Per VLAN Spanning Tree Protocol
Video Activity
Per VLAN Spanning Tree Protocol Now that we've completed our discussion and demonstrations of Spanning Tree Protocol, in this next lesson we look at all the other protocols Cisco devices run.
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Time
27 hours 46 minutes
Difficulty
Intermediate
CEU/CPE
31
Video Description
Per VLAN Spanning Tree Protocol Now that we've completed our discussion and demonstrations of Spanning Tree Protocol, in this next lesson we look at all the other protocols Cisco devices run. You'll learn the configuration of PVST, how blocking occurs within each link, and why the one Spanning Tree configuration demonstrates better load balancing. We'll also demonstrate how the Bridge ID segments map differently and what impact that has on the VLAN ID and priority.
Video Transcription
00:04
now, a little caveat. Cisco switches don't exactly just run spanning tree Cisco switches don't run just spanning tree protocol.
00:14
Cisco devices
00:16
run what is called per villain spanning tree particle or P V S t.
00:25
Now,
00:26
if I have five villains in my network,
00:30
I will have five spanning trees running per switch in my network. So Cisco does not run one spanning tree protocol for all villains. Cisco chooses to run one spanning tree per villain. So if you have 10 villains, you're gonna have 10 spanning trees running automatically.
00:49
Why no Cisco do this and this little five minute
00:53
lecture is beyond the scope of C CNN. But since you're learning
00:58
that Cisco switches run pervy land spanning tree, I think you guys should know why.
01:03
And this is why.
01:04
Let's say we have force, which is connected to each other. This is which one is the switch three.
01:12
Is this switch to
01:15
and
01:15
switch for
01:19
Let's say we had Vreeland
01:21
10
01:22
and 20.
01:25
No,
01:26
if you were running what is called regular spanning tree or common spanning tree CST,
01:33
what would happen is this port maybe, and let's say this is the route
01:37
for both of the land
01:38
this port would become blocking, which means this link would go unused
01:46
for both Bill and 10 and 20
01:49
now. In Sister's case,
01:51
Cisco switches would run a separate spanning tree for villain 10 and a separate spanning tree for villain 20. So, in theory, what I could do
02:00
is I could make switch one root for
02:02
villain 10 and makes which four root for villain 20. This way.
02:13
Since this is route for villain 20
02:15
and this is route for villain 10
02:17
villain 10 traffic would be blocking here
02:21
and Dylan 20 traffic might be blocking here,
02:24
so each link is blocking for a separate villain. However, it is being used for some traffic. So you're not wasting the band with completely. You're at least using each link for Dilantin or real and 20 traffic. So since this is route for villain 10
02:42
this would be blocking for Dilantin. But at least 20. And since this is route for villain 20 this would be blocking for Bill and 20 but at least the land 10 traffic would be flowing through it.
02:53
This you. This is better load balancing and better bandwith utilization
03:00
with common spanning tree. This link might be blocking for both the land, and then it's completely unused.
03:06
So that is why Cisco chose to
03:07
used one spanning tree per villa for load balancing.
03:14
Now what else is different with pervy land spanning tree?
03:17
Well, the Bridge I D Field has been modified, so bridge I d.
03:23
Now consists off
03:25
a four bit villain I D. Plus 12 bit
03:32
priority. And then you're 48 bit Mac. So your priority field actually has been split in two with the four bit villain I D. And then a 12 it. So let's draw these 16 bits out, and I'm doing this for a reason, which will come in about five minutes. One too
03:53
three,
03:53
456789 10 11 12 13 14 15 16.
04:02
So on the right most side or my right hand side, start with the one
04:09
double it to keep doubling it. 48 16 32 64 1 28 to 56 5 12
04:21
10 24
04:25
2048
04:27
40 96
04:30
81 92 16 3 84 and 32 768
04:38
If I add all these numbers up, I would get 65 5 36
04:42
Okay, so you have a four bit
04:46
villain i D and a 12 bit priority field.
04:50
So
04:51
keep this 40 96 number in mind. For now.
04:56
Keep this 40 96 in mind.
05:00
The default priority for a Cisco switch is
05:04
32 768 And let's say we are running villain 10 in art Apology.
05:12
So the villain I D Field would say eight plus two is 10 and the rest of the bits would be
05:18
turned off. So zero is off and the one is on.
05:24
So my total priority field, actually, if looking at it the old fashioned, where the priority field will actually say
05:30
32 768 plus 10 which gives me 32 778
05:41
Okay, this is how the bridge I D is kept separate for each villain
05:46
by adding the villain I d to the priority field.
05:49
Now I'm going to go ahead and show you how spanning three works on the lab.
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