Time
31 hours 29 minutes
Difficulty
Beginner
CEU/CPE
30

Video Description

Spanning Tree Protocol This lesson covers spanning tree protocol, which prevents a packet from doing an endless loop in a network. This is accomplished by turning certain links to blocking mode. It sends out a packet that lets spanning tree protocol know not to send data on certain links; a certain bridge can block off its link to other bridges which prevents loops.

Video Transcription

00:04
our next concept that we've mentioned a little bit earlier in a little bit of a no overview is called Spanning Tree protocol. So what spanning tree protocol? Well,
00:16
in our network, we may have different bridges or different. We may have different devices, which connects second network segments. So we may have multiple segments of our network that we have these devices that connect these different segments together. And these different devices are different. Bridges
00:32
may also connect to each other and connect in multiple different ways in order to provide reliability
00:37
as well as provide quick connections.
00:40
Now, this is good,
00:41
because we have We have an example of a messed apology over here where all of our devices are connected to or all of our bridges are connected to all of the other bridges on our network, which in a way, is good, because if this
00:58
bridge goes down, then all of our other bridges can still communicate with each other,
01:02
or if this bridge need to communicate with this bridge over here, it doesn't have to go all the way around the world in order to get there.
01:08
But
01:10
well, is there a downside to this Why do we need spanning tree protocol? While our downside is
01:15
if our bridges form any loops,
01:18
then
01:19
we may have situations where our network goes down because of broadcast packets.
01:25
So what do you mean by that?
01:27
Let's take a look at our network here.
01:30
Let's imagine that are certain. Let's imagine that all of our different computer nodes are on the same network and our bridges air just connecting are different parts of our network.
01:38
And let's imagine that our server here sends out a broadcast packet. That broadcast packet is essentially a packet that goes out to everyone and says, Hey, pass this along to everybody. Everybody needs to hear this.
01:49
So our server here sends out a broadcast packet.
01:53
Now that broadcast packet is going to go to
01:57
our first bridge here.
02:00
Who's this guy? Who's then gonna say, Oh, this is broadcast packet.
02:02
So I need to send this
02:05
So we have bridge, eh?
02:07
Be
02:08
See
02:09
the
02:12
and E
02:14
bridge A. Sends the packet to bridge, see
02:17
our bridge, Be sorry and also to bridge, see, So it sends it to both of them
02:23
and then bridge, see and bridge be say, Okay, I'm not gonna say this is a broadcast back in. I'm not gonna send it back to the person who sent it to me, but I'm gonna pass it along to everyone else I'm connected to.
02:34
So well, actually, bridge is connected to everybody, so it's going to send it to everybody.
02:39
So bridge a sins. The packets, Everybody.
02:43
Now each of those bridges have a broadcast packet.
02:46
They're not going to send it back to the link they got it from,
02:50
but they're going to send it back to older other links.
02:53
So now bridge, See is going to send this broadcast packet
02:58
to bridge be
03:00
to bridge e into bridge D.
03:04
Enbridge B is going to send its broadcast packet to bridge. See Bridge E and Bridge D in bridge. He's going to send his broadcasts packet to bridge. Be rich. See bridge D
03:14
you get the idea?
03:15
And as soon as those devices receive a broadcast packet from a different bridge, say when bridge see now receives the broadcast. The same broadcast packet from birds E bridge. See says, Oh, I need to send this to everybody else so it sends it to bridge. Be bridge a bridge D
03:31
who then send it to bridge. Be bridge E bridge D.
03:36
So that broadcast packet is going toe loop forever.
03:39
And our network can only handle so much traffic.
03:43
So what we have now going on is a dying. It's just going to increase increase forever the amount of traffic on your network, because this data is never going to stop anywhere. And this is going to cause a an issue. This is gonna cause a complete denial of service on our network because they're going to be so much traffic
04:01
from our bridges saying, Oh, here's a broadcast back it, he's a broadcast. Pack it in just pushing these packets everywhere.
04:06
We're not gonna be able to get anything done on our network.
04:10
So we need a way to prevent this. We need some sort of protocol that says, OK,
04:15
if you receive this packet
04:18
and you're going to start sending this package, everybody there needs to be some blocks in place to make sure that you don't receive this packet again and then go and send it out to everybody again and receive it again and send it out to everybody again. We need a way to prevent these loops.
04:33
So it's a race. Our broadcast packets here
04:39
and this way that we prevent. This is called spanning tree protocol.
04:46
So spanning tree protocol prevents loops by it turns certain links to blocking mode. Now,
04:55
let's take a look at that.
04:57
So what happens with spanning tree protocol? Well spanning tree protocol essentially sends out a packet that
05:02
let's
05:03
are spanning tree protocol. No. Okay. Hey,
05:09
I'm gonna send out this test packet
05:12
and be receives the packet
05:15
and sends it T e, c and D,
05:19
who all want to send it back to
05:23
everywhere else who want to send it back to me and wants to rescind the back to be. And once it's in the back to e. And so this packet determines that, Okay, we have a lot of loops going on here Now, we wouldn't set up our bridges necessarily in a complete total meshed apology. But just in our hypothetical situation,
05:40
um, we have
05:42
So
05:43
what are spanning tree protocol is going to do is it's going to determine.
05:46
Okay,
05:48
I'm going to set some of these links in blocking mode,
05:51
which means these links are not going to send. We're not gonna send any data on these links.
05:58
So
06:00
bridge, eh
06:01
is going to maintain all of its links. So it's going to be able to send everybody
06:08
bridge See, in order to help prevent loops is going to block off. It's linked to be
06:15
It is goingto block off its length e.
06:17
And it's going to block off its link to D.
06:21
So bridge, see if it needs to communicate to bridge D needs to go through bridge A. So if bridge A sends a broadcast packet to bridge, see, it's going to stop there.
06:32
Bridge, See? Isn't going to propagate that broadcast packet to everybody because they're spanning tree protocol knows. Okay, if bridge A gets a packet
06:42
and sends it out broadcast, everybody's already gonna get it. I don't need to send this on to anybody else.
06:48
So bridge. See, we're eliminating our links here
06:53
between bridge see bridge between bridge C and B and E indeed,
06:59
so bridge, they can still send everybody,
07:01
But we also need to eliminate our links between bridge e and bridge, eh?
07:08
As well as bridge D in bridge, eh?
07:11
Because if a sent a packet to be who sent it to e they could send it back to a and we would have an A B E loop going on or ace in a packet to be who sent it to D. They could send it back to a and we would have an A B de le Loup going on.
07:26
So now
07:28
a consent to be
07:30
and be skin sin to E.
07:32
And that's why broadcast pack it would stop. But it isn't going to send it on to anybody else
07:38
they could send to B and B kits into D.
07:42
And then it wouldn't go any wells because D isn't going to Ford that packet on to anybody else because if it receives it from B, then it's going to say, OK, I'm not going to send this broadcast back back where it came from and there's nowhere else Why can there's no other links that I have to any other devices other than B to D.
08:01
So I'm just gonna hold on to this broadcast packet
08:07
and then we just have our link between A and C,
08:11
so
08:11
we have these different links and blocking mode, and this is now prevented our loops and I'm trying to look over this. I keep looking back at this just to make sure I don't have any other hidden loops going on that I haven't identified. But you get you get the gist or essentially trying to block links to block off loops.
08:28
Now,
08:31
the upside of this is we prevented loops. We've prevented our network from going down because of loops.
08:35
The downside of this is our most direct path to certain parts of our network. Now may not be available.
08:43
So
08:45
if a
08:48
needs to send a packet
08:50
all the way over to D
08:52
A can no longer send directly to D
08:56
because of our loops. If we allowed a to send directly to D, then we have the potential for a D B loop.
09:03
So we blocked off that link.
09:05
But a has to send to be who then sends to D.
09:11
So
09:11
we're eliminating some of our most direct paths, and this is a downside, and this may cause a little bit of slowness, but it's better than our network going down.
09:20
We also may have the another upside where when devices go off line, others remove their blocks.
09:28
So what do we mean by this? Well, let's say,
09:33
uh, someone someone disgruntled, goes into our network closet and takes a tire iron and
09:39
puts a hole in bridge Be
09:41
so
09:43
bridge be
09:43
probably isn't going to be sending any packets right now.
09:48
So does that mean that we can't talk between bridge A bridge E and bridge D no.
09:54
When are spanning tree protocol
09:58
realizes that. Oh, I just tried to send a packet to D.
10:03
And I just tried to send a packet E or Di and I can't get there. Can I send a packet to be no bees off line? So I'm going to remove some of my blocks in order to make my network start working again.
10:15
So as soon as B goes off line, A might remove its block between A and E
10:22
and then e removes its block between e and D.
10:30
Not not C and D. But between e and D. There we go.
10:35
So now even with be off line because we had our messed apology here. Now unblocked
10:41
Achon get to e directly and a can get to see who can get to D. But Deacon, still not can still cannot get back to a directly because we would have an a e d loop,
10:52
so
10:54
that's how it's made. Entry protocol works in a nutshell.
10:58
Bridges, because they connect different network segments they could cause
11:03
caused blank during broadcast that can cause loops
11:09
during block broadcast Spain entry protocol helps prevent these loops
11:15
they turn off. They turned certain links into blocking mode,
11:18
and that blocking mode means that they're not going to send. They're not gonna send traffic on that link.
11:26
The most direct path may not be used. That's our downside of spanning tree protocol. We may not have a direct path between a nd we have to go
11:35
a couple other steps in order to get there.
11:37
But the good side of that is that we're not. We're not causing any loops.
11:43
And then our last point, Our last upside, is that when certain devices go off line like we had someone put a tire iron through bridge be we were still able to simply have those blocks removed using spanning tree protocol, which automatically determined that I need to remove these blocks in order for my network to continue functioning without bridge be,
12:03
we're now able to communicate between all of our other network components. Still
12:07
and again, this is all this also is determined by if you're able to communicate with all your other network components based on what type of topology that you have. If you don't have a complete mess, Topology say, say, for example, I never wired a direct link between
12:26
a.
12:28
T, e
12:30
or E to D or eat anybody else, for that matter.
12:37
If my network topology is such that I can on Lee get T E through B,
12:43
then I'm not gonna get be able to get through e just because I have spanning tree protocol spanning tree protocol and these devices coming back off online either even after others are offline,
12:56
um
12:56
still has the limitations of physical connections between devices. So we have to realize that that's our spanning tree protocol. And that's how we keep bridges from forming loops. And so hopefully we'll be able to utilize that, and we'll be able to prevent that cross talk, and we'll be able to prevent those broadcast storms on our network

Up Next

CompTIA Network+

This CompTIA Network+ certification training provides you with the knowledge to begin a career in network administration. This online course teaches the skills needed to create, configure, manage, and troubleshoot wireless and wired networks.

Instructed By

Instructor Profile Image
Anthony Harris
Systems Analyst and Administrator at SAIC
Instructor