because if it's boring, it's stable. 00:18:58.000 --> 00:19:01.000 If it's not boring, that means there's changes. 00:19:01.000 --> 00:19:05.000 If there's changes, that means our spanning tree environment is not stable. 00:19:05.000 --> 00:19:07.000 We want a stable environment. 00:19:07.000 --> 00:19:16.000 So we really want the BPDUs to be the same all the time, every two seconds, because if anything changes, that means we have to converge. 00:19:16.000 --> 00:19:17.000 We have to adjust. 00:19:17.000 --> 00:19:23.000 We have to make maybe unblock this, block this, okay, or unblock this and block this. 00:19:23.000 --> 00:19:29.000 So when it's all said and done, the end result is this shouldn't be changing all the time. 00:19:29.000 --> 00:19:33.000 If it is, you've got stability issues in your Layer 2 environment. 00:19:34.000 --> 00:19:50.000 So things we'll be focused on here as we continue our discussion are things like the root bridge ID, the root path cost, the sending bridge ID, the sending port ID, the max, the message age, max age, right, definitely the max age, 20 seconds. 00:19:50.000 --> 00:19:52.000 We'll talk about that more later. 00:19:52.000 --> 00:19:55.000 The hello time is two seconds, so that's what it is right there, hello time. 00:19:55.000 --> 00:19:58.000 And then our forward delay time is 15 seconds. 00:19:58.000 --> 00:20:01.000 So numbers I would want you to know for the certification exam. 00:20:01.000 --> 00:20:03.000 So they'll come up again later on. 00:20:03.000 --> 00:20:08.000 So all of this information is going to be used to build our spanning tree topology. 00:20:08.000 --> 00:20:16.000 Now, we have to take a moment to understand that Cisco's implementation of spanning tree is different than 802.1D. 00:20:16.000 --> 00:20:25.000 For example, you've got 802.1D, which is the industry standard for spanning tree protocol, and rapid spanning tree protocol is 802.1W. 00:20:25.000 --> 00:20:28.000 Cisco doesn't refer to them this way. 00:20:28.000 --> 00:20:34.000 We refer to them as per-VLAN spanning tree plus and rapid per-VLAN spanning tree plus. 00:20:34.000 --> 00:20:40.000 Now, what you need to recognize about this is that they are significantly different, meaning they are not compatible with each other. 00:20:40.000 --> 00:20:51.000 So if there are products out there you are using that are using these versions, well, they're not compatible with Cisco's versions, and Cisco does not use these versions at all. 00:20:51.000 --> 00:20:54.000 Cisco uses these versions. 00:20:55.000 --> 00:21:03.000 So you would never mix and match Cisco switches and non-Cisco switches together because Cisco switches use these, non-Cisco switches would use these. 00:21:03.000 --> 00:21:11.000 Now, there is another protocol we'll be talking about today that allows you to mix and match different vendor switches like Cisco switches with non-Cisco switches. 00:21:11.000 --> 00:21:12.000 We'll mention that later. 00:21:12.000 --> 00:21:16.000 We brought it up at the end of yesterday's session, but we'll definitely talk about it today. 00:21:16.000 --> 00:21:29.000 But the biggest difference you need to realize here is that when it comes to Cisco's implementation, Cisco is not blocking ports physically. 00:21:29.000 --> 00:21:31.000 That's right. 00:21:31.000 --> 00:21:32.000 They're not. 00:21:32.000 --> 00:21:41.000 See, what Cisco does is they block ports logically on a VLAN by VLAN basis. 00:21:42.000 --> 00:21:53.000 So with 802.1D and 802.1W, you would end up with one giant tree like you see here, regardless of the number of VLANs you have. 00:21:53.000 --> 00:22:02.000 So if I had 10 VLANs, this is what the topology would look like for all 10 VLANs, and you can't change that. 00:22:03.000 --> 00:22:11.000 Okay, but with Cisco, we can manipulate our trees on a VLAN by VLAN basis. 00:22:11.000 --> 00:22:15.000 Let me jump ahead to a slide we'll talk about later or show you later. 00:22:15.000 --> 00:22:19.000 But for right now, you'll see that I could have two different trees. 00:22:19.000 --> 00:22:23.000 I could have my purple tree here, and I could have my red tree. 00:22:23.000 --> 00:22:25.000 Two totally different trees. 00:22:25.000 --> 00:22:31.000 Look how some ports are blocked for red but unblocked for purple. 00:22:31.000 --> 00:22:36.000 So if it has an RP, it means it's unblocked for red, but it's blocked for purple. 00:22:36.000 --> 00:22:42.000 If it has an RP, it's unblocked for purple but blocked for red. 00:22:42.000 --> 00:22:45.000 So we're blocking logically. 00:22:45.000 --> 00:22:56.000 So I can use a multitude of links based on a VLAN by VLAN basis in a Cisco environment because we're not blocking physically. 00:22:56.000 --> 00:23:02.000 We're blocking virtually with Cisco's implementation. 00:23:02.000 --> 00:23:07.000 So what we have now is we have a multitude of different protocols. 00:23:07.000 --> 00:23:13.000 So 802.1D, not functional on Cisco devices, right? 00:23:14.000 --> 00:23:18.000 This is the original industry standard implementation spanning tree protocol. 00:23:18.000 --> 00:23:24.000 You get one tree, one spanning tree topology blocking your physical ports. 00:23:24.000 --> 00:23:26.000 Doesn't matter how many VLANs you have. 00:23:26.000 --> 00:23:27.000 One VLAN, there's your tree. 00:23:27.000 --> 00:23:30.000 A hundred VLANs, you get one tree. 00:23:30.000 --> 00:23:32.000 Every VLAN follows that exact same tree. 00:23:32.000 --> 00:23:33.000 Can't change that. 00:23:33.000 --> 00:23:35.000 Can't adjust that, okay? 00:23:35.000 --> 00:23:43.000 Where Cisco's implementation says we're going to give you one tree for each and every single one of your VLANs. 00:23:43.000 --> 00:23:48.000 So if you have 200 VLANs, we're going to give you 200 trees. 00:23:48.000 --> 00:23:58.000 And as a result of that, you can manipulate each tree any way you want, and it only affects that tree. 00:23:58.000 --> 00:24:01.000 So if I want this tree to use these links, I can. 00:24:01.000 --> 00:24:03.000 If I want this tree to use these links, I can. 00:24:03.000 --> 00:24:11.000 If I want these, you can control the flow of traffic on your Cisco devices in your layer 2 network on a VLAN by VLAN basis. 00:24:11.000 --> 00:24:19.000 So these are not compatible with each other, as I was saying, because they see the trees differently, 00:24:19.000 --> 00:24:26.000 and the numbers they use for electing, for example, like the root bridge are calculated differently, 00:24:26.000 --> 00:24:33.000 and therefore, you can't use a non-Cisco switch that's running 802.1d with a Cisco switch running Pre-VLAN Spanning Tree Plus. 00:24:33.000 --> 00:24:37.000 Now, you can plug them together, and it's going to look like they're working properly, 00:24:37.000 --> 00:24:43.000 but the problem is that your Cisco switch sees the spanning tree topology differently than the non-Cisco switch, 00:24:43.000 --> 00:24:53.000 and that's why it doesn't work, which means you could have loops in some of your VLANs and not even realize it. 00:24:53.000 --> 00:24:55.000 These two are slow. 00:24:55.000 --> 00:24:58.000 We really don't use them today anymore. 00:24:58.000 --> 00:25:01.000 We would use something like 802.1w and Rapid Pre-VLAN Spanning Tree Plus. 00:25:01.000 --> 00:25:03.000 So, again, same idea. 00:25:03.000 --> 00:25:10.000 The industry standard version of Rapid Spanning Tree Protocol, but you still only get one tree. 00:25:10.000 --> 00:25:11.000 That's it. 00:25:11.000 --> 00:25:13.000 All the VLANs are bound by that tree. 00:25:13.000 --> 00:25:21.000 We're blocking our ports physically, whereas Rapid Spanning Tree Protocol for Cisco is called Rapid Pre-VLAN Spanning Tree Plus, 00:25:21.000 --> 00:25:28.000 and so this is what you would use on your Cisco devices, and you get one tree for each and every single VLAN, 00:25:28.000 --> 00:25:33.000 and you can manipulate each and every single VLAN, every tree, however you want to to control your flow of traffic. 00:25:33.000 --> 00:25:36.000 So, again, not compatible. 00:25:36.000 --> 00:25:47.000 However, these two are compatible with each other, and these two are compatible with each other. 00:25:47.000 --> 00:25:53.000 All that simply means is if you have a Cisco switch running Rapid Pre-VLAN Spanning Tree Plus connected to a Cisco switch running 00:25:53.000 --> 00:26:02.000 Per-VLAN Spanning Tree Plus, then this switch, when communicating with this switch, will use Per-VLAN Spanning Tree Plus and not Rapid, 00:26:02.000 --> 00:26:04.000 right, backwards compatible. 00:26:04.000 --> 00:26:06.000 This one's backwards compatible with this one. 00:26:06.000 --> 00:26:13.000 Likewise, this one's backwards compatible with this one, but anytime you have a Rapid device connected to a non-Rapid device, 00:26:13.000 --> 00:26:16.000 the Rapid device will behave in a non-Rapid way with that device. 00:26:16.000 --> 00:26:19.000 Keep that in mind for the exam, all right? 00:26:19.000 --> 00:26:26.000 Now, the bottom example here, you can see this was created by Cisco, multiple Spanning Tree Protocol, 00:26:26.000 --> 00:26:30.000 but then it became an industry standard called 802.1S. 00:26:30.000 --> 00:26:39.000 So, if you use multiple Spanning Tree Protocol or 802.1S on a Cisco device or a non-Cisco device, they behave the exact same way. 00:26:40.000 --> 00:26:49.000 So, this would be the protocol you would use anytime you have a Cisco device and a non-Cisco device connected together in a Layer 2 environment, right? 00:26:49.000 --> 00:26:54.000 If you're using any of these other options, you're not going to get the results you think you are getting. 00:26:54.000 --> 00:26:59.000 You want to go with 802.1S, multiple Spanning Tree Protocol, in those scenarios. 00:26:59.000 --> 00:27:07.000 We'll also talk about the benefit of using multiple Spanning Tree Protocol when we get to the discussion that is specifically dedicated to it later on today. 00:27:07.000 --> 00:27:09.000 But for right now, we'll leave it alone. 00:27:09.000 --> 00:27:11.000 We'll leave it alone. 00:27:11.000 --> 00:27:14.000 And what we're going to do is we're going to focus on our algorithm now. 00:27:14.000 --> 00:27:17.000 That is the same algorithm used for all of these. 00:27:17.000 --> 00:27:20.000 So, that way there, you'll understand them and you'll know them. 00:27:20.000 --> 00:27:28.000 And then we'll get into some discussions on some of the features and functions of some of these different protocols as we go along and how we can manipulate them. 00:27:28.000 --> 00:27:33.000 So, for each and every single loop, we got to find a port to block. 00:27:33.000 --> 00:27:34.000 We block that port. 00:27:34.000 --> 00:27:35.000 We break the loop. 00:27:35.000 --> 00:27:38.000 And so, you got your block ports and then you've got other types of ports. 00:27:38.000 --> 00:27:43.000 You got root ports that are in the forwarding state, designated ports that are in the forwarding state. 00:27:43.000 --> 00:27:45.000 So, let's talk about our algorithm. 00:27:45.000 --> 00:27:48.000 So, it's basically a four-step process. 00:27:48.000 --> 00:27:50.000 You have to elect the root bridge. 00:27:50.000 --> 00:27:56.000 Now, notice I'm saying bridge, bridge, bridge, bridge, protocol, data unit, root, bridge, right? 00:27:56.000 --> 00:27:57.000 Why? 00:27:57.000 --> 00:28:01.000 Because Spanning Tree Protocol was invented when bridges existed. 00:28:01.000 --> 00:28:06.000 So, Ratio Perlman created Spanning Tree Protocol when bridges existed, not switches. 00:28:06.000 --> 00:28:07.000 There were no switches at the time. 00:28:07.000 --> 00:28:13.000 But switches are just more elaborate versions of bridges from back in the day. 00:28:13.000 --> 00:28:15.000 Bridges had a small number of ports. 00:28:15.000 --> 00:28:16.000 Everything was software-based. 00:28:16.000 --> 00:28:21.000 Switches, lots and lots of ports today, hardware-based products and stuff like that. 00:28:21.000 --> 00:28:25.000 So, root bridge, root switch, interchangeable nowadays. 00:28:25.000 --> 00:28:28.000 Like you can see here, I said root bridge, put a star so you can see that. 00:28:28.000 --> 00:28:30.000 But no, non-root switch, right? 00:28:30.000 --> 00:28:34.000 Ratio Perlman's days, it was non-root bridge, okay, non-root bridge. 00:28:34.000 --> 00:28:36.000 So, just showing you terms interchangeably. 00:28:36.000 --> 00:28:43.000 So, if you do see bridge on the exam, then they're just following the actual technological name of things. 00:28:43.000 --> 00:28:44.000 But you could see switch. 00:28:44.000 --> 00:28:48.000 They could say switch on the exam, and I wouldn't want you to get thrown for a loop based on that. 00:28:48.000 --> 00:28:50.000 Bridge and switch, interchangeable. 00:28:50.000 --> 00:28:51.000 All right. 00:28:51.000 --> 00:28:53.000 So, we're going to go through all of this together here. 00:28:53.000 --> 00:28:55.000 First one, elect a root bridge. 00:28:55.000 --> 00:29:03.000 Elect a root bridge on Cisco devices for each and every single VLAN, all right? 00:29:03.000 --> 00:29:08.000 So, elect a root bridge for each and every single VLAN, therefore a root bridge for each and every single tree. 00:29:08.000 --> 00:29:12.000 If these were non-Cisco switches, then they'd just be electing one root bridge. 00:29:12.000 --> 00:29:13.000 That's it. 00:29:13.000 --> 00:29:16.000 Now, what is the root bridge? 00:29:16.000 --> 00:29:17.000 Why does it exist? 00:29:17.000 --> 00:29:18.000 Why do we care about it? 00:29:18.000 --> 00:29:19.000 Well, read this. 00:29:19.000 --> 00:29:26.000 It says the switch that all other switches use as the reference point when building the spanning tree topology. 00:29:26.000 --> 00:29:33.000 So, if you think about this for a second, if I had these four switches, and they were all interconnected together. 00:29:33.000 --> 00:29:34.000 Let's just do this for fun. 00:29:34.000 --> 00:29:38.000 Let's say they're all interconnected together. 00:29:38.000 --> 00:29:52.000 If they don't all agree on who the reference point is or the root bridge is, then they're all going to come up with different results, okay? 00:29:52.000 --> 00:30:06.000 So, for example, if each and every single one of them said, I'm the root bridge, then they would all come up with different results of which port should be blocked and which port should not be blocked. 00:30:06.000 --> 00:30:11.000 If they all elected themselves as the root bridge, what would happen is no ports would be blocked. 00:30:11.000 --> 00:30:14.000 That's what would happen in that case, okay? 00:30:14.000 --> 00:30:16.000 But here's the deal, right? 00:30:16.000 --> 00:30:26.000 All of these switches have to ultimately figure out, hey, we need a single reference point that we can then do our calculations from. 00:30:26.000 --> 00:30:36.000 And if we all use that same reference point, then we'll all get the results we need that ultimately build a loop-free topology. 00:30:36.000 --> 00:30:38.000 That's why we have the root bridge. 00:30:38.000 --> 00:30:45.000 It's just a reference point for all of the calculations to start from. 00:30:45.000 --> 00:30:53.000 So, that way there, the results are going to be consistent across all the switches in that environment. 00:30:53.000 --> 00:30:55.000 How do we figure out who the root bridge is? 00:30:55.000 --> 00:31:02.000 It's going to be the device with the lowest bridge ID, the lowest bridge ID. 00:31:02.000 --> 00:31:05.000 So, what is the bridge ID? 00:31:05.000 --> 00:31:12.000 It's a value that every single one of our switches will have in relation to spanning tree protocol. 00:31:12.000 --> 00:31:15.000 And the bridge ID is made up of two parts. 00:31:15.000 --> 00:31:22.000 It's made up of a bridge priority number and a MAC address. 00:31:22.000 --> 00:31:31.000 You'll see here the bridge priority is a value from 0 to 65,535 with the default value being 32,768. 00:31:31.000 --> 00:31:33.000 And then you have a MAC address. 00:31:33.000 --> 00:31:40.000 This is known as the base Ethernet MAC address on our devices. 00:31:40.000 --> 00:31:44.000 So, we type in show version and we scroll right down to the bottom. 00:31:44.000 --> 00:31:50.000 Well, not right down to the bottom, but you'll see here something called the base Ethernet MAC address. 00:31:50.000 --> 00:31:58.000 This is used on our devices for things like spanning tree protocol. 00:31:58.000 --> 00:32:06.000 It's also used for LACP and a whole bunch of other protocols whenever we need to make decisions like we're trying to make right now. 00:32:06.000 --> 00:32:08.000 So, this is not associated with any single port. 00:32:08.000 --> 00:32:10.000 It's not associated with anything like that. 00:32:10.000 --> 00:32:18.000 It is basically a chassis MAC address to represent this particular device for certain decision-making processes. 00:32:18.000 --> 00:32:23.000 So, that is what's used in this particular case, that base Ethernet MAC address. 00:32:23.000 --> 00:32:28.000 So, you have this bridge priority combined with the base Ethernet MAC address. 00:32:28.000 --> 00:32:35.000 Now, whenever we're trying to do our election here to figure out who's going to be the root bridge, what's going to happen? 00:32:35.000 --> 00:32:39.000 Well, we're going to send out BPDUs, send out BPDUs, BPDUs, BPDUs, BPDUs. 00:32:39.000 --> 00:32:41.000 All the switches are sending out BPDUs. 00:32:41.000 --> 00:32:44.000 So, they're all going to get each other's BPDUs. 00:32:44.000 --> 00:32:47.000 And in the end, what's going to happen is they're going to be able to compare. 00:32:47.000 --> 00:32:48.000 Compare what? 00:32:48.000 --> 00:32:51.000 Compare things like the priority. 00:32:51.000 --> 00:32:53.000 So, look, it says priority first. 00:32:53.000 --> 00:32:56.000 So, we compare priority first. 00:32:56.000 --> 00:33:00.000 And what's going to happen is the switches are going to try to figure out who has the what? 00:33:00.000 --> 00:33:03.000 Lower priority. 00:33:03.000 --> 00:33:05.000 The lower priority first. 00:33:05.000 --> 00:33:13.000 If one switch stands out with the lowest priority, then it is the root bridge. 00:33:13.000 --> 00:33:17.000 By default, they're all 32,768, the priorities. 00:33:17.000 --> 00:33:19.000 So, what happens then? 00:33:19.000 --> 00:33:24.000 To break the tie, to break the tie, they look at that base Ethernet MAC address. 00:33:24.000 --> 00:33:27.000 Who has the lowest base Ethernet MAC address? 00:33:27.000 --> 00:33:33.000 And the odds of you having two switches with the same base Ethernet MAC address are worse than you winning the lottery. 00:33:33.000 --> 00:33:37.000 You have a better chance of winning the lottery than having two switches with the same base Ethernet MAC address. 00:33:37.000 --> 00:33:40.000 Let's be clear about that. 00:33:40.000 --> 00:33:42.000 And your odds of winning the lottery are pretty bad. 00:33:42.000 --> 00:33:49.000 So, in the end, right, that lowest MAC address will be guaranteed to break your tie. 00:33:49.000 --> 00:34:06.000 Now, before we actually do an example of this, let's understand why something like 802.1d is not compatible with Cisco's Purveyland Spanning Tree Plus. 00:34:06.000 --> 00:34:09.000 It's because of the bridge priority. 00:34:09.000 --> 00:34:11.000 It's one of the things. 00:34:11.000 --> 00:34:18.000 When you look at an 802.1d bridge priority, it is 16 bits. 00:34:18.000 --> 00:34:25.000 So, therefore, it's a value from 0 all the way up to 65,535. 00:34:25.000 --> 00:34:28.000 And it can be any of those values. 00:34:28.000 --> 00:34:33.000 And you can manipulate it any way you want. 00:34:33.000 --> 00:34:39.000 So, it doesn't matter if it's VLAN 10 or 20 or 30 or 40. 00:34:39.000 --> 00:34:42.000 That bridge priority is the same. 00:34:42.000 --> 00:34:49.000 The bridge priority, right, is going to be for the entire device regardless of the number of VLANs you have. 00:34:49.000 --> 00:34:50.000 Why? Because you only get one tree. 00:34:50.000 --> 00:34:52.000 You only get one tree. 00:34:52.000 --> 00:34:55.000 And all the VLANs are just going to follow this tree. 00:34:55.000 --> 00:34:57.000 Okay, but remember, Cisco doesn't do that. 00:34:57.000 --> 00:35:01.000 Cisco says, hey, we're going to give you a different tree for each and every single VLAN. 00:35:01.000 --> 00:35:12.000 So, they need to be able to differentiate the different spanning tree messages for each and every single tree on a VLAN by VLAN basis. 00:35:12.000 --> 00:35:17.000 So, their entire bridge priority is still 16 bits. 00:35:17.000 --> 00:35:20.000 Okay, it's still 16 bits. 00:35:20.000 --> 00:35:26.000 But now, look at the VLAN number is thrown in there. 00:35:26.000 --> 00:35:37.000 So, your bridge priority on a Cisco device is going to be a bridge priority number with the VLAN number that is part of it. 00:35:37.000 --> 00:35:42.000 So, how many possible VLAN numbers are there, folks, based on our discussion yesterday? 00:35:42.000 --> 00:35:45.000 How many possible VLAN numbers are there? 00:35:45.000 --> 00:35:47.000 Well, if you don't know, take 2 to the power of 12. 00:35:47.000 --> 00:35:49.000 That will give you your answer. 00:35:49.000 --> 00:35:52.000 2 to the power of 12 is 4096. 00:35:52.000 --> 00:35:57.000 0 through 4095, as Conrad has pointed out. 00:35:57.000 --> 00:35:59.000 0 through 4095. 00:35:59.000 --> 00:36:11.000 So, when you now look at the bridge priority, I cannot manipulate this bridge priority like I could manipulate this. 00:36:11.000 --> 00:36:14.000 I can manipulate this in increments of 1. 00:36:15.000 --> 00:36:21.000 But here, because it's the top 4 bits, I can't do that anymore. 00:36:21.000 --> 00:36:23.000 That's why we're no longer compatible. 00:36:23.000 --> 00:36:33.000 Because if we look, for example, here, on a Cisco device, this might be my priority, 32,778. 00:36:33.000 --> 00:36:35.000 How do we get that priority? 00:36:35.000 --> 00:36:39.000 32,768 plus 10. 00:36:39.000 --> 00:36:41.000 The VLAN number. 00:36:41.000 --> 00:36:42.000 The VLAN number. 00:36:42.000 --> 00:36:43.000 You see VLAN 10? 00:36:43.000 --> 00:36:44.000 VLAN 10. 00:36:44.000 --> 00:36:45.000 Okay? 00:36:45.000 --> 00:36:51.000 So, 32,778 for a Cisco device means VLAN 10. 00:36:51.000 --> 00:36:52.000 That's right. 00:36:52.000 --> 00:36:55.000 32,778 actually means VLAN 10. 00:36:55.000 --> 00:36:57.000 There is no other option. 00:36:57.000 --> 00:36:59.000 That actually means VLAN 10. 00:36:59.000 --> 00:37:12.000 Just like if we had 32,768 plus VLAN 20, then that would be 32,788. 00:37:13.000 --> 00:37:17.000 This actually means VLAN 20 for our Cisco device. 00:37:17.000 --> 00:37:19.000 But what about a non-Cisco device? 00:37:19.000 --> 00:37:26.000 A non-Cisco device, something like 32,768, means 32,768. 00:37:29.000 --> 00:37:30.000 Sorry, I meant 78. 00:37:30.000 --> 00:37:31.000 Did I say 68? 00:37:31.000 --> 00:37:32.000 78. 00:37:32.000 --> 00:37:34.000 32,778, that means what? 00:37:34.000 --> 00:37:37.000 That means 32,778. 00:37:37.000 --> 00:37:43.000 On a Cisco device, that would mean 32,768 plus VLAN 10. 00:37:43.000 --> 00:37:44.000 That's the difference. 00:37:44.000 --> 00:37:48.000 So, they don't use those numbers the same way. 00:37:48.000 --> 00:37:52.000 That's why you can't connect them together and get it to work properly. 00:37:52.000 --> 00:37:56.000 Because one device is going to see the network one way, the other device is going to see the network the other way. 00:37:56.000 --> 00:37:57.000 They're going to make different decisions. 00:37:57.000 --> 00:38:01.000 You're probably going to end up with ports that should be blocked that aren't blocked, ports that are blocked that shouldn't be blocked. 00:38:01.000 --> 00:38:03.000 Therefore, you maybe have some loops. 00:38:08.000 --> 00:38:15.000 So, because you can only manipulate four bits in the 16, because you can't manipulate these, 00:38:15.000 --> 00:38:19.000 those are set in stone based on the VLAN. 00:38:19.000 --> 00:38:23.000 That means there's only so many possible combinations for this. 00:38:23.000 --> 00:38:32.000 And those combinations would be this, either 0, which would be 0, 0, 0, 0, or 4,096 would be 0, 0, 0, 1, 00:38:32.000 --> 00:38:41.000 or 8,192, which would be 0, 0, I want to go with 1, 0, and 12, 28 would be 0, 0, 1, 1, and so on and so forth. 00:38:41.000 --> 00:38:45.000 So, you only have 16 possible values that you can manipulate or play around with. 00:38:45.000 --> 00:38:47.000 And then you combine that with the VLAN number. 00:38:47.000 --> 00:38:53.000 So, if it's VLAN 10, if it was 0 plus 10, then it would be 10, which is for VLAN 10. 00:38:53.000 --> 00:38:55.000 If it's VLAN 20, it would be 20. 00:38:55.000 --> 00:39:07.000 But if you've got 4,096, well, then you've got 4,096 plus 10, that would bring you up to 4,106, as an example. 00:39:07.000 --> 00:39:12.000 And so, the device knows, because these are all the only combinations that exist. 00:39:12.000 --> 00:39:16.000 It looks at a number, essentially, let's put it this way. 00:39:16.000 --> 00:39:22.000 Let's say you had the number 20,490. 00:39:23.000 --> 00:39:28.000 Your device looks at this number and says, okay, which one of these is closest? 00:39:28.000 --> 00:39:29.000 It's this one right here. 00:39:29.000 --> 00:39:30.000 Lowest, but closest. 00:39:30.000 --> 00:39:32.000 Lower than this number, but closest. 00:39:32.000 --> 00:39:33.000 It's that one. 00:39:33.000 --> 00:39:34.000 Okay? 00:39:34.000 --> 00:39:38.000 So, subtract 20,480 from 20,490. 00:39:38.000 --> 00:39:41.000 So, 20,480, subtract it, and you get what? 00:39:41.000 --> 00:39:42.000 10. 00:39:42.000 --> 00:39:43.000 So, this is VLAN 10. 00:39:43.000 --> 00:39:44.000 There you go. 00:39:44.000 --> 00:39:45.000 That's the only option. 00:39:45.000 --> 00:39:47.000 No other combination can give you VLAN 10. 00:39:47.000 --> 00:39:48.000 It's not possible. 00:39:48.000 --> 00:39:49.000 Right? 00:39:50.000 --> 00:39:56.000 So, if it's 24,586, that would also be VLAN 10. 00:39:56.000 --> 00:40:02.000 If it's, as we said, 4,106, that's also VLAN 10. 00:40:02.000 --> 00:40:03.000 So, that's how it makes the reference. 00:40:03.000 --> 00:40:04.000 That's how it does it. 00:40:04.000 --> 00:40:05.000 Right? 00:40:05.000 --> 00:40:09.000 You're not going to have to necessarily do that on the exam, but I'm just trying to really 00:40:09.000 --> 00:40:15.000 get you knowledgeable about how all these decisions are being made in regards to our 00:40:15.000 --> 00:40:17.000 algorithm when it works out. 00:40:18.000 --> 00:40:26.000 So, let's go ahead and test you now on your understanding of electing the root bridge. 00:40:26.000 --> 00:40:27.000 So, remember what we said. 00:40:27.000 --> 00:40:29.000 Look at priority first. 00:40:29.000 --> 00:40:32.000 If one has a lower priority, that's the root bridge. 00:40:32.000 --> 00:40:36.000 If priorities are tied, then you got to look at the MAC address. 00:40:36.000 --> 00:40:38.000 So, give it a try here. 00:40:38.000 --> 00:40:43.000 Is ASW1 or is ASW2 going to be the root bridge if they were battling it out? 00:40:43.000 --> 00:40:44.000 Chat in. 00:40:48.000 --> 00:40:58.000 Remember, lower is better. 00:40:58.000 --> 00:41:08.000 So, number one, ties are not allowed. 00:41:08.000 --> 00:41:11.000 So, you always got to be able to break a tie with Spanning Tree Protocol. 00:41:11.000 --> 00:41:12.000 All right. 00:41:12.000 --> 00:41:22.000 So, let's look at our priority first, 32,778 versus 32,778. 00:41:22.000 --> 00:41:27.000 When you look at the output of Show Spanning Tree, whenever you look at the bridge ID information, 00:41:27.000 --> 00:41:29.000 this is the local device information. 00:41:29.000 --> 00:41:32.000 Whenever you look, local device information. 00:41:32.000 --> 00:41:38.000 That's what you're looking at. 00:41:38.000 --> 00:41:39.000 All right. 00:41:39.000 --> 00:41:41.000 Now, if you don't know how to look at the MAC address, you just read it left to right. 00:41:41.000 --> 00:41:44.000 So, since we're tied, priorities tied, we got to use the MAC address to break the tie. 00:41:44.000 --> 00:41:46.000 Start at the left. 00:41:46.000 --> 00:41:49.000 D versus D, tied. 00:41:49.000 --> 00:41:51.000 4 versus 4, tied. 00:41:51.000 --> 00:41:52.000 A versus A, tied. 00:41:52.000 --> 00:41:54.000 0 versus 0, tied. 00:41:54.000 --> 00:41:56.000 2 versus 2, tied. 00:41:56.000 --> 00:41:58.000 A versus A, tied. 00:41:58.000 --> 00:42:00.000 6 versus 6, tied. 00:42:00.000 --> 00:42:03.000 3 versus E, we have a winner. 00:42:03.000 --> 00:42:05.000 E versus 3. 00:42:05.000 --> 00:42:08.000 Now, those are hexadecimal values. 00:42:08.000 --> 00:42:13.000 So, with hexadecimal, we got 0 through 9 and then A, B, C, D, E, F. 00:42:13.000 --> 00:42:15.000 So, which one's lower? 00:42:15.000 --> 00:42:18.000 Is 3 lower or is E lower? 00:42:18.000 --> 00:42:21.000 Is 3 lower or is E lower? 00:42:33.000 --> 00:42:34.000 Yep, 3 is lower. 00:42:34.000 --> 00:42:36.000 So, this is our winner. 00:42:37.000 --> 00:42:39.000 This is our winner. 00:42:39.000 --> 00:42:40.000 All right. 00:42:40.000 --> 00:42:43.000 So, definitely something that you need to be able to do for the certification exam. 00:42:43.000 --> 00:42:49.000 You need to be able to go through this process of identifying first priority. 00:42:49.000 --> 00:42:52.000 And if one of them stands out with the lowest priority, then that's your winner. 00:42:52.000 --> 00:42:53.000 Don't even go any further than that. 00:42:53.000 --> 00:43:05.000 But if two or more have the exact same lowest priority, then you're going to have to go ahead and start looking at the MAC addresses from left to right and knock them off one at a time, one at a time, until you get to the differentiator. 00:43:05.000 --> 00:43:07.000 And then there's your winner. 00:43:07.000 --> 00:43:08.000 And there's your winner. 00:43:08.000 --> 00:43:09.000 All right. 00:43:09.000 --> 00:43:10.000 So, let's go ahead and do this one. 00:43:10.000 --> 00:43:20.000 Of these four devices now, ASW1, ASW2, DSW1, DSW2, who is going to be our root bridge in our demo topology? 00:44:06.000 --> 00:44:07.000 All right. 00:44:07.000 --> 00:44:14.000 If you don't know where to begin, and if you get overwhelmed on certification exams, the easiest thing for you to do is divide and conquer. 00:44:14.000 --> 00:44:16.000 Divide and conquer. 00:44:16.000 --> 00:44:24.000 And the best thing to do is to pick two at a time. 00:44:24.000 --> 00:44:26.000 Just compare two at a time. 00:44:26.000 --> 00:44:30.000 So, for example, let's do ASW1 versus ASW2. 00:44:30.000 --> 00:44:32.000 So, priority, priority, tied, tied. 00:44:32.000 --> 00:44:36.000 Let's start tied, tied, tied, tied, tied, tied, tied, tied, tied, tied, tied, tied. 00:44:36.000 --> 00:44:37.000 Three versus E. 00:44:37.000 --> 00:44:38.000 Okay. 00:44:38.000 --> 00:44:45.000 So, now that we figured out that ASW1 is better than ASW2, all I have to do is knock that off the list. 00:44:45.000 --> 00:44:50.000 Now, I go with ASW1, and let's go down to DSW2 as an example. 00:44:50.000 --> 00:44:51.000 And let's look at priority. 00:44:51.000 --> 00:44:53.000 Priority's tied. 00:44:53.000 --> 00:44:55.000 So, we got to look at MAC address now. 00:44:55.000 --> 00:44:57.000 So, we got four versus D. Stop right there. 00:44:57.000 --> 00:44:58.000 Stop right there. 00:44:58.000 --> 00:45:02.000 Four is lower than D, so now you are the weakest link. 00:45:02.000 --> 00:45:03.000 Goodbye. 00:45:03.000 --> 00:45:06.000 So, now we can look at DSW2 versus DSW1. 00:45:06.000 --> 00:45:08.000 Look at your priority. 00:45:08.000 --> 00:45:09.000 In this case, it's tied. 00:45:09.000 --> 00:45:12.000 So, now we look at our MAC two versus four. 00:45:12.000 --> 00:45:16.000 We can stop right there because two is lower than four. 00:45:16.000 --> 00:45:17.000 Now we have our winner. 00:45:17.000 --> 00:45:22.000 There's our root bridge. 00:45:22.000 --> 00:45:26.000 See how quickly I was able to do that? 00:45:26.000 --> 00:45:29.000 You want to be able to do it that fast for a certification exam, 00:45:29.000 --> 00:45:31.000 be able to look at all the devices, look at that information, 00:45:31.000 --> 00:45:36.000 and just go through it as quickly as possible and identify who that root bridge is. 00:45:36.000 --> 00:45:39.000 You don't want to waste time on that. 00:45:39.000 --> 00:45:43.000 Cisco expects you at this level to be able to do that without even thinking, essentially, 00:45:43.000 --> 00:45:48.000 expanding to protocol. 00:45:48.000 --> 00:46:03.000 So, what if we had this, for example? 00:46:03.000 --> 00:46:10.000 The root bridge. 00:46:10.000 --> 00:46:13.000 So, now this one is super simple because all you're doing is looking at priorities 00:46:13.000 --> 00:46:15.000 and you're like, okay, wait a second, all the priorities are different. 00:46:15.000 --> 00:46:16.000 Which one's lower? 00:46:16.000 --> 00:46:18.000 We're good to go. 00:46:18.000 --> 00:46:19.000 Now what? 00:46:19.000 --> 00:46:22.000 Let's try this. 00:46:22.000 --> 00:46:35.000 Who's the root bridge now? 00:46:35.000 --> 00:46:37.000 So, when we look at priorities, well, you're too high. 00:46:37.000 --> 00:46:40.000 You're too high compared to the other two. 00:46:40.000 --> 00:46:41.000 So, now we're looking at these two. 00:46:41.000 --> 00:46:42.000 They're tied. 00:46:42.000 --> 00:46:43.000 Can't have ties. 00:46:43.000 --> 00:46:44.000 So, now we've got to break the tie. 00:46:44.000 --> 00:46:50.000 If we're working left to right, left to right, three versus E, three is lower, you're gone. 00:46:50.000 --> 00:46:51.000 So, there you go. 00:46:51.000 --> 00:46:57.000 There's some different examples of how you're going to approach it. 00:46:57.000 --> 00:47:02.000 And then we'll see here that our root bridge is definitely DSW1. 00:47:02.000 --> 00:47:07.000 And now I'm going to expand out and show you everything else that goes along. 00:47:07.000 --> 00:47:10.000 In the previous screen, all we saw was this information here. 00:47:10.000 --> 00:47:12.000 And we had to make our decision based on that. 00:47:12.000 --> 00:47:18.000 But if you look here, when we look at the output of show spanning tree, when you look at all the information, look at the top. 00:47:18.000 --> 00:47:20.000 See where it says root ID? 00:47:20.000 --> 00:47:22.000 Root ID says what? 00:47:22.000 --> 00:47:26.000 Well, this is the information of the root bridge. 00:47:26.000 --> 00:47:30.000 It actually says the root bridge has this priority and this is its MAC address. 00:47:30.000 --> 00:47:34.000 So, when you have all this information here, you can see that it's the same as this and this. 00:47:35.000 --> 00:47:42.000 So, if you're looking at this output and that is the same as that, well, you can conclude that it's the root bridge. 00:47:42.000 --> 00:47:44.000 But wait a second, it actually tells you, doesn't it? 00:47:44.000 --> 00:47:47.000 Yeah, it says this bridge is the root bridge. 00:47:47.000 --> 00:47:50.000 So, just different ways that you can identify that it's the root bridge. 00:47:50.000 --> 00:47:52.000 Another way is all of this. 00:47:52.000 --> 00:47:58.000 Every port on the root bridge is going to be a designated port. 00:47:58.000 --> 00:47:59.000 We're going to get to that shortly. 00:47:59.000 --> 00:48:01.000 And the forwarding state. 00:48:01.000 --> 00:48:02.000 That is a must. 00:48:03.000 --> 00:48:09.000 So, if they decided not even to give you this, and they said, which of the following is the root bridge? 00:48:09.000 --> 00:48:13.000 Look for the device with every port as a designated port. 00:48:13.000 --> 00:48:14.000 That is your root bridge. 00:48:14.000 --> 00:48:15.000 That is your root bridge. 00:48:19.000 --> 00:48:23.000 And so, let's just take a quick peek at our switches in our environment. 00:48:23.000 --> 00:48:27.000 And first of all, we'll type in show VLAN. 00:48:27.000 --> 00:48:31.000 Nope, show spanning tree on ASW1. 00:48:31.000 --> 00:48:37.000 And what I want you to see, first and foremost, you have a tree for every VLAN on your Cisco devices. 00:48:37.000 --> 00:48:39.000 So, there's my tree for VLAN1. 00:48:39.000 --> 00:48:41.000 Now, here's a tree for VLAN10. 00:48:41.000 --> 00:48:44.000 Now, here's a tree for VLAN20. 00:48:44.000 --> 00:48:47.000 Now, here's a tree for VLAN50 and VLAN100. 00:48:47.000 --> 00:48:52.000 Because remember, we created a bunch of VLANs yesterday just for fun as we were manipulating different things. 00:48:52.000 --> 00:48:59.000 So, all the VLANs you create, spanning tree will automatically create a tree for it for you as we go along. 00:48:59.000 --> 00:49:03.000 So, we're just going to focus, though, on VLAN10 here. 00:49:03.000 --> 00:49:08.000 So, I'll type in spanning tree for VLAN10. 00:49:08.000 --> 00:49:10.000 We're running per VLAN spanning tree plus. 00:49:10.000 --> 00:49:12.000 That's what IEEE means. 00:49:12.000 --> 00:49:16.000 If you were running rapid per the VLAN spanning tree plus on your Cisco device, it would be RSTP. 00:49:16.000 --> 00:49:17.000 That would show up there. 00:49:17.000 --> 00:49:19.000 I'll show you that later, a little bit more. 00:49:19.000 --> 00:49:24.000 But you'll see here that the root ID information is different than the bridge information. 00:49:24.000 --> 00:49:29.000 Remember, the bridge information is the local device's information in this case. 00:49:29.000 --> 00:49:37.000 So, if the root ID information is different than the local ID information, then that means you're not the root bridge. 00:49:37.000 --> 00:49:39.000 That's one way that you can figure that out pretty quick. 00:49:39.000 --> 00:49:41.000 Also, it doesn't say that you're the root bridge. 00:49:41.000 --> 00:49:42.000 Instead, what does it say? 00:49:42.000 --> 00:49:47.000 It says the root bridge has this priority and this MAC address, which is different than this. 00:49:47.000 --> 00:49:48.000 Well, the priority is the same. 00:49:48.000 --> 00:49:49.000 The MAC address is different. 00:49:50.000 --> 00:49:55.000 But it also says your cost to get to the root bridge is four. 00:49:55.000 --> 00:49:56.000 We'll talk about cost here momentarily. 00:49:56.000 --> 00:49:59.000 But it tells you your cost to get to that root bridge is four. 00:49:59.000 --> 00:50:04.000 The port you're going to use to get to the root bridge is port 47. 00:50:04.000 --> 00:50:08.000 So, you can give it one size zero, size 47 on the device. 00:50:08.000 --> 00:50:13.000 And if you come down here, you'll see that that port is actually known as the root port in this particular case. 00:50:13.000 --> 00:50:14.000 More on that later. 00:50:14.000 --> 00:50:17.000 These ports are designated ports in this case. 00:50:17.000 --> 00:50:21.000 And notice how this one's blocked in this example. 00:50:21.000 --> 00:50:27.000 But being able to read this and understand what you're looking at would be important for the certification exam. 00:50:27.000 --> 00:50:31.000 So, so far, if this is the same as this, then you know you're the root bridge. 00:50:31.000 --> 00:50:34.000 So, even if they omitted a lot of other information, how can you tell? 00:50:34.000 --> 00:50:36.000 It's got the same information. 00:50:36.000 --> 00:50:38.000 Or all the ports are designated as the root bridge. 00:50:38.000 --> 00:50:39.000 So, we can look at this and say you're not the root bridge. 00:50:39.000 --> 00:50:40.000 Why? 00:50:40.000 --> 00:50:44.000 Because not all ports are designated, number one. 00:50:44.000 --> 00:50:46.000 Number two, it doesn't say you're the root bridge. 00:50:46.000 --> 00:50:48.000 Number three, the priority and addresses aren't the same. 00:50:48.000 --> 00:50:50.000 So, a lot of different ways you can verify that. 00:50:50.000 --> 00:50:54.000 And they might only give you one piece of the puzzle or one piece of that information. 00:50:54.000 --> 00:50:58.000 But knowing that, all of those are possible ways to do it. 00:50:58.000 --> 00:51:00.000 It will help you answer questions on the certification exam. 00:51:00.000 --> 00:51:04.000 So, you never know how they're going to want to trick you on the exam. 00:51:04.000 --> 00:51:05.000 All right. 00:51:05.000 --> 00:51:06.000 So, that is our root bridge. 00:51:06.000 --> 00:51:09.000 So, now that we've figured out who the root bridge is, what's our next step? 00:51:09.000 --> 00:51:16.000 The next step is to find on all the remaining switches in that Layer 2 environment the root ports. 00:51:16.000 --> 00:51:19.000 The thing about the root port is you always got to remember this. 00:51:19.000 --> 00:51:20.000 And it's a way to check your work. 00:51:20.000 --> 00:51:23.000 There can only be one root port for every other switch. 00:51:23.000 --> 00:51:24.000 Okay. 00:51:24.000 --> 00:51:25.000 So, the root bridge has zero root ports. 00:51:25.000 --> 00:51:27.000 The root bridge, zero root ports. 00:51:27.000 --> 00:51:33.000 But every other switch is going to have one and only one root port. 00:51:33.000 --> 00:51:35.000 And what does the root port do? 00:51:36.000 --> 00:51:43.000 The root port is the port on the switch that is the quickest way to get to the root bridge. 00:51:43.000 --> 00:51:44.000 That's what it is. 00:51:44.000 --> 00:51:47.000 It's the best way to get to the root bridge. 00:51:47.000 --> 00:51:51.000 So, the best way to diagram this out is for you to stand on your switch. 00:51:51.000 --> 00:51:54.000 Stand on it. 00:51:54.000 --> 00:52:02.000 And then ask yourself, should I go this way, this way, this way, this way, this way, this way, or this way? 00:52:02.000 --> 00:52:08.000 Well, we already know that we can't go this way because it's a dead end, a dead end, a dead end. 00:52:08.000 --> 00:52:13.000 So, those are not going to be root ports at all. 00:52:13.000 --> 00:52:14.000 And your switch would know that. 00:52:14.000 --> 00:52:15.000 Why? 00:52:15.000 --> 00:52:18.000 Because, remember, we've already gone through Step 1, electing the root bridge. 00:52:18.000 --> 00:52:22.000 So, we know the root bridge is reachable out of one of these four ports. 00:52:22.000 --> 00:52:26.000 We just have to figure out now which port is the best one to use to get to it. 00:52:26.000 --> 00:52:28.000 And we'll label that as our root port. 00:52:28.000 --> 00:52:35.000 So, when you look at this switch here, which is not going to be the same as this one because this one has different values pertaining to it 00:52:35.000 --> 00:52:43.000 because this network is actually different than this one when it comes to MAC addresses and things like that. 00:52:43.000 --> 00:52:49.000 So, the default MAC addresses, the bases in it on these actual devices are different than the ones that this diagram was built based on. 00:52:49.000 --> 00:52:53.000 Just that way there we can show you how to do things later on. 00:52:53.000 --> 00:52:57.000 But what I want you to realize is you can see, right, this one has a root port. 00:52:57.000 --> 00:52:58.000 So, we've got to figure that out. 00:52:58.000 --> 00:53:02.000 In this case, it was 47, but it's not going to be 47 in this diagram. 00:53:02.000 --> 00:53:05.000 And we'll see why that is when we just simply do the math. 00:53:05.000 --> 00:53:08.000 So, what do we have to do first and foremost? 00:53:08.000 --> 00:53:13.000 We have to understand that the root port is selected based on something known as cost. 00:53:13.000 --> 00:53:19.000 So, it's important for you to understand this concept of cost in regards to spanning tree protocol. 00:53:19.000 --> 00:53:23.000 So, cost is based on interface values. 00:53:23.000 --> 00:53:25.000 So, you'll see here that there's this thing called the short method. 00:53:25.000 --> 00:53:29.000 The short method says if it's a 100-meg interface, it gets a cost of 19. 00:53:29.000 --> 00:53:31.000 If it's a 1-gig interface, it gets a cost of 4. 00:53:31.000 --> 00:53:34.000 If it's a 10-gig interface, it gets a cost of 2. 00:53:34.000 --> 00:53:38.000 And then we have the long method that says if it's a 100-meg interface, it gets a cost of 200,000. 00:53:38.000 --> 00:53:46.000 A 1-gig interface gets a cost of 20,000, and a 10-gig interface gets a cost of 2,000. 00:53:46.000 --> 00:53:54.000 Your equipment, based on its age, the version of the operating system will determine whether the default short method or long method. 00:53:55.000 --> 00:53:59.000 Short method was the original method that was used before long method existed. 00:53:59.000 --> 00:54:02.000 So, short method is not actually a good option for today. 00:54:02.000 --> 00:54:04.000 So, if you have older switches that are using short method, 00:54:04.000 --> 00:54:09.000 you definitely want to make sure they're using the long method along with all your other switches. 00:54:09.000 --> 00:54:10.000 And here's the reason why. 00:54:10.000 --> 00:54:15.000 The short method, there's not enough room for higher-speed interfaces when it comes to cost. 00:54:15.000 --> 00:54:16.000 Look at this. 00:54:16.000 --> 00:54:19.000 1-gig is 4, 10-gig is 2. 00:54:19.000 --> 00:54:20.000 So, let me ask you this. 00:54:20.000 --> 00:54:23.000 What do you think the cost would be of a 2-gig interface? 00:54:23.000 --> 00:54:26.000 What would the cost be of a 2-gig interface? 00:54:26.000 --> 00:54:28.000 So, take a look at this number. 00:54:28.000 --> 00:54:34.000 If a 1-gig is 4 and a 10-gig is 2, what would a 2-gig interface be? 00:54:34.000 --> 00:54:35.000 What do you think? 00:54:39.000 --> 00:54:40.000 Remember, lower is better. 00:54:40.000 --> 00:54:42.000 Lower cost is better. 00:54:42.000 --> 00:54:43.000 Yeah, right? 00:54:43.000 --> 00:54:45.000 3, because it would have to fit in between, right? 00:54:45.000 --> 00:54:48.000 2-gig is 3. 00:54:49.000 --> 00:54:53.000 Okay, so what do you think the cost of a 3-gig interface is? 00:54:57.000 --> 00:54:58.000 It's 3. 00:54:58.000 --> 00:55:01.000 What about a 4-gig interface? 00:55:01.000 --> 00:55:03.000 It's 3. 00:55:03.000 --> 00:55:08.000 What about a 5-gig, 6-gig, 7-gig, 8-gig, 9-gig? 00:55:08.000 --> 00:55:10.000 It is 3. 00:55:10.000 --> 00:55:12.000 Well, that's a problem, right? 00:55:12.000 --> 00:55:14.000 And you might be saying, well, why is it a problem? 00:55:14.000 --> 00:55:15.000 Super simple. 00:55:15.000 --> 00:55:17.000 I'll diagram it out for you. 00:55:18.000 --> 00:55:25.000 Folks, right now, I have 2 1-gig links. 00:55:25.000 --> 00:55:28.000 That's what I have, 2 1-gig links. 00:55:30.000 --> 00:55:33.000 So, the cost of each of these links is what? 00:55:33.000 --> 00:55:37.000 Each link has a cost of 4. 00:55:37.000 --> 00:55:38.000 Pretty straightforward. 00:55:38.000 --> 00:55:39.000 Cost of 4, cost of 4. 00:55:39.000 --> 00:55:40.000 Pretty straightforward. 00:55:40.000 --> 00:55:41.000 All right. 00:55:41.000 --> 00:55:47.000 So, what happens, though, if we do an Ether Channel Bundle? 00:55:47.000 --> 00:55:50.000 So, we did an Ether Channel Bundle now. 00:55:50.000 --> 00:55:56.000 Spanning Tree Protocol sees 1 2-gig link. 00:55:56.000 --> 00:56:02.000 So, what's the cost of our 2-gig link, according to Spanning Tree Protocol? 00:56:02.000 --> 00:56:04.000 Cost is 3, we said. 00:56:04.000 --> 00:56:05.000 Okay. 00:56:05.000 --> 00:56:06.000 Let's go over here. 00:56:06.000 --> 00:56:20.000 If we Ether Channel Bundle all of these together, we now have 1 4-gig link, according to Spanning Tree Protocol. 00:56:20.000 --> 00:56:23.000 What's the cost of a 4-gig link? 00:56:23.000 --> 00:56:24.000 It's 3. 00:56:24.000 --> 00:56:32.000 Oops, that's no good now, because Spanning Tree Protocol is going to make a bad decision based on bad information. 00:56:32.000 --> 00:56:34.000 What if we had 8 links? 00:56:34.000 --> 00:56:35.000 What if we had 8 links? 00:56:35.000 --> 00:56:39.000 1, 2, 3, 4, 5, 6, 7, 8. 00:56:39.000 --> 00:56:40.000 Bundle them all together. 00:56:40.000 --> 00:56:46.000 Now, you got 1 8-gig link, which equals a cost of 3. 00:56:46.000 --> 00:56:49.000 You see the problem? 00:56:49.000 --> 00:56:52.000 They're all treated the exact same way. 00:56:52.000 --> 00:57:03.000 So, this is garbage nowadays for most of us, because most of us have links between our switches that are anywhere from 1-gig to 4-gigs to 8-gigs. 00:57:04.000 --> 00:57:08.000 to 10-gigs to 40-gigs to 20-gigs, right? 00:57:08.000 --> 00:57:11.000 And all of those would be treated with really low costs. 00:57:11.000 --> 00:57:13.000 Really low costs. 00:57:13.000 --> 00:57:16.000 Or, sorry, a cost of 3, not low cost, a cost of 3. 00:57:16.000 --> 00:57:18.000 There's not enough room, okay? 00:57:18.000 --> 00:57:20.000 So, long method is the way to go nowadays. 00:57:20.000 --> 00:57:23.000 And how would you change to a long method? 00:57:23.000 --> 00:57:29.000 Well, you'd want to do it on all your switches, because if you have short method on some, long method on the other, the costs don't make sense. 00:57:29.000 --> 00:57:30.000 Right? 00:57:30.000 --> 00:57:31.000 Because lower is always better. 00:57:31.000 --> 00:57:36.000 So, if it's 4 on this device, but 200,000 on another device, that doesn't work. 00:57:36.000 --> 00:57:37.000 That doesn't work. 00:57:37.000 --> 00:57:39.000 It's got to be doing it the same way. 00:57:39.000 --> 00:57:46.000 So, configure terminal, spanning tree, path, cost, method, and you just change it to long. 00:57:46.000 --> 00:57:49.000 That's all you have to do. 00:57:49.000 --> 00:57:53.000 Now, if you're using multiple spanning tree protocol, it automatically uses long. 00:57:53.000 --> 00:57:54.000 Automatically. 00:57:54.000 --> 00:57:59.000 So, as soon as you change this multiple spanning tree protocol, it uses long automatically for you. 00:57:59.000 --> 00:58:03.000 But if you're using per-VLAN spanning tree plus or rapid per-VLAN spanning tree plus, it's using the short method. 00:58:03.000 --> 00:58:05.000 Most cases by default. 00:58:05.000 --> 00:58:08.000 But many of your new switches might be just defaulting to long. 00:58:08.000 --> 00:58:09.000 Right? 00:58:09.000 --> 00:58:21.000 But for the exam, right, be aware that if the scenario is about adjusting your cost to make room for your ether channel links, your higher speed links, 00:58:21.000 --> 00:58:24.000 then just switching to a long method will solve your problem. 00:58:24.000 --> 00:58:27.000 Do show span VLAN 10 now, you'll see. 00:58:27.000 --> 00:58:28.000 And this is for the whole device. 00:58:28.000 --> 00:58:29.000 It's not on a VLAN. 00:58:29.000 --> 00:58:31.000 It'll affect all VLANs. 00:58:31.000 --> 00:58:34.000 Now, you'll see that the cost is $20,000 for my what? 00:58:34.000 --> 00:58:37.000 For my 10-gig links. 00:58:37.000 --> 00:58:39.000 Oh, 1-gig link. 00:58:39.000 --> 00:58:45.000 So, now that it's $20,000 for my 1-gig link, you'll see here that I have room in here now. 00:58:45.000 --> 00:58:49.000 So, a 2-gig link, a 2-gig link is $10,000. 00:58:49.000 --> 00:58:53.000 A 4-gig link would get a cost of $5,000. 00:58:53.000 --> 00:58:58.000 An 8-gig link would get a cost of $2,500. 00:58:58.000 --> 00:59:01.000 And then you got your 10-gig at $2,000. 00:59:01.000 --> 00:59:08.000 And then your 100-gig would be $200, giving you room in between $200 and $2,000 for what? 00:59:08.000 --> 00:59:12.000 20-gig links, 40-gig links, 80-gig links, up to 100-gig. 00:59:12.000 --> 00:59:14.000 So, 100-gig gets $200. 00:59:14.000 --> 00:59:19.000 And then your 1,000-gig links would get $20. 00:59:19.000 --> 00:59:24.000 And your 10,000-gig links would get a cost of $2. 00:59:24.000 --> 00:59:29.000 And then once we get higher than 10,000-gig links, then we're going to have to come up with a new calculation. 00:59:29.000 --> 00:59:34.000 We're going to have to go to the super long method then and adjust the whole thing. 00:59:34.000 --> 00:59:35.000 All right. 00:59:35.000 --> 00:59:36.000 So, there you go. 00:59:36.000 --> 00:59:41.000 So, cost is on an interface-by-interface basis. 00:59:41.000 --> 00:59:43.000 So, we know we have a cost of 4. 00:59:43.000 --> 00:59:45.000 We know we have a cost of 4. 00:59:45.000 --> 00:59:46.000 We know we have a cost of 4. 00:59:46.000 --> 00:59:47.000 Why? 00:59:47.000 --> 00:59:51.000 They're all 1-gig links in this particular case. 00:59:51.000 --> 00:59:52.000 All right. 00:59:52.000 --> 00:59:55.000 What's another thing we have to be aware of then? 00:59:55.000 --> 00:59:56.000 Let me clean this up. 00:59:56.000 --> 00:59:58.000 Another thing we have to be aware of. 00:59:58.000 --> 01:00:02.000 So, I'm just going to use short method for my calculation today. 01:00:02.000 --> 01:00:08.000 But be aware that you might have to use either short or long on the certification exam for your calculations. 01:00:08.000 --> 01:00:15.000 So, now what we need to recognize is that our cost is going to be cumulative. 01:00:15.000 --> 01:00:22.000 Meaning that cost has to be added from there all the way back to the root bridge. 01:00:22.000 --> 01:00:27.000 So, it's cumulative from you to the root bridge. 01:00:28.000 --> 01:00:43.000 So, if we go to our BPDU really quickly, recognize that we have our root path cost listed in there. 01:00:43.000 --> 01:00:47.000 Our root path cost listed in there. 01:00:48.000 --> 01:00:50.000 Our root path cost listed in there. 01:00:50.000 --> 01:01:01.000 So, when I send a BPDU, the root path cost indicates how far I am away from the root bridge. 01:01:01.000 --> 01:01:06.000 So, DSW1 sends a BPDU, the root path cost is going to be what? 01:01:06.000 --> 01:01:09.000 It's going to be a big old 0 in this case. 01:01:09.000 --> 01:01:13.000 The root path cost is 0. 01:01:13.000 --> 01:01:14.000 Why? 01:01:14.000 --> 01:01:16.000 Because it is the root bridge. 01:01:16.000 --> 01:01:19.000 You can't get any closer than being the root bridge. 01:01:19.000 --> 01:01:24.000 So, anytime the root bridge sends out a BPDU, it says my root path cost is 0. 01:01:24.000 --> 01:01:27.000 So, now I can take my number 4. 01:01:27.000 --> 01:01:30.000 So, 4 plus 0. 01:01:30.000 --> 01:01:34.000 That is my cumulative cost. 01:01:34.000 --> 01:01:37.000 That's how I get that cumulative cost. 01:01:37.000 --> 01:01:39.000 So, same thing here. 01:01:39.000 --> 01:01:41.000 BPDU comes out this interface. 01:01:41.000 --> 01:01:43.000 Root path cost of 0. 01:01:43.000 --> 01:01:50.000 So, you've got 4 plus 0 for a grand total of what? 01:01:50.000 --> 01:01:51.000 Total cost of 4. 01:01:51.000 --> 01:01:53.000 Total cost of 4. 01:01:53.000 --> 01:01:56.000 So, now I know that there's a total cost of 4 here. 01:01:56.000 --> 01:01:59.000 There's a total cost of 4 here. 01:01:59.000 --> 01:02:03.000 But now, this is going to be different. 01:02:03.000 --> 01:02:04.000 Why? 01:02:04.000 --> 01:02:10.000 Because we have to realize that the BPDU is coming from DSW2. 01:02:10.000 --> 01:02:17.000 So, my decision about going this way has to be based on decisions that DSW2 has made. 01:02:17.000 --> 01:02:23.000 So, DSW2, let's say, has chosen to go this way, over to the root bridge. 01:02:23.000 --> 01:02:28.000 So, what is the root path cost from DSW2 to get to the root bridge? 01:02:28.000 --> 01:02:33.000 Well, root bridge would be saying my root path cost is 0. 01:02:33.000 --> 01:02:36.000 DSW2 would have a value of 4 for its interface. 01:02:36.000 --> 01:02:39.000 So, 4 plus 0 equals 4. 01:02:39.000 --> 01:02:46.000 So, the cost to go from DSW2 to DSW1, DSW2 figured out is 4, just like over here. 01:02:46.000 --> 01:02:55.000 Now, DSW2, when it sends its BPDU, it says my root path cost equals 4. 01:02:55.000 --> 01:02:57.000 My root path cost equals 4. 01:02:57.000 --> 01:03:03.000 So, now my 4 plus your 4 is the cumulative cost of 8. 01:03:03.000 --> 01:03:08.000 And then my 4, because you're going to send out two BPDUs, my 4 plus your 4 equals a grand total of 8. 01:03:08.000 --> 01:03:11.000 So, that's how the cumulative cost is calculated. 01:03:11.000 --> 01:03:18.000 So, the root path cost has to be sent in that BPDU telling the downstream switch, hey, I am so far away from the root bridge. 01:03:18.000 --> 01:03:25.000 So, then you take that number plus your interface number, you add them together, and you got your results. 01:03:26.000 --> 01:03:44.000 So, we now know, based on this information, that we have a total cost, a cumulative cost of 4, of 4, of 8, and of 8. 01:03:44.000 --> 01:03:47.000 That's what we have. 01:03:47.000 --> 01:03:54.000 So, with that in mind now, we have to pick the lowest cost path. 01:03:54.000 --> 01:04:01.000 So, the lowest cost path is going to be this one, and this one, not this one, and not this one. 01:04:01.000 --> 01:04:05.000 So, we no longer have to worry about 47 and 48. 01:04:05.000 --> 01:04:10.000 They are not good enough based on cost. 01:04:10.000 --> 01:04:12.000 Now, we're left with two. 01:04:12.000 --> 01:04:13.000 Why? 01:04:13.000 --> 01:04:14.000 You cannot have ties. 01:04:14.000 --> 01:04:16.000 No ties. 01:04:16.000 --> 01:04:19.000 Anybody ever watch The Incredibles? 01:04:19.000 --> 01:04:20.000 No capes. 01:04:20.000 --> 01:04:21.000 Right? 01:04:21.000 --> 01:04:23.000 Well, no ties right here. 01:04:24.000 --> 01:04:31.000 So, if there's a tie in cost, now you have to look at this tiebreaker right here. 01:04:31.000 --> 01:04:37.000 This tiebreaker says, look at the upstream bridge ID. 01:04:37.000 --> 01:04:39.000 Upstream bridge ID. 01:04:39.000 --> 01:04:41.000 So, upstream would mean up here. 01:04:41.000 --> 01:04:47.000 So, what is the bridge ID of DSW1? 01:04:47.000 --> 01:04:50.000 What is the upstream bridge ID? 01:04:50.000 --> 01:05:02.000 Well, if we go back to this slide from before, we'll see that the bridge ID is 32,778 plus this MAC address. 01:05:06.000 --> 01:05:09.000 That information is going to be in here. 01:05:09.000 --> 01:05:12.000 Sending bridge ID. 01:05:15.000 --> 01:05:17.000 Sending bridge ID. 01:05:17.000 --> 01:05:21.000 So, sending bridge ID. 01:05:21.000 --> 01:05:26.000 Sending bridge ID, because there's two different BPDUs, one for each of those links. 01:05:28.000 --> 01:05:35.000 So, folks, which of these is going to have the lower bridge ID? 01:05:36.000 --> 01:05:49.000 Well, if it's the bridge ID of DSW1, in this scenario, it's the same. 01:05:49.000 --> 01:05:51.000 It's the same in this scenario. 01:05:51.000 --> 01:05:53.000 That's right. 01:05:53.000 --> 01:05:58.000 So, we have a tie again in this scenario. 01:05:58.000 --> 01:06:02.000 So, to break the tie, we now have to look at what? 01:06:02.000 --> 01:06:08.000 Lowest upstream port ID. 01:06:08.000 --> 01:06:13.000 The lowest upstream port ID. 01:06:13.000 --> 01:06:15.000 So, is that in the BPDU as well? 01:06:15.000 --> 01:06:16.000 Yes, it is. 01:06:16.000 --> 01:06:17.000 Look at this. 01:06:17.000 --> 01:06:20.000 Sending port ID. 01:06:20.000 --> 01:06:24.000 So, when DSW1 sends out its BPDU, there's a port ID in there. 01:06:24.000 --> 01:06:28.000 For example, this port will be 128.21. 01:06:28.000 --> 01:06:35.000 For example, this port will be 128.21, and this one's going to be 128.22. 01:06:35.000 --> 01:06:37.000 Let's take a peek at that. 01:06:37.000 --> 01:06:39.000 Go to DSW1 here as an example. 01:06:47.000 --> 01:06:49.000 You'll see that it's listed right here. 01:06:51.000 --> 01:06:55.000 128.21, 128.22. 01:06:55.000 --> 01:07:00.000 If we go back to our slides, we'll see in our slides from this example, same thing. 01:07:00.000 --> 01:07:04.000 128.21, 128.22 for this. 01:07:04.000 --> 01:07:09.000 So, it sends its port ID in its BPDU. 01:07:09.000 --> 01:07:16.000 So, now, when ASW1 has those BPDUs, it's looking at what? 01:07:16.000 --> 01:07:20.000 Well, it figured out with the route path cost that its total cost was four. 01:07:20.000 --> 01:07:22.000 Total cost was four, so we have a tie. 01:07:22.000 --> 01:07:24.000 It then looked at the sending bridge ID. 01:07:24.000 --> 01:07:28.000 It figured out that, oh, the bridge ID is tied in those two different BPDUs. 01:07:28.000 --> 01:07:32.000 Then it looks at the sending port ID, and it says, oh, this one says 128.1. 01:07:32.000 --> 01:07:34.000 This one says 128.2. 01:07:34.000 --> 01:07:38.000 Therefore, this one is the winner and becomes my root port. 01:07:43.000 --> 01:07:44.000 There you go. 01:07:44.000 --> 01:07:45.000 That's it. 01:07:45.000 --> 01:07:46.000 We're done. 01:07:46.000 --> 01:07:51.000 The tie will always be broken with this every time because no two interfaces can have the same port ID. 01:07:52.000 --> 01:08:07.000 Now, you might be asking, well, is it possible that the upstream bridge ID could be the deciding factor? 01:08:07.000 --> 01:08:14.000 Well, the only way the upstream bridge ID could be the deciding factor, for example, would be in a situation if we had, 01:08:14.000 --> 01:08:19.000 let's say this was an Ether Channel bundle here, this was an Ether Channel bundle here, 01:08:19.000 --> 01:08:24.000 and let's say the total cost this way was eight and the total cost this way was eight. 01:08:24.000 --> 01:08:31.000 So, if the total cost of each of those is eight, we have a tie. 01:08:31.000 --> 01:08:40.000 Now, when we look at the upstream bridge ID, this bridge ID would be compared to this bridge ID. 01:08:40.000 --> 01:08:42.000 Now, who would win in this case? 01:08:42.000 --> 01:08:45.000 Well, obviously, it would be this one. 01:08:45.000 --> 01:08:48.000 So, then this is the loser and we have our winner. 01:08:48.000 --> 01:08:49.000 It's over. 01:08:49.000 --> 01:08:55.000 So, you can see how bridge ID must be checked. 01:08:55.000 --> 01:09:01.000 I've had in the past students say, well, we don't have to check the bridge ID because it is tied, but it's not always going to be tied. 01:09:01.000 --> 01:09:02.000 Look at this scenario. 01:09:02.000 --> 01:09:05.000 It wouldn't be tied if it was done this way. 01:09:05.000 --> 01:09:07.000 So, pay attention to that on the exam. 01:09:07.000 --> 01:09:12.000 Always go through cost, bridge ID, then port ID every time. 01:09:12.000 --> 01:09:17.000 Don't cut corners unless you know for sure that you did not need to look at the bridge ID. 01:09:17.000 --> 01:09:19.000 Be careful with that. 01:09:19.000 --> 01:09:21.000 So, that's ASW1 down. 01:09:21.000 --> 01:09:23.000 Let's go down to ASW2. 01:09:23.000 --> 01:09:24.000 Stand on it. 01:09:24.000 --> 01:09:26.000 I'm going to do this really, really quick now. 01:09:26.000 --> 01:09:31.000 I'm going to do this really, really quick because it's the same step over and over and over again. 01:09:31.000 --> 01:09:33.000 So, all you have to do is learn the steps. 01:09:33.000 --> 01:09:34.000 That's it. 01:09:34.000 --> 01:09:35.000 Just learn the steps. 01:09:35.000 --> 01:09:36.000 You're done. 01:09:36.000 --> 01:09:38.000 And then you just repeat them over and over and over again. 01:09:38.000 --> 01:09:41.000 And you can practice them on the same picture, same diagram. 01:09:41.000 --> 01:09:43.000 Because all you do is learn the steps. 01:09:43.000 --> 01:09:44.000 Learn the steps. 01:09:44.000 --> 01:09:45.000 Memorize the steps. 01:09:45.000 --> 01:09:47.000 That's how you get through the algorithm. 01:09:47.000 --> 01:09:49.000 So, in the end here now, what do we got? 01:09:49.000 --> 01:09:50.000 I can go this way. 01:09:50.000 --> 01:09:51.000 I can go this way. 01:09:51.000 --> 01:09:52.000 I can go this way. 01:09:52.000 --> 01:09:53.000 I can go this way. 01:09:53.000 --> 01:09:57.000 If I go this way, it's 4 plus 0, 4 plus 0, 4 plus 4, 4 plus 4. 01:09:57.000 --> 01:09:58.000 Right? 01:09:58.000 --> 01:09:59.000 So, we know that we got 8. 01:09:59.000 --> 01:10:00.000 We got 8. 01:10:00.000 --> 01:10:05.000 We got – whoa, what did I do? 01:10:05.000 --> 01:10:07.000 I hit the subtitles button. 01:10:07.000 --> 01:10:08.000 There we go. 01:10:08.000 --> 01:10:09.000 All right. 01:10:09.000 --> 01:10:10.000 Never done that before. 01:10:10.000 --> 01:10:11.000 All right. 01:10:11.000 --> 01:10:12.000 So, we got 4. 01:10:12.000 --> 01:10:13.000 We got 4. 01:10:13.000 --> 01:10:14.000 We got 8. 01:10:14.000 --> 01:10:15.000 We got 8. 01:10:15.000 --> 01:10:16.000 That's what we have. 01:10:16.000 --> 01:10:18.000 So, we can eliminate these. 01:10:18.000 --> 01:10:19.000 We don't matter anymore. 01:10:19.000 --> 01:10:23.000 So, we got cost is tied, upstream bridge ID. 01:10:23.000 --> 01:10:24.000 So, we know. 01:10:24.000 --> 01:10:26.000 Look, this is when you know 100% it's going to be tied. 01:10:26.000 --> 01:10:27.000 Why? 01:10:27.000 --> 01:10:30.000 Because both of them are attached to the same device. 01:10:30.000 --> 01:10:33.000 So, immediately you know that that's a tie. 01:10:33.000 --> 01:10:35.000 There's no way that cannot be a tie in this scenario. 01:10:36.000 --> 01:10:40.000 If both links go to the exact same switch, it's tied every time. 01:10:40.000 --> 01:10:41.000 The bridge ID. 01:10:41.000 --> 01:10:42.000 All right. 01:10:42.000 --> 01:10:44.000 So, now we've got this, which is the port ID. 01:10:44.000 --> 01:10:49.000 So, we got 128.23 and 128.24. 01:10:49.000 --> 01:10:50.000 So, 23 is the winner. 01:10:50.000 --> 01:10:52.000 So, you come back down to this side. 01:10:52.000 --> 01:10:53.000 You follow it. 01:10:53.000 --> 01:10:56.000 And you're like, this is the root port in this case. 01:10:56.000 --> 01:10:57.000 All right. 01:10:57.000 --> 01:10:58.000 So, then the same thing for this one. 01:10:58.000 --> 01:10:59.000 So, we're going to stand on this switch. 01:10:59.000 --> 01:11:03.000 We can go this way, this way, this way, this way, this way, this way, this way, this way. 01:11:03.000 --> 01:11:07.000 We've got 4, 4, 4, 4, 8, 8, 8, 8. 01:11:07.000 --> 01:11:09.000 Based on what we talked about before. 01:11:09.000 --> 01:11:11.000 So, no good, no good. 01:11:11.000 --> 01:11:12.000 We can eliminate those. 01:11:12.000 --> 01:11:14.000 So, now it's between these four here. 01:11:14.000 --> 01:11:16.000 So, again, they're all going to the same device. 01:11:16.000 --> 01:11:17.000 So, this is going to be tied. 01:11:17.000 --> 01:11:19.000 So, port IDs. 01:11:19.000 --> 01:11:23.000 We got 128.17.18.19.20. 01:11:23.000 --> 01:11:25.000 17 is the winner because it's the lowest. 01:11:25.000 --> 01:11:27.000 There is my root port. 01:11:27.000 --> 01:11:29.000 Let's solidify those right there. 01:11:29.000 --> 01:11:31.000 Okay. 01:11:31.000 --> 01:11:33.000 That's what you want to be able to do. 01:11:33.000 --> 01:11:35.000 So, figure out who the root bridge is. 01:11:35.000 --> 01:11:37.000 Figure out who the root ports are. 01:11:37.000 --> 01:11:39.000 The only way to do that is come back. 01:11:39.000 --> 01:11:40.000 If you're not familiar with that, come back. 01:11:40.000 --> 01:11:41.000 Review. 01:11:41.000 --> 01:11:42.000 Go through the steps. 01:11:42.000 --> 01:11:43.000 Pause the video. 01:11:43.000 --> 01:11:45.000 Write some things out. 01:11:45.000 --> 01:11:46.000 Draw your own little diagram. 01:11:46.000 --> 01:11:47.000 Do the math yourself. 01:11:47.000 --> 01:11:51.000 That's how you're going to practice it. 01:11:51.000 --> 01:11:52.000 Okay. 01:11:52.000 --> 01:11:53.000 So, the 120. 01:11:53.000 --> 01:11:55.000 We're going to come back to that Conrad a little later. 01:11:55.000 --> 01:11:56.000 All right. 01:11:56.000 --> 01:11:57.000 What does that actually mean? 01:11:57.000 --> 01:11:58.000 But it's a number. 01:11:58.000 --> 01:12:02.000 That number there is something you can manipulate to control the decision-making process, 01:12:02.000 --> 01:12:03.000 and I'll teach you that today. 01:12:03.000 --> 01:12:04.000 So, we're going to come back to that number. 01:12:07.000 --> 01:12:08.000 All right. 01:12:08.000 --> 01:12:11.000 So, now that we've figured out who the root port is, we're going to go ahead, 01:12:11.000 --> 01:12:13.000 and we are going to take a look at the designated port now. 01:12:13.000 --> 01:12:16.000 So, once you're done, you move on. 01:12:16.000 --> 01:12:17.000 Once you're done, you move on. 01:12:17.000 --> 01:12:19.000 Now we're on phase three, step three. 01:12:19.000 --> 01:12:21.000 So, what is a designated port? 01:12:21.000 --> 01:12:26.000 Well, first of all, you need one designated port for every single one of these. 01:12:26.000 --> 01:12:28.000 You need one designated port for every single one of these. 01:12:28.000 --> 01:12:29.000 What are these? 01:12:29.000 --> 01:12:30.000 Links. 01:12:30.000 --> 01:12:31.000 Connections. 01:12:31.000 --> 01:12:32.000 Call them collision domains if you want to. 01:12:32.000 --> 01:12:34.000 Call them segments if you want to. 01:12:34.000 --> 01:12:36.000 Call them links if you want to. 01:12:36.000 --> 01:12:37.000 Right? 01:12:37.000 --> 01:12:41.000 Call them cables between switches if you want to. 01:12:41.000 --> 01:12:44.000 The idea here is every single one of these need a designated port. 01:12:44.000 --> 01:12:45.000 So, what does that mean? 01:12:45.000 --> 01:12:50.000 It means one side has to have a designated port, and that's what you're figuring out. 01:12:50.000 --> 01:12:54.000 Which side gets the designated port? 01:12:56.000 --> 01:12:59.000 One, and only one per link. 01:13:02.000 --> 01:13:08.000 So, when you position yourself, for example, on the link, you say to yourself, 01:13:08.000 --> 01:13:12.000 is this going to be the designated port, or is this going to be the designated port? 01:13:12.000 --> 01:13:15.000 And it's based on what? 01:13:15.000 --> 01:13:22.000 Again, the lowest cumulative cost to the root bridge, just like we did with our root ports. 01:13:27.000 --> 01:13:30.000 There's a few shortcuts here I want you to know about. 01:13:32.000 --> 01:13:39.000 All ports on a root bridge are designated, and we saw that earlier. 01:13:48.000 --> 01:13:52.000 All ports are designated. 01:13:52.000 --> 01:13:56.000 So, immediately on an exam in the real world, 01:13:56.000 --> 01:14:00.000 you don't have to do any math on any links that are connected. 01:14:00.000 --> 01:14:04.000 Designated port, designated port, designated port, designated port, 01:14:04.000 --> 01:14:07.000 designated, designated, designated, designated, designated. 01:14:07.000 --> 01:14:08.000 Done. 01:14:08.000 --> 01:14:10.000 You just eliminated all these links. 01:14:10.000 --> 01:14:12.000 Now, you figured out the designated port here. 01:14:12.000 --> 01:14:13.000 You figured it out here. 01:14:13.000 --> 01:14:14.000 You figured it out here. 01:14:14.000 --> 01:14:17.000 Here, here, here, here, here. 01:14:17.000 --> 01:14:18.000 You don't have to worry about those links anymore. 01:14:18.000 --> 01:14:20.000 It was that easy. 01:14:21.000 --> 01:14:27.000 Also, it says here designated ports are always on the opposite side of root ports. 01:14:27.000 --> 01:14:30.000 So, if you look at this scenario, I have a root port here. 01:14:30.000 --> 01:14:33.000 This makes that automatically a designated port. 01:14:33.000 --> 01:14:34.000 I have a root port here. 01:14:34.000 --> 01:14:36.000 This makes that automatically a designated port. 01:14:36.000 --> 01:14:38.000 I have a root port here. 01:14:38.000 --> 01:14:40.000 This makes that automatically a designated port. 01:14:40.000 --> 01:14:42.000 You might be saying, well, of course it does, Raymond. 01:14:42.000 --> 01:14:43.000 It's the root bridge. 01:14:43.000 --> 01:14:45.000 That's so silly. 01:14:45.000 --> 01:14:52.000 No, but you got to think of a bigger environment. 01:14:52.000 --> 01:14:59.000 What if I had another switch here? 01:14:59.000 --> 01:15:01.000 So, we have three layers now. 01:15:01.000 --> 01:15:02.000 Why? 01:15:02.000 --> 01:15:04.000 Because we did. 01:15:04.000 --> 01:15:11.000 Now, for this switch, we have a bunch of PCs connected here. 01:15:11.000 --> 01:15:15.000 Which way are we going to go to get to the root bridge? 01:15:15.000 --> 01:15:18.000 Well, we have no choice but to go this way. 01:15:18.000 --> 01:15:19.000 It's the only way to go. 01:15:19.000 --> 01:15:24.000 So, we know that this is going to be our root port. 01:15:24.000 --> 01:15:28.000 Designated ports are always on the opposite side of root ports. 01:15:28.000 --> 01:15:29.000 There you go. 01:15:29.000 --> 01:15:30.000 Shortcut. 01:15:30.000 --> 01:15:46.000 So, in the exam, if you see a topology that's something like this, 01:15:46.000 --> 01:15:53.000 and this is the root bridge, and you figure out that this is a root port, 01:15:53.000 --> 01:15:55.000 then you know this is a designated port. 01:15:55.000 --> 01:15:58.000 You figure out that this is a root port, you know this is a designated port. 01:15:58.000 --> 01:16:00.000 You know that automatically. 01:16:00.000 --> 01:16:10.000 Shortcuts to help you get through questions faster and in the real world. 01:16:10.000 --> 01:16:17.000 So, let's go ahead and actually figure out our designated ports in this scenario here. 01:16:17.000 --> 01:16:18.000 That we have left. 01:16:18.000 --> 01:16:19.000 So, we have this one left. 01:16:19.000 --> 01:16:20.000 We have this one left. 01:16:20.000 --> 01:16:21.000 We have this one left and this one left. 01:16:21.000 --> 01:16:26.000 That's it. 01:16:26.000 --> 01:16:27.000 So, what are we going to do? 01:16:27.000 --> 01:16:31.000 We're going to stand on our link and we're going to ask ourselves, 01:16:31.000 --> 01:16:44.000 if I walk this way, how far is it from here, this dot, how far from here to here? 01:16:44.000 --> 01:16:50.000 How far? 01:16:50.000 --> 01:16:51.000 That's what I want to know. 01:16:51.000 --> 01:16:54.000 How far from there to there? 01:16:55.000 --> 01:16:57.000 I don't care about the distance from here to here. 01:16:57.000 --> 01:16:58.000 This doesn't matter. 01:16:58.000 --> 01:17:00.000 I don't calculate that. 01:17:00.000 --> 01:17:07.000 How far from ASW2 to DSW1 to the root bridge? 01:17:07.000 --> 01:17:10.000 Well, we know based on the root port calculation that it's four. 01:17:10.000 --> 01:17:12.000 So, that's already figured out. 01:17:12.000 --> 01:17:14.000 It's four. 01:17:14.000 --> 01:17:22.000 So, the next question is, if I walk this way, how far from here to here? 01:17:25.000 --> 01:17:27.000 Again, it's four. 01:17:27.000 --> 01:17:29.000 It might be different. 01:17:29.000 --> 01:17:30.000 It could be three. 01:17:30.000 --> 01:17:32.000 If it was three, let's pretend it's three. 01:17:32.000 --> 01:17:35.000 If it's three, who's the winner? 01:17:35.000 --> 01:17:36.000 We've already done, right? 01:17:36.000 --> 01:17:40.000 Because cost is four versus three. 01:17:40.000 --> 01:17:42.000 Lower is better. 01:17:42.000 --> 01:17:43.000 We know it's three. 01:17:43.000 --> 01:17:44.000 We're going to use three then. 01:17:44.000 --> 01:17:48.000 So, this would be the winner and this would be the designated port. 01:17:48.000 --> 01:17:49.000 That's how you figure it out. 01:17:49.000 --> 01:17:52.000 But, in our case, cost is tied. 01:17:53.000 --> 01:17:56.000 So, if cost is tied, what do you do? 01:17:56.000 --> 01:17:58.000 Upstream bridge ID. 01:17:58.000 --> 01:18:01.000 So, with the upstream bridge ID, who are we comparing here? 01:18:01.000 --> 01:18:08.000 We're comparing the bridge ID of DSW2 to the bridge ID of ASW2. 01:18:08.000 --> 01:18:10.000 That's what we're comparing right now. 01:18:10.000 --> 01:18:13.000 So, cost is tied, but let's compare our bridge ID. 01:18:13.000 --> 01:18:17.000 So, we said ASW2 and DSW2. 01:18:17.000 --> 01:18:18.000 All right, folks. 01:18:18.000 --> 01:18:20.000 Here's your chance to shine. 01:18:22.000 --> 01:18:25.000 ASW2 versus DSW2. 01:18:25.000 --> 01:18:28.000 Who has the lower bridge ID? 01:18:28.000 --> 01:18:29.000 Go. 01:18:42.000 --> 01:18:44.000 Absolutely, DSW2. 01:18:44.000 --> 01:18:45.000 Perfect. 01:18:45.000 --> 01:18:51.000 So, since it's DSW2, we have found our winner. 01:18:51.000 --> 01:18:56.000 We don't have to look at port ID in this particular scenario. 01:18:56.000 --> 01:19:05.000 So, we now know that this is the designated port. 01:19:05.000 --> 01:19:11.000 Now, the cool thing is is that, by default, both of these links are going to be the exact same answer. 01:19:11.000 --> 01:19:15.000 So, since we figured out one, we automatically know the other one in this particular case. 01:19:15.000 --> 01:19:17.000 So, that's good. 01:19:17.000 --> 01:19:18.000 All right. 01:19:18.000 --> 01:19:21.000 So, we have now figured out those two links. 01:19:21.000 --> 01:19:30.000 Let's now stand on this link and say, if I go this way, how far from here to here? 01:19:30.000 --> 01:19:34.000 If I go this way, how far from here to here? 01:19:34.000 --> 01:19:40.000 Well, we know, based on our route path calculation before, that it was four and four over here. 01:19:40.000 --> 01:19:43.000 So, we have a tie again. 01:19:43.000 --> 01:19:47.000 So, to break the tie, what do we do? 01:19:47.000 --> 01:19:48.000 We look at the upstream bridge ID. 01:19:48.000 --> 01:19:52.000 Now, if it wasn't tied, if one of them was lower, that's the one we would use. 01:19:52.000 --> 01:19:55.000 But in this case, we have a tie, so let's look at our upstream bridge ID next. 01:19:55.000 --> 01:20:01.000 So, now it's DSW2 versus ASW1. 01:20:01.000 --> 01:20:03.000 So, folks, who is it? 01:20:03.000 --> 01:20:04.000 Chat in. 01:20:04.000 --> 01:20:09.000 Is it DSW2 or is it ASW1? 01:20:18.000 --> 01:20:20.000 Yeah, it's DSW2 again. 01:20:20.000 --> 01:20:21.000 Why? 01:20:21.000 --> 01:20:27.000 Priority is tied, but if we look at the MAC address, four is lower than D. 01:20:27.000 --> 01:20:28.000 Excellent job. 01:20:28.000 --> 01:20:30.000 Excellent job. 01:20:30.000 --> 01:20:35.000 So, we know now that DSW2 is going to be the preferred option. 01:20:35.000 --> 01:20:39.000 So, designated port over here, designated port over here. 01:20:39.000 --> 01:20:40.000 Okay. 01:20:40.000 --> 01:20:41.000 So, let's fill those in. 01:20:41.000 --> 01:20:42.000 Perfect. 01:20:42.000 --> 01:20:51.000 So, remember we said one designated port per link. 01:20:51.000 --> 01:20:59.000 So, we figured out our designated port here, here, here, here, here, here, here, here, here, here, here, and here. 01:20:59.000 --> 01:21:01.000 I think we've got them all. 01:21:01.000 --> 01:21:02.000 We have them all. 01:21:02.000 --> 01:21:03.000 All right. 01:21:03.000 --> 01:21:10.000 Now, let's not forget about those ports that go down to our end stations. 01:21:10.000 --> 01:21:14.000 They're still participating in spanning tree protocol. 01:21:14.000 --> 01:21:17.000 Now, right now, we're only focusing on VLAN10. 01:21:17.000 --> 01:21:21.000 So, I want you to remember that these ports are in VLAN10. 01:21:21.000 --> 01:21:24.000 These ports are in VLAN20. 01:21:24.000 --> 01:21:27.000 These ports are in VLAN50. 01:21:27.000 --> 01:21:28.000 What does that mean? 01:21:28.000 --> 01:21:37.000 It means that if this is the tree for VLAN10, this port doesn't matter, this port doesn't matter, this port doesn't matter. 01:21:37.000 --> 01:21:38.000 Why? 01:21:38.000 --> 01:21:39.000 Because they're not in VLAN10. 01:21:39.000 --> 01:21:45.000 Only the user ports that are in VLAN10 matter. 01:21:45.000 --> 01:21:47.000 Now, my question to you folks is this. 01:21:47.000 --> 01:21:54.000 Do you think they will be root ports, designated ports, or blocked ports? 01:21:54.000 --> 01:21:55.000 What do you think it is? 01:21:55.000 --> 01:22:02.000 If you don't know the answer, I want you to guess. 01:22:02.000 --> 01:22:10.000 What are these going to be? 01:22:10.000 --> 01:22:22.000 Root port, designated port, or blocked port? 01:22:22.000 --> 01:22:23.000 All right. 01:22:24.000 --> 01:22:29.000 Conrad in with one answer. 01:22:29.000 --> 01:22:32.000 Designated port. 01:22:32.000 --> 01:22:36.000 Louisa's designated port, I think. 01:22:36.000 --> 01:22:37.000 Come on, folks. 01:22:37.000 --> 01:22:38.000 I need you to guess here. 01:22:38.000 --> 01:22:42.000 Do you think it's going to be designated port, root port, or blocked port? 01:22:42.000 --> 01:22:45.000 Well, here's the thing. 01:22:45.000 --> 01:22:47.000 Ask yourself this. 01:22:48.000 --> 01:22:53.000 Does it make sense for it to be a blocked port? 01:22:53.000 --> 01:22:58.000 Yes or no? 01:22:58.000 --> 01:23:02.000 Because if it's a blocked port, do your users get to communicate? 01:23:02.000 --> 01:23:03.000 No, they don't. 01:23:03.000 --> 01:23:05.000 So, definitely not a blocked port. 01:23:05.000 --> 01:23:06.000 Okay. 01:23:06.000 --> 01:23:08.000 Can it be a root port? 01:23:08.000 --> 01:23:11.000 Well, what is a root port based on my definition? 01:23:11.000 --> 01:23:19.000 The port on the switch that is the fastest way or the best way to get to the root bridge. 01:23:19.000 --> 01:23:22.000 And you can only have one. 01:23:22.000 --> 01:23:27.000 So, is the fastest way to the root bridge through the PCs? 01:23:27.000 --> 01:23:28.000 Do we already have one? 01:23:28.000 --> 01:23:30.000 Yeah. 01:23:30.000 --> 01:23:31.000 Okay. 01:23:31.000 --> 01:23:32.000 So, it can't be blocked. 01:23:32.000 --> 01:23:33.000 It can't be root port. 01:23:33.000 --> 01:23:34.000 Now, remember what I said. 01:23:34.000 --> 01:23:42.000 Every single link in Spanning Tree must have a designated port. 01:23:42.000 --> 01:23:44.000 Remember? 01:23:44.000 --> 01:23:50.000 Every single link in Spanning Tree Protocol must have a designated port. 01:23:50.000 --> 01:23:52.000 So, what do you think? 01:23:52.000 --> 01:23:59.000 The ports that go down to the end users has to be designated ports. 01:23:59.000 --> 01:24:00.000 There you go. 01:24:00.000 --> 01:24:05.000 There you go. 01:24:05.000 --> 01:24:06.000 All right. 01:24:06.000 --> 01:24:11.000 And then, once we're done going through these steps, we're down to step four. 01:24:11.000 --> 01:24:16.000 Transition the root port and the designated ports to the forwarding state so we can send 01:24:16.000 --> 01:24:22.000 and receive traffic, and every other port goes to the blocking state. 01:24:22.000 --> 01:24:27.000 So, you'll see here that right now all these root ports and designated ports will now be 01:24:27.000 --> 01:24:34.000 able to send and receive frames, and everything else is going to get blocked that doesn't 01:24:34.000 --> 01:24:38.000 have RP or DP associated with it. 01:24:38.000 --> 01:24:45.000 And we're done. 01:24:45.000 --> 01:24:51.000 So, your root ports and designated ports transition to the forwarding state, are able to send 01:24:51.000 --> 01:24:53.000 and receive frames. 01:24:53.000 --> 01:25:03.000 They'll populate the MAC address table based on source MACs of received frames. 01:25:03.000 --> 01:25:10.000 They'll forward frames based on the destination MAC address. 01:25:10.000 --> 01:25:14.000 All those in the blocking state do not send or receive frames. 01:25:14.000 --> 01:25:19.000 They do not populate the MAC address table. 01:25:19.000 --> 01:25:26.000 They will accept BPDUs and send them to the switch system module. 01:25:26.000 --> 01:25:30.000 They'll receive and respond to network. 01:25:30.000 --> 01:25:32.000 I'm going to erase this word. 01:25:32.000 --> 01:25:33.000 I don't like that word. 01:25:33.000 --> 01:25:42.000 Control messages, meaning things like CDP, LLDP, DTP, VTP. 01:25:42.000 --> 01:25:44.000 What else? 01:25:45.000 --> 01:25:47.000 STP, Spanish Reporter. 01:25:47.000 --> 01:25:52.000 All those control protocol messages are still allowed in and out of blocked ports. 01:25:52.000 --> 01:25:54.000 So, what are we really blocking here? 01:25:54.000 --> 01:25:57.000 We're blocking end user traffic. 01:25:57.000 --> 01:26:01.000 We're blocking data traffic, voice traffic, that type of traffic. 01:26:01.000 --> 01:26:03.000 Traffic that is not control traffic. 01:26:03.000 --> 01:26:04.000 Even management traffic. 01:26:04.000 --> 01:26:13.000 For example, like Telnet and SSH traffic can only flow through the network on the unblocked ports. 01:26:13.000 --> 01:26:17.000 So, management messages, I don't like there because it makes you think, 01:26:17.000 --> 01:26:19.000 like, oh, well, Telnet and SSH can go through blocked ports. 01:26:19.000 --> 01:26:20.000 No, it can't. 01:26:20.000 --> 01:26:23.000 No, it can't because that would be a type of end user traffic still. 01:26:23.000 --> 01:26:28.000 So, it's the control traffic we're talking about here. 01:26:28.000 --> 01:26:33.000 So, end user traffic only allowed on root ports and designated ports. 01:26:33.000 --> 01:26:36.000 Management traffic only root ports, designated ports, stuff like that. 01:26:36.000 --> 01:26:38.000 Control traffic is allowed on all ports. 01:26:38.000 --> 01:26:41.000 Because the control traffic doesn't cause a loop, okay? 01:26:41.000 --> 01:26:42.000 Remember that. 01:26:42.000 --> 01:26:46.000 So, control traffic is generated from here, sent to here, it's done. 01:26:46.000 --> 01:26:51.000 Control traffic would not be in a loop. 01:26:51.000 --> 01:26:56.000 Andrew says, are you still a designated port if we're using port fast and client? 01:26:56.000 --> 01:26:57.000 So, you're talking about here. 01:26:57.000 --> 01:27:02.000 If we enable port fast, are these still designated ports? 01:27:02.000 --> 01:27:04.000 Absolutely, yes. 01:27:04.000 --> 01:27:13.000 Because even though you're port fast, you're still participating in Spanning Tree Protocol. 01:27:13.000 --> 01:27:15.000 Right. 01:27:15.000 --> 01:27:16.000 That was a lot. 01:27:16.000 --> 01:27:18.000 That was a lot. 01:27:18.000 --> 01:27:22.000 And so, if you are unfamiliar with that algorithm, you struggle with that algorithm, 01:27:22.000 --> 01:27:25.000 you got to go back and review, go back and review, go back and review, 01:27:25.000 --> 01:27:30.000 and make sure that you become more comfortable with that algorithm. 01:27:31.000 --> 01:27:40.000 So, with that said, I would encourage you to make sure that you have the ability to figure out 01:27:40.000 --> 01:27:44.000 who the root bridge is for the exam, who the designated port is for the exam, 01:27:44.000 --> 01:27:47.000 who the root ports are for the exam, and ultimately the block ports. 01:27:47.000 --> 01:27:49.000 Now, here's the kicker. 01:27:49.000 --> 01:27:53.000 They may not be asking you, who is the root bridge? 01:27:53.000 --> 01:27:55.000 Who is or what is the root port? 01:27:55.000 --> 01:27:56.000 Or what is the designated? 01:27:56.000 --> 01:27:58.000 They might not be asking you that. 01:27:58.000 --> 01:28:05.000 They might be asking you a more involved question where they stipulate that they want traffic to flow 01:28:05.000 --> 01:28:08.000 from here to there. 01:28:08.000 --> 01:28:12.000 But right now it's flowing from here to there. 01:28:12.000 --> 01:28:14.000 How are you going to manipulate that? 01:28:14.000 --> 01:28:20.000 That is where you have to understand that algorithm and how you would manipulate that algorithm 01:28:20.000 --> 01:28:23.000 to change the way the environment looks. 01:28:23.000 --> 01:28:31.000 Because by manipulating certain aspects that the algorithm depends on, you can change the tree. 01:28:31.000 --> 01:28:36.000 And that's more than likely what they would be testing you on when it's all said and done. 01:28:36.000 --> 01:28:39.000 So, you really have to know the algorithm.