# Spanning Tree Protocol (Part 1)

Video Activity

In this lesson we introduce you to Spanning Tree Protocol. We explain the purpose behind Spanning Tree Protocol, and examine in great detail, the Spanning Tree Protocol (STP) Decision Process.

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Time
27 hours 46 minutes
Difficulty
Intermediate
CEU/CPE
31
Video Description

Spanning Tree Protocol (part 1) In this lesson we introduce you to Spanning Tree Protocol. We explain the purpose behind Spanning Tree Protocol, and examine in great detail, the Spanning Tree Protocol (STP) Decision Process. We'll demonstrate how it prevents looping between switches when redundant links are present. We also explain at how loops can happen, explain what a root bridge is, and why it's necessary to have a designated port for each segment.

Video Transcription
00:04
the next topic of discussion
00:06
that we're gonna cover is spanning. Tree particle spanning tree protocol
00:11
was created by this lady named Roddy a Pearlman back in the 19
00:17
late 19 seventies, I believe when she was working for Xerox.
00:21
Um, and
00:23
it prevents
00:24
loops or frames from looping around a network
00:28
when redundant links are present between switches.
00:32
Now, what does that mean?
00:34
If you look at this diagram over here on the left hand side.
00:38
You see that PC one is connected to switch one
00:41
and PC three is connected to switch three.
00:45
Now, between switch one and switch three, there are two paths. They are redundant links present,
00:52
or they are redundant. Paths present.
00:55
One is directly from switch one to switch three and the other one is from switch one to switch to two Swiss four and again, back to switch three. So there are two paths between switch one and switch 31 going this way.
01:11
One
01:12
going this way.
01:15
So less
01:17
pretend that PC one sends a frame out too.
01:21
PC three.
01:23
Now switch. One knows that PC three exists going out Port
01:30
one,
01:30
and also PC three exists going out
01:34
port to
01:37
now Let's pretend as that frame is being sent. This link between switched three and PC three failed.
01:46
So
01:49
the friend makes it to switch one.
01:53
And at this point, switch one does not know where PC three years previously, switch one knew that PC three is available either out port one or port to
02:02
So what does this switch do with the frame
02:07
where it doesn't know whether destination Mac it's or namely an unknown, unique *** frame. A unique *** frame that's being sent where the destination is unknown.
02:17
The switch floods it out.
02:19
The switch floods it out port one
02:23
and floods it out for two.
02:24
This frame that's moving in the clockwise direction Let's look at that first
02:30
guest who switched 333 also does not know where pc three years. So the switch floods it out
02:37
Port to
02:38
this friend gets to switch for switch for also doesn't know where pc three s Swiss four floods it out
02:46
and sport to switch to
02:49
the frame will get to switch to and to its two will again flooded out back to switch one and this process will continue and this frame will continue looping in a clockwise direction in this network.
03:01
The same thing will happen to the frame that was originally sent out. Switch one poor, too, and it will start looping
03:08
between switch to 43 and one in a counterclockwise direction
03:14
so this can eat up unnecessary. This could need a band. What band? With unnecessarily
03:21
and eventually If PC one keeps transmitting frames to PC three. Not knowing that PC three is gone, this link is gone. Enough frames will
03:30
enough frames will gather and start looping around the network. That's which. 134 and two will start trashing our eventually will crash.
03:39
This is called a layer to loop or broadcast storm, actually sometimes called the broadcast storm. If PC one sends out a broadcast, what happens to that again? A broadcast is flooded out all ports and a frame the loop in a clockwise direction
03:57
and a framed loop in a counterclockwise direction and continue to luv
04:00
till these switches crash.
04:03
Now let's look at switch three. What's happening at switch three?
04:06
Well, Swiss three gets this first frame from switch one
04:11
that PC one sent out and Swiss Tree learns reading the source Mac address field in the frame that PC one is available out. Port one
04:20
immediately after other frame. That's which one sent out. Port to arrives at Swiss Report to. After looping around the network
04:30
and switch three learns that no PC one is available out port to
04:34
so switch one modifies its Mac address table to reflect that PC one is available out for two
04:41
immediately after the frame looping in the counter clockwise direction arrives and switch three
04:46
again modifies its Mac address table and say is uh, PC one is available out of Port one.
04:51
So what's happening is switched to me. Is constantly updating it smack at this table and
04:59
modifying it to say that PC one is either available out of Port one and then a second later. No, it's available at a port to then a second later. No, it's available out of Port one. This is called Trashing the Mac Table.
05:11
When you were trashing the Mac table, you're not actually making any forwarding decisions. Eventually, enough traffic will queue up on this wish that this which will crash
05:19
so to prevent against this spanning tree protocol, shuts down redundant links between switches.
05:27
So let's say if this apology was running spanning three spanning tree would maybe shut down this link.
05:34
Now, when this link is shut down, there's only one path between PC one and PC three, assuming that you know this link is up and running
05:44
and there can't be any loops in this topology.
05:47
So
05:50
when we move over to this apology, keep in mind our end game, which spanning tree is that some of these links are going to get shut down to prevent against layer to lose from happening. This was just drawn to show you the need for spanning tree and how loops can happen.
06:09
It's not really part of your CCN a knowledge that you're supposed to know how these loops happen. You're just supposed to.
06:16
I know that they can happen. You don't have to know how they happen.
06:20
However, I felt the need to describe how
06:26
now, spanning through protocol,
06:30
as I said, works
06:31
to prevent loops between switched devices or switches when they are redundant links present. Do we have redundant links here? In this apology,
06:42
we have switched 123 and four
06:45
connected together,
06:46
so a loop conform going this way or the other way
06:53
and then we have switched two and four connected together on port to and port three on both switches and you may get a loop going this way.
07:03
Now, upon power up, when all these switches are powered up, they start sending spanning three protocol frames to each other by default. Spanning tree is on on all switches by default,
07:15
and it is highly recommended that you don't turn it off. You may turn it off under circum certain circumstances. And I will tell you how at the end of this lecture, but it is recommended that you don't turn spanning tree off.
07:29
Now when these switches power up, and just to quickly elaborate on this apology, which one is connected to switch three on port to
07:38
on both sides on switch one ends with three,
07:41
so it's two is connected to switch four on port port to port three
07:46
and switch one is connected to switch to on Port one, and so it's three is connected to switch four. Both sides on Port one,
07:54
the A, A, C, C, C, C, B, B, B, B and D D dee dee on the Mac addresses of these respective switches. Once again I know this is only 16 bits. Each hex character is four bits making 16 bits instead of 48. But it just makes the diagram cleaner on board instead of me having to write 48 bits worth of A's.
08:15
Now, when these were just power up
08:16
by default, they start sending each other
08:20
spanning tree protocol frames called bpd Use
08:24
bpd. You stands for a bridge protocol data unit.
08:28
These bpd use are sent every two seconds out all ports. So for examples, which one will be sending out BP to use out port?
08:39
Fascinating. It's, you know, too. And port fast in a 01
08:43
every two seconds. Keeping in mind that switches send BP to use every two seconds to each other upon power now, spanning tree works
08:54
Bye
08:54
in three steps.
08:56
So for spanning three do you go from
09:00
nothing from just starting up to it converging? And by converging, I mean, some of these links will get shut down, Does that in three steps, so step one for spanning tree is elect one rude bridge part earlier to domain.
09:16
Step two is elect one route port per non brute switch.
09:24
And I know these don't make sense yet. They're not supposed to. I'm just going over the three steps that you need to
09:31
Mick spanning, tree, converge. And then we're gonna explore each step in detail. So the third step is elect one designated port per segment.
09:43
So step one is elect one route bridge per layer to domain. So this is our later to domain. Out of this, four switches were gonna elect one switch to be something called the Route Bridge,
09:54
then elect one route port per non route switch. So for the switches that are not the root bridge, we're gonna elect something called a route port,
10:03
and then we're gonna elect one designated port per segment. And a segment is the link between two switches. So this is a segment between switch one and Swiss three. This is a segment between switch one and switch to. This is a segment between Swiss three and switch four. And these are two segments between switch to and switch four.
10:20
So there are a total of 12345 segments in this network.
10:26
Now, each one of these fanning three steps
10:28
needs to follow this four steps spanning three decision process
10:35
So for each one of these steps, we're gonna follow the spanning three decision process, which is a four step process. And we're going to stop at the process which works
10:46
so rude Bridge election we're going to stop
10:48
at, We're gonna go top down, and we're gonna stop at the process that elects the route bridge
10:54
for Rue Port Election. We're gonna go top down, and we're gonna stop at the process that elects the route port
11:00
and same with the designated port. We're gonna go top down and stop at the process that elects the designated port.
11:07
So the first step is electing a rude bridge.
11:11
Now, I said that each one of these switches, when their power up start sending Ridge Protocol data units to each other every two seconds.
11:20
I hope you have taken note that there are three steps for spanning tree to convert. Step one is electing a rude bridge
11:26
part earlier to domain. Once again, set to is electing a route port for non roots, which for all switches that are not the root bridge, we're gonna elect something called the Route port. And Step three is elect one designated port per segment
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