now, a little caveat. Cisco switches don't exactly just run spanning tree Cisco switches don't run just spanning tree protocol.
run what is called per villain spanning tree particle or P V S t.
if I have five villains in my network,
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.
Why no Cisco do this and this little five minute
lecture is beyond the scope of C CNN. But since you're learning
that Cisco switches run pervy land spanning tree, I think you guys should know why.
Let's say we have force, which is connected to each other. This is which one is the switch three.
Let's say we had Vreeland
if you were running what is called regular spanning tree or common spanning tree CST,
what would happen is this port maybe, and let's say this is the route
for both of the land
this port would become blocking, which means this link would go unused
for both Bill and 10 and 20
now. In Sister's case,
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
is I could make switch one root for
villain 10 and makes which four root for villain 20. This way.
Since this is route for villain 20
and this is route for villain 10
villain 10 traffic would be blocking here
and Dylan 20 traffic might be blocking here,
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
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.
This you. This is better load balancing and better bandwith utilization
with common spanning tree. This link might be blocking for both the land, and then it's completely unused.
So that is why Cisco chose to
used one spanning tree per villa for load balancing.
Now what else is different with pervy land spanning tree?
Well, the Bridge I D Field has been modified, so bridge I d.
a four bit villain I D. Plus 12 bit
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
456789 10 11 12 13 14 15 16.
So on the right most side or my right hand side, start with the one
double it to keep doubling it. 48 16 32 64 1 28 to 56 5 12
81 92 16 3 84 and 32 768
If I add all these numbers up, I would get 65 5 36
Okay, so you have a four bit
villain i D and a 12 bit priority field.
keep this 40 96 number in mind. For now.
Keep this 40 96 in mind.
The default priority for a Cisco switch is
32 768 And let's say we are running villain 10 in art Apology.
So the villain I D Field would say eight plus two is 10 and the rest of the bits would be
turned off. So zero is off and the one is on.
So my total priority field, actually, if looking at it the old fashioned, where the priority field will actually say
32 768 plus 10 which gives me 32 778
Okay, this is how the bridge I D is kept separate for each villain
by adding the villain I d to the priority field.
Now I'm going to go ahead and show you how spanning three works on the lab.