Cisco CCNA ICND2 640-816

Switch STP: Understanding the Spanning-Tree Protocol

by Jeremy Cioara

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Video Title Duration

Review: Rebuilding the Small Office Network, Part 1

Review: Rebuilding the Small Office Network, Part 2

Review: Rebuilding the Small Office Network, Part 3

Switch VLANs: Understanding VLANs

Switch VLANs: Understanding Trunks and VTP

Switch VLANs: Configuring VLANs and VTP, Part 1

Switch VLANs: Configuring VLANs and VTP, Part 2

Switch STP: Understanding the Spanning-Tree Protocol

00:00:00 - It is time to arm ourselves with another tool that we can use
00:00:05 - in our networks and that is the spanning tree protocol. We're
00:00:09 - going to take a look at what spanning tree protocol is all
00:00:12 - about and how it helps in a good switch design. We'll start
00:00:16 - off by taking a look at some good switch design practices. I mean,
00:00:18 - we've moved from the CCENT level now into the CCNA.
00:00:22 - So our networks have grown from the small business network
00:00:26 - to a medium to enterprise class local area network environment
00:00:30 - where our switches tend to be like rabbits. You leave them in a
00:00:33 - room for a while and you come back and there's just switches
00:00:35 - everywhere and you go, ah. How do I manage this? We'll take a look at some
00:00:39 - good switching design practices to handle the multiplying switches.
00:00:43 - We'll then look at what switch loops are because that's one
00:00:46 - of the crises that you run into when you're setting up a large
00:00:50 - switch network. And then we'll see how to stop the switch loops.
00:00:55 - I'll give you the answer right now. It's spanning tree. Spanning tree
00:00:57 - protocol. So after we talk about what spanning tree does we'll
00:01:01 - get into the specifics of spanning tree operation. Spanning
00:01:04 - tree is one of those protocols that has a very simple goal but there's
00:01:08 - a lot of complexity in how it accomplishes that goal.
00:01:13 - When you design a switch network it's best to approach it
00:01:16 - in layers and to take a layered approach. What you see right here
00:01:20 - is a picture of an extremely large network that is designed
00:01:24 - in three separate layers that CISCO has dictated as the access layer,
00:01:28 - distribution layer, and the core layer. It's kind of hard for me
00:01:32 - to talk about this picture without really getting into the
00:01:35 - growth of a network. Imagine yourself. Imagine
00:01:39 - you created a company that makes iPhone accessories.
00:01:44 - You know, the Apple iPhone they make accessories for that. Now
00:01:47 - when you create this company you start off with you and
00:01:50 - ten employees. Are you going to deploy a switch network that looks
00:01:54 - like this? No. Not for ten people unless you're just going for
00:01:58 - way overkill. You're going to deploy a network that looks something
00:02:02 - like, oh, where's my pen? Something like this, where we have one switch
00:02:07 - with our ten employees attached and maybe some little, you know, not even a
00:02:11 - CISCO. You've got like a little NetGear or something connecting you off to the
00:02:14 - internet. Now, your iPhone accessory store starts to grow and
00:02:17 - you hire a few more people to produce those accessories. And so
00:02:20 - you don't move to a model again, like this; you just plug in another
00:02:23 - switch with the crossover cable and attach those users right
00:02:27 - there. And you continue to grow and continue to grow and eventually
00:02:30 - max out those two switches and, you know, plug-in another switch
00:02:34 - in there and now you've got your official servers on your network
00:02:37 - and maybe, well, you're still probably using a NetGear. You've got
00:02:40 - fifty, sixty different employees that are working out of the
00:02:43 - office and you're starting to get a little nervous feeling
00:02:46 - in your system because you know you've been daisy chaining these
00:02:49 - switches. They're almost all maxed out. But you also know if any
00:02:53 - one of those switches go down you're going to cut your network
00:02:56 - in half and you're going to have a pretty catastrophic
00:02:59 - outage. Which, when your company has grown to this point people
00:03:02 - are depending on those systems to be in place. So
00:03:06 - what you do is you take all the money you've been making
00:03:09 - out in your store, you take all those switches, put them in a
00:03:11 - closet somewhere and you buy one of these switches which is
00:03:16 - maybe a CISCO 4000 series or 6500 series
00:03:19 - switch that has a bunch of blades. You've got the dual fan tray action.
00:03:23 - You've got the dual redundant power supply down here. This
00:03:26 - is where the plugs go in, and you've got the fans and the power supply
00:03:29 - that kicks on it makes a lot of noise. You know, you got the big mother
00:03:32 - of all switch, and all these devices are plugging in there. You now
00:03:35 - have upgraded to a CISCO router that connects you off to the
00:03:38 - internet. And eventually you just, you know, keep on going and max out this
00:03:42 - switch and you start daisy chaining your old switches that you
00:03:46 - pull of the closet from this guy to get some more ports.
00:03:49 - And, you know, at this point you're probably in the hundreds of employees
00:03:52 - at the company. And you start to get that nervous feeling
00:03:55 - in your system again, thinking, well this is a great switch,
00:03:58 - you know, it's got the dual power supplies and fan trays and
00:04:01 - supervisor modules and all that kind of stuff. But what if that
00:04:05 - switch goes down. That's the core of my whole company. So that's where
00:04:08 - you buy a second one. You got money at this point, right? So you get
00:04:11 - a second one. Hook them up to where they're redundant and they back
00:04:14 - each other up. And now you're starting to approach a model that
00:04:19 - looks like this. Where you have distribution layer switches
00:04:23 - which are providing core services or essentially routing services,
00:04:28 - VLAN services and so on, that are attached on here to access
00:04:32 - layer switches. Now with this layered approach it allows you
00:04:36 - to have easy and manageable growth. This is CISCO's model. They
00:04:39 - created this model right here. The access layer is where the
00:04:43 - devices actually plug into the network. We have servers connecting
00:04:46 - to the network on their access layer switch. PC's plugging into
00:04:50 - the network on their switches. The distribution layer is where
00:04:53 - your major segments happen. You have kind of these modules that
00:04:57 - are created here. You know, maybe you've got your sales
00:05:00 - group over here which has a 100 or 200, 300
00:05:03 - different sales people inside of it. Or it could be floor
00:05:06 - four of your building that you own. You've segmented into
00:05:10 - one big group. You've got the dual redundant 4500
00:05:14 - or 6500 series switches there. Over here, you've got your
00:05:17 - server farm network, where all your servers are isolated.
00:05:20 - You have a separate VLAN over here for that. And now you eventually
00:05:24 - reach a core layer and when you get to this point you're probably
00:05:27 - at thousands of employees. The core layer is considered the
00:05:30 - backbone of a network.
00:05:34 - Now think of this as a campus model. And when I say campus
00:05:39 - I'm talking about a literal college campus. Or maybe you have
00:05:43 - the college of business over here. You've got the college of
00:05:47 - art over here. And you know, all the different colleges and they all
00:05:50 - tie back to the backbone of the entire campus network which
00:05:54 - is the core and everything goes through the core and you have other
00:05:56 - buildings that branch off of here with modules that look like
00:05:59 - this. So that's the idea of
00:06:03 - designing a network in a layered approach is designing it
00:06:06 - with modules. Now you see down here a new term, etherchannel. Can provide
00:06:10 - for more bandwidth on key links. What etherchannel is, is where
00:06:14 - you can do something like this. You see these three lines going
00:06:17 - between these. Maybe you've got so much traffic passing through
00:06:22 - the backbone that just one, one hundred megabit per second
00:06:25 - or one thousand megabit per second port isn't enough. What
00:06:29 - etherchannel can do is it can actually take between two
00:06:33 - and eight ports, you can go up to eight and bundle them together into
00:06:37 - a single pipe. So you could have your two switches right here. And if these were two one
00:06:41 - hundred megabit per second ports, etherchannel allows you to tie them together and get two
00:06:45 - hundred mega bits per second of throughput. So that way you can have
00:06:49 - you know, add a bunch of links in there and channel them all to get
00:06:52 - super high bandwidth links between your switches. Now the last
00:06:57 - point I want to make. And the reason we're talking about this ideal
00:07:00 - design right here in this video section, is the redundant
00:07:05 - connection. The redundant connections eliminate a single point
00:07:10 - of failure. So when I have my access layer switch, notice, I always
00:07:15 - have a crossover cable going to both distribution layer switches.
00:07:19 - The distribution layer switches always has a cable going to
00:07:22 - both core layer switches so that if any one device fails in
00:07:27 - the network, we have a backup path that can get us around
00:07:30 - that failure. So redundancy is good, we've determined that. But
00:07:34 - unless you design it the right way redundancy can be very bad.
00:07:39 - To understand why that redundancy is very bad or can be bad
00:07:43 - you have to understand how the switches are going to treat
00:07:46 - those redundant links. By default the switch will send broadcast
00:07:50 - packets out all ports. That's how they're designed. So when this computer
00:07:53 - sends a broadcast packet into the network, the switch says oh,
00:07:57 - I don't know where that goes. I'll send it out every single port
00:07:59 - except the one I received it on. So out those two ports it goes. This
00:08:03 - switch receives it and sends it out to this computer because
00:08:06 - that's the port and then as it receives it on this port, it loops it
00:08:09 - right back to that one. And as it receives it on this board it loops it right back to
00:08:12 - that one. Again the switch receives it on both those ports and
00:08:16 - sends it back out to this PC and then loops right back around.
00:08:19 - This will keep happening eternally. Meaning this broadcast
00:08:23 - will cycle the network blowing up all the devices on the network
00:08:28 - until you shut down the whole network. Meaning turning everything
00:08:30 - off. A lot of you might be thinking, well wait a sec. I
00:08:34 - heard inside of a packet is this field called the TTL.
00:08:37 - That's the time to live. How long a packet survives but
00:08:42 - the TTL is a layer three field, meaning it's up at the network
00:08:46 - layer. Only a router can subtract time from the time to
00:08:51 - live. Now looking at that picture right there, do you see any routers?
00:08:54 - Nope. So what's going to happen is that packet is going to
00:08:57 - cycle the network round and round and round and round destroying
00:09:00 - everything that's out there. So while the redundant connections
00:09:03 - are necessary in business networks we have to have a system
00:09:07 - to manage them so they're not all active at the same time.
00:09:11 - Becaue if they were we'd have that broadcast problem. So the place
00:09:15 - of spanning tree is to drop trees on redundant links. That's my little
00:09:21 - way of remembering it because they're so often, you get all
00:09:24 - these turns, and then spanning tree comes out there, and you're like, well what
00:09:27 - did that do? Just think of a falling tree. It's falling on all
00:09:30 - the redundant links in the network. What spanning tree does
00:09:34 - is find all the best links in your organization. For example,
00:09:39 - it looked at that link and for some reason or another, we'll
00:09:42 - talk about those reasons in a moment; it said that link is better
00:09:46 - than that link. So I want that one to be active. So, the
00:09:50 - tree falls on this redundant link and it goes inactive to
00:09:53 - where it's not in use. So this one's forwarding along life is
00:09:57 - good, until you know, maybe the cable goes bad
00:10:01 - or the port gets shut down or something like that. Spanning tree's
00:10:04 - always watching the network and if something occurs it's going
00:10:08 - to take that tree
00:10:10 - and lift it right back up. And it's going to say okay life is good let's go ahead and
00:10:13 - unblock our redundant connection and allow traffic to forward
00:10:17 - here until we see our main link, which is better for some reason.
00:10:20 - Our main link come back on line then we can go ahead and drop
00:10:23 - the tree on our redundant link again to make it inactive.
00:10:27 - Spanning tree is one of those concepts that is sometimes difficult
00:10:31 - for me to talk about. Because what I described to you right then
00:10:35 - is all it does. That's the whole goal of spanning tree.
00:10:39 - But, go to Google and type in spanning tree protocol
00:10:43 - and just see how many pages and pages and books of information
00:10:47 - are out there on spanning tree. You might think, well, if that's all it does
00:10:51 - is block the redundant links, what's the point of writing
00:10:53 - a book about it. Well networks are not as simple as what I
00:10:58 - just showed you where we have two individual switches connected
00:11:01 - together. Networks have hundreds of switches and spanning tree
00:11:04 - has to find what's the best ones to block. Because if it blocks
00:11:07 - the wrong one you're going to have a very inefficient network.
00:11:10 - So here's the facts. The original spanning tree protocol it's
00:11:14 - official standard is 802.1D. That's the
00:11:18 - technical name for it. Was created to prevent loops. The switches
00:11:23 - will send probes into the network called BPDU's or bridge
00:11:27 - protocol data units to discover loops. So what's going to happen
00:11:30 - and again, take little topology there of our two switches. If we have
00:11:33 - two switches that run spanning tree, they will send probes on
00:11:38 - the network. Now those probes are kind of like broadcast
00:11:41 - packets. They're actually technically multicast. And they'll come into
00:11:44 - the switch and all this probe is, this BPDU is a
00:11:48 - little identifier for the switch. So say this is switch one
00:11:52 - and this is switch two. Switch one sends a probe into the network
00:11:55 - and says, let's say, think of like sonar sounds. And sends out
00:11:59 - a little probe. Now that message comes into the switch and
00:12:02 - since it's a multicast or it's designed, kind of like a broadcast
00:12:06 - packet. It will loop it back around. Switch one will get the probe, open
00:12:10 - it back up and look inside and go wow, this is my probe.
00:12:14 - I sent this out, meaning I see my name inside of this probe
00:12:18 - that means there's some kind of loop in the network. So that's
00:12:21 - when they go into action. These probes also help elect the
00:12:25 - core switch of the network known as the root bridge. There's
00:12:29 - our key term. In our switch network, say we've got, you know, bunches
00:12:33 - and bunches of switches all around the network that are all
00:12:35 - tied together with redundant connections going everywhere. In that
00:12:39 - case we will have one switch which will be elected the root.
00:12:44 - Now that is considered the core switch of the network because
00:12:48 - all the switches will find the best way to reach that root
00:12:52 - bridge then block all the redundant links. Meaning this one's
00:12:56 - going to look and say well the easiest and fastest way for
00:12:58 - me to get there is this link so it's good but I've noticed that
00:13:01 - I can also reach the root through this link and well, not
00:13:05 - that link down there. So I'll block that redundant connection.
00:13:08 - So all the switches find the best way to reach the root
00:13:11 - and then block redundancy. So the key of spanning tree protocol
00:13:16 - is to make sure you're very accurate in the one that you elect
00:13:19 - as the root bridge. The thing that could work out horrible
00:13:23 - for your network is if you just let spanning tree treat everything
00:13:27 - at default. I'm jumping a little ahead of myself but
00:13:32 - by default spanning tree will elect the oldest switch in your
00:13:36 - network as the root bridge. Now I don't mean oldest as in how
00:13:39 - long it's been running. I mean oldest as in manufacturing
00:13:43 - date. So some little, you know, ten megabit per second 1992
00:13:47 - switch that you have stuck in a wiring closet that you
00:13:50 - forgot about gets elected as the root and everybody is like, hey, that's the
00:13:54 - root. Let's find the best way to get there, because that must
00:13:57 - be the center of the network. Everybody finds the best way to
00:13:59 - the closet switch and your network performs horribly. And the
00:14:04 - problem is, is nobody really knows why. Nobody can really put
00:14:07 - a finger on it because it looks like everything's designed
00:14:10 - right, I mean, you're using great equipment and servers are working,
00:14:13 - it's just, things just run slow and people just finally, ah
00:14:18 - I guess that's that's the way it is and you run this beautifully
00:14:21 - expensive network that you've invested all this
00:14:24 - money into that never performs quite like it should.
00:14:28 - Now, I should also mention that every CISCO switch runs spanning
00:14:32 - tree by default. So you can plug in networks together with redundant
00:14:36 - connections and it's not going to have any loops. It's just that it's going
00:14:39 - to be a very inefficient network. Now, you should also keep in mind
00:14:42 - that you are at a CCNA topic at this point. It's not; this
00:14:47 - topic isn't in the CCENT material or the ICND 1
00:14:51 - video series you might have already seen because this is something
00:14:55 - that works for enterprise networks. Meaning, in a CCENT
00:14:59 - level network you're not going to have hundreds of switches.
00:15:03 - You won't even have ten to twenty switches because CCENT
00:15:06 - is focused around small businesses. So when you get to the larger
00:15:10 - networks, the mid-size the enterprise networks, that's where spanning
00:15:14 - tree really plays a role, and a big role at that. It has to elect
00:15:17 - the best switch as the root bridge. So when we're understanding
00:15:21 - our spanning tree network,
00:15:24 - we need to understand more about these BPDU's and how the
00:15:27 - elections work. The way that the elections will happen is all
00:15:32 - the different devices in the network have something known as
00:15:36 - a bridge id.
00:15:40 - That's their name to spanning tree. Remember I said we had like
00:15:44 - switch 1, switch 2. Well it's not that, the switches don't really
00:15:47 - care about their host name, they care about their bridge id. Now that
00:15:51 - bridge id is built of two pieces. You have a priority
00:15:57 - and you have the switch's Mac address. I'll just put Mac add.
00:16:04 - You can see I put priority dot Mac address. Now, by default every
00:16:10 - switch when you pull it out of the box has the same priority.
00:16:13 - 32768. You know somebody just took a dart and threw
00:16:18 - it at the dart board, and they were like ah, let's make it 32768.
00:16:22 - Now I should mention the priority is kind of counterintuitive. When
00:16:25 - you think of priority you think, oh, bigger priority that's better right?
00:16:29 - Not so. Lower is better. So the lower your priority the more chance
00:16:34 - you have to be elected as the root bridge. Now since all the
00:16:39 - priorities are tied when you pull these switches out of the box,
00:16:43 - it has to resort to the Mac address to break the tie.
00:16:47 - Now we're getting to the reason why the oldest switch in the network
00:16:50 - becomes the root bridge. When the manufacturers manufacture
00:16:55 - switches, they will start with the Mac address that's lowest
00:17:00 - in their range. Meaning, you know, say you're CISCO. You'll
00:17:03 - go to the powers that be and acquire a Mac address range that
00:17:07 - you can use for all your devices. You'll start manufacturing devices
00:17:11 - and start from the lowest Mac address and just keep increasing
00:17:13 - the Mac addresses of your devices as it goes. Now keep in mind
00:17:17 - I'm talking about the Mac address of the switch, not one of
00:17:20 - the Mac addresses that it's learned about that's from a device
00:17:23 - that's plugged in. The Mac at the switch itself has a Mac address. So,
00:17:27 - when we have this kind of scenario obviously I just made up these
00:17:30 - Mac addresses aaa bbb ccc. This one will end up being the
00:17:35 - root bridge because it has the lowest Mac address of the three.
00:17:39 - So,
00:17:42 - once that root election happens all the switches in the network
00:17:46 - settle and they say okay that one is the root bridge. Now let's
00:17:49 - find the best way to get to that root bridge. Now, you notice right
00:17:54 - in the middle of the
00:17:56 - picture right here I have a link cost equals nineteen. There's
00:18:00 - a table, I'm going bring up that table in just a few that shows
00:18:04 - all the different speeds of links that you have like a
00:18:07 - 1000 megs per second, a 100 megs per second, 10
00:18:10 - megs per second and down and down and out it goes. Now if you
00:18:14 - have a 10 meg per second link the cost to spanning tree
00:18:17 - is actually 100. A 100 megabit per second link is
00:18:20 - nineteen. And I'll show you the chart later on but that's how
00:18:24 - it determines the best way around the network is based on the
00:18:27 - link costs. So it says okay if this is a 100 mega link it's
00:18:31 - nineteen. 19, 19, we'll just say it's all a 100 meg link everywhere.
00:18:35 - All the switches in the network try and find the best way to get
00:18:38 - to the root. This one looks and it says, well, I see a cost of
00:18:42 - nineteen to get that way, a cost of nineteen plus nineteen
00:18:46 - that way so my root port; I'll just out RP on there, is going to be
00:18:51 - right here because that is the most efficient way to reach
00:18:54 - the root bridge. This one does the same and elects this one
00:18:58 - as its root port. That's the best way to get there. Now the root
00:19:01 - bridge, you know, it's the king of the network and as its reward
00:19:05 - it will never block one of its ports. And by the way a root
00:19:08 - bridge will never have a root port because those are used to reach the root
00:19:12 - bridge. Obviously if it is the root bridge it doesn't need to reach
00:19:16 - itself. So down below you can see the next type of port is
00:19:21 - a designated port. The designated port is just a forwarding port.
00:19:25 - I wish, I wish they would have chose that name instead of saying
00:19:28 - designated port. Designated port means there's one port that
00:19:32 - is forwarding.
00:19:34 - Now there is one designated port per link. This gets kind
00:19:38 - of confusing. A link is a segment between switches, or I guess
00:19:43 - you could say a link is just a switch port.
00:19:46 - Or a link between the switches. If I have a PC attached to this
00:19:50 - switch, this would be a designated port. Meaning, a port that is
00:19:54 - forwarding, because it works. And on another PC over here that's a designated
00:19:59 - port. It's plugged into to a port that is forwarding. Now when it comes to
00:20:02 - a link between switches you'll have one forwarding port
00:20:06 - per link. Now you're kind of looking at this picture and I guess I should back
00:20:10 - up a step. Is that picture a demonstration of a redundant topology?
00:20:15 - Yeah, sure it is. Because if this is, we'll just say
00:20:20 - this is switch 1, switch 2, and switch 3. If switch 1 goes
00:20:24 - down and dies well, switch 3 can still reach switch 2 and
00:20:29 - still has active connections. If switch 2 were to die switch
00:20:32 - 1 could still reach switch 3. It's redundant. But remember,
00:20:35 - a broadcast if we don't block something will
00:20:38 - circle around and around that network everywhere blowing
00:20:41 - up all the devices. So when you are talking about these links;
00:20:45 - we know one of them has to go and we know now that these are
00:20:48 - using these paths to reach the root. We know that it's
00:20:51 - going to be this guy. We know that link is going to die. But before
00:20:55 - it dies we have to realize that every link has to have
00:20:59 - one designated port. Meaning one of these must stay forwarding.
00:21:03 - In this case it will be this one. DP, designated port. The
00:21:10 - other side will assume a blocking state. Meaning,
00:21:15 - switch 3 blocks its port. Now, it's kind of obnoxious because
00:21:20 - if you're sitting on switch 1 doing show commands it's going
00:21:24 - to be like, oh yeah, the port is great, it's forwarding, life is good and
00:21:27 - you're going to think to yourself, wow, I thought I understood spanning tree. Why is that
00:21:31 - forwarding? I thought that link would go down. Well it is down because
00:21:35 - if you block one side of the link, the other side can't communicate,
00:21:38 - so it's disabled but you have to be on switch 3 to be able
00:21:41 - to figure that out.
00:21:43 - Now I want to answer a question that I think is probably circling around
00:21:46 - some of your minds. You might be thinking, well, why is that side
00:21:51 - the designated port and this side get blocked? Why didn't this side get blocked
00:21:56 - and that one be designated? Well, take a guess. Knowing what you
00:21:59 - know about spanning tree, why do you think that would be so?
00:22:06 - Remember these. Bridge id's. The bridge id is that combination
00:22:11 - of priority and Mac address. It's not only used to elect the
00:22:14 - root bridge. Whoever has the lowest one in the whole network becomes
00:22:17 - the root but it's also used to determine who's going to block
00:22:21 - the link. Again lower is better so since this guy is lower he's
00:22:25 - like I'm not blocking my link. Switch 3, you're lower than me or
00:22:29 - higher so your priority is lower or, wait a sec, I'm just, I'm going to stop talking.
00:22:33 - Switch 3 is not as good as switch 2, so it's going to block its link
00:22:37 - and disable that
00:22:39 - effectively taking the whole thing down. Now if any one of these
00:22:42 - links ever dies, spanning tree will recognize that and it will
00:22:46 - unblock this link to resume the connectivity even though
00:22:50 - one of the switches failed in your network.
00:22:53 - I want to point out that even in a network this small, the
00:22:58 - switch that becomes the root effects what links get used. So
00:23:02 - let me just clear off all this chicken scratch. We said this one..woop...
00:23:07 - let me get to where I can draw again. We said this one is going to be the root
00:23:11 - so these links stay active and these switches go through this
00:23:16 - switch to reach each other because this one gets blocked. But
00:23:19 - if let's say this one over here gets elected the root
00:23:23 - then which link gets blocked?
00:23:26 - This one. Because these switches will say these are my root ports,
00:23:29 - that's the best way to get to the root bridge and they will go
00:23:31 - through this switch to reach each other. And now I think you can
00:23:35 - start to see why it's so critical that you play a role
00:23:40 - in who becomes the root bridge. Because if this is the oldest
00:23:44 - switch in your network and these two switches have to go through
00:23:47 - that switch to reach each other then this guy is going to become
00:23:50 - a bottleneck because it's not going to have the speed and capacity
00:23:53 - that some of the newer bigger switches in the network are going
00:23:56 - to have. And I think you can already begin pondering how you're
00:24:00 - able to influence the root election. You know how a root
00:24:04 - bridge is elected it's the priority plus the Mac address. Well
00:24:08 - you can't change the Mac address that's set on the switch. It's
00:24:10 - hard coded but you can change the priority. So by dropping
00:24:15 - that number down and lowering that you can influence the election.
00:24:20 - At this point I think we're getting the feel for spanning tree
00:24:23 - in that what it does and how it does what it does. So here are
00:24:27 - the official two major steps to how spanning tree finds the
00:24:30 - best path. Step one elect the root. If we had our little three switch
00:24:35 - networks, and by the way, anytime you're learning spanning tree that's
00:24:38 - the normal topology that you have. Once you elect the root
00:24:41 - then the switches will find the lowest cost path to the
00:24:46 - root. So they're going to look at the link speeds that they
00:24:48 - have and say, well, the lowest cost path, these are all 100 megabits
00:24:52 - per second. They're all a cost of 19,19, 19, and
00:24:56 - they'll all find the best way to the root. Now just to give you an idea.
00:24:59 - If we had a low speed link; let's say this one was a 10 meg
00:25:03 - link then it will be a cost of 100. So this switch will say
00:25:06 - well, it's either a 100 to go that way or 38 to go
00:25:10 - that way so that's the more efficient way. I'm going to block
00:25:13 - that link. So the cost or the speed of the links
00:25:17 - really do have an influence. Now, I want to add in a couple pieces
00:25:21 - here because I know some of you might be thinking of the bigger
00:25:24 - more advanced topologies. What if you had a situation like
00:25:29 - this. What if that was the root and this switch was trying
00:25:36 - to figure out what link to block. Now, remember this is redundant. We have
00:25:39 - to block one of these links but if all of these were equals, let's
00:25:42 - say they were all 1 gigabit per second connection. Well
00:25:46 - then both paths are a cost of eight for that switch down here
00:25:50 - to reach the root. What it's going to do to break the tie,
00:25:54 - is it's going to choose the switch, the upstream switch, with
00:25:58 - the lower bridge id. Meaning, again remember that priority
00:26:01 - we had, 32768. and then the Mac address.
00:26:06 - If this Mac address is lower than this one over here then it's
00:26:10 - going to say you're the best route. If the other side was lower it would
00:26:13 - say you're the best route and block the opposite one.
00:26:18 - There's only one more topology that can throw a fork in the
00:26:20 - wrench and that was the original one I showed you when I was saying what
00:26:24 - spanning tree did. What if you got a situation like that. Both
00:26:28 - of these we'll say are a gigabit per second so they're both the
00:26:30 - cost of four. That guy's the root. This one, you remember, the root never
00:26:34 - blocks a port, so this switch has to figure out which port to block
00:26:37 - to stop this redundancy. The costs are tied; the
00:26:42 - upstream switch is the same so it can't use the method we used
00:26:45 - over here to break the tie. So what it's going to do is prefer
00:26:51 - the lower port. Just remember in spanning tree lower is better
00:26:55 - on everything. It's going to say I have to block one so since you are
00:26:59 - the higher port I will block you. And this one becomes
00:27:03 - the active link. This one gets the tree dropped on it.
00:27:07 - That is the basic concepts of spanning tree. If you want to
00:27:10 - have a good well-designed redundant network, then spanning tree
00:27:14 - has to play a role. Because you have to have something to disable
00:27:17 - those redundant connections until you need them. So we talked about
00:27:21 - what some good switching practices were. Some design pictures of
00:27:25 - how we should design our network in the three layers,
00:27:28 - access, distribution, and core. With redundant connections across
00:27:31 - all of them. We saw what switch loops are and how to stop them. The
00:27:36 - switch loops happen any time you have redundancy inside of the
00:27:39 - switch or layer two environment. And we stop them by using the spanning
00:27:43 - tree protocol. Finally we looked at some of the specifics of
00:27:46 - spanning tree operation like the root bridge election. Like the bridge
00:27:50 - priority and the bridge and Mac address, combining to create
00:27:54 - the bridge id that's used to elect the root. And then how all the switches
00:27:58 - find the best way to reach that root bridge. Those
00:28:02 - concepts are critical for when we get into the next video which
00:28:05 - is going to be configuring a little bit of spanning tree and
00:28:08 - then we'll get into some of the enhancements that make spanning
00:28:11 - tree better and faster for our networks of today. I hope this has been
00:28:15 - informative for you and I'd like to thank you for viewing.

Switch STP: Configuring Basic STP

Switch STP: Enhancements to STP

General Switching: Troubleshooting and Security Best Practices

Subnetting: Understanding VLSM

Routing Protocols: Distance Vector vs. Link State

Routing Protocols: OSPF Concepts

Routing Protocols: OSPF Configuration and Troubleshooting

Routing Protocols: EIGRP Concepts and Configuration

Access-Lists: The Rules of the ACL

Access-Lists: Configuring ACLs

Access-Lists: Configuring ACLs, Part 2

NAT: Understanding the Three Styles of NAT

NAT: Command-line NAT Configuration

WAN Connections: Concepts of VPN Technology

WAN Connections: Implementing PPP Authentication

WAN Connections: Understanding Frame Relay

WAN Connections: Configuring Frame Relay

IPv6: Understanding Basic Concepts and Addressing

IPv6: Configuring, Routing, and Interoperating

Certification: Some Last Words for Test Takers

Advanced TCP/IP: Working with Binary

Advanced TCP/IP: IP Subnetting, Part 1

Advanced TCP/IP: IP Subnetting, Part 2

Advanced TCP/IP: IP Subnetting, Part 3

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Jeremy Cioara

Jeremy Cioara

CBT Nuggets Trainer

Certifications:
Cisco CCNA, CCDA, CCNA Security, CCNA Voice, CCNP, CCSP, CCVP, CCDP, CCIE R&S; Amazon Web Services CSA; Microsoft MCP, MCSE, Novell CNA, CNE; CompTIA A+, Network+, iNet+

Area Of Expertise:
Cisco network administration and development. Author or coauthor of numerous books, including: CCNA Voice 640-461 Official Cert Guide; CCNA Voice Official Exam Certification Guide (640-460 IIUC); CCENT Exam Prep (Exam 640-822); CCNA Exam Cram (Exam 640-802) 3rd Edition; and CCNA Voice 640-461 Official Cert Guide.


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