Alright, now it's time to dive into our final spanning tree protocol discussion, and that
is on multiple spanning tree protocol, MSTP, also known as, what did we say earlier, 802.1S,
or multiple spanning tree, you might see MST, MSTP, 802.1S, if you see any of those, referring
to the exact same thing on the certification exam.
Alright, so there's many different reasons why we want to use multiple spanning tree protocol, so one of the reasons that we talked about earlier was the fact that this is the only version of spanning tree protocol that works the same way on non-Cisco equipment and Cisco equipment.
So on a certification exam, if they ever have a scenario that mentions Cisco and non-Cisco equipment in the exact same scenario, and you're dealing with layer two, and you are dealing with spanning tree protocol, immediately think multiple spanning tree protocol.
That's right where I want you to go with that, because that is the best solution possible, and that's what they're going to be testing you on in that type of situation.
So right away, multi-vendor support, multiple spanning tree protocol is the way to go.
Alright, however, what is the other reason why we want to use multiple spanning tree protocol in pretty much most of our modern day environments?
It is because there are some limitations in regards to spanning tree protocol.
Does anybody know how many trees we can have when we're using Pervyland Spanning Tree Plus or Rapid Pervyland Spanning Tree Plus?
Does anybody know how many trees?
What do I say by trees?
I mean, how many different spanning trees can we have running on a single device?
Okay, well, it typically depends on the platform, so nobody's going to have the perfect answer here.
But usually, it's around 256 or less.
So that means in this part of my network here, I cannot have more than 256 trees.
Therefore, how many VLANs can I have then in that part of the network?
256.
Because what happens if I create 260 VLANs?
Well, if I create 260 VLANs, 256 will have a spanning tree instance running.
But the other four will not.
They will be those four extra VLANs 100% looping all the time.
Oh, what if you had 300 VLANs?
Well, now it's getting worse, right?
You could only have 256.
So typically, it's 256.
I have seen a few platforms that only support 128 VLANs, okay?
So the number you're not going to be testing on, that's the key.
You're not going to be testing on the actual number of trees you could have on the exam.
But I want you to realize that if you have an excessive number of VLANs, 500, 600, 700,
on the certification exam or in the real world, these really are not your go-to options at all.
And they are the defaults, right?
You really need to go with multiple spanning tree protocol.
Because multiple spanning tree protocol is going to allow us to circumvent this limitation.
So consider right now the fact that we have, let's say I'm going to use 1,000 VLANs, okay?
Now, we know based on what I just said that with 802.1W, right, we would have one tree.
That's it, just one tree for all 1,000 VLANs based on what we talked about earlier today, okay?
But rapid per VLAN spanning tree plus, we'd have one tree per VLAN, therefore 1,000 trees,
which is not even possible, as we just said, because it would max out at 256.
But just to keep the story going, right, think about that for a second.
One tree with 802.1W, all the VLANs follow that tree, that diagram, okay?
Rapid per VLAN spanning 1,000 trees, okay?
But with multiple spanning tree protocol, I pick the number of trees.
I pick the number of trees.
So if I have 1,000 VLANs, I could turn around and I could have two trees and put certain VLANs in each tree.
I control the number of trees in which VLANs are in the trees.
Now, let's just look at this diagram for a second.
Based on our conversation earlier today in our spanning tree protocol discussion,
we said typically we're going to place our root bridges where, folks?
Where are we going to do that?
As close as possible to the layer three.
So in our case here, our what?
Our distribution layer switches.
That's where we're going to put it.
So let me ask you this.
How many devices in total do I have, in this case, to be root bridges?
This too.
I got one here and I got one here because we're not going to use these two, right?
All right.
So if I could have this as a root bridge for a tree and this as a root bridge for a tree,
how many trees, how many unique trees can we have?
So if I could have a root bridge here for a tree and a root bridge here for a tree,
how many unique trees can we have in this topology?
It's two, right?
One tree for each root bridge.
One tree for each distribution layer switch.
So really, two trees, two unique trees, one anchored here, one anchored here.
Just think about earlier when we did VLAN 10 and VLAN 50.
Do you remember what it looked like?
Well, let me remind you.
So one of them was here.
One of them was here.
VLAN 10, VLAN 50.
Okay, but what about VLAN 20?
Where's VLAN 20's root bridge?
Well, by default, it's here.
So VLAN 20 has a tree anchored to DSW1.
Okay, what about VLAN 200 by default?
Well, it's also anchored here by default.
So I have four trees right now in this environment,
but there's really only two unique trees we can have, right?
One that is anchored here and one that is anchored there.
So meaning right now, I've got three trees that are all anchored here.
Well, with multiple spanning tree protocol, if that's what I want,
I could have one tree with all three VLANs in it.
That's what I can do while having this one here.
That's the difference between running PVST plus or rapid PVST plus
versus something like multiple spanning tree protocol.
I now have the ability to ensure that I'm running the minimum number of trees
based on my architecture.
So instead of having 1,000 trees, again, we know it's not possible.
You can only like basically 256 max, okay?
But 1,000 trees that are going to be all anchored here or anchored here.
Why not just have two trees
and put all the VLANs in this tree and put the other VLANs in this tree, right?
Half here, half there.
We can do that with multiple spanning tree protocol.
So now my router only has to send what?
Enough BPDUs for what?
Those two trees instead of 1,000 trees, right?
Performance is increased here as well.
So a lot of us are running rapid PVST plus in our environment.
And we have like 20 VLANs down here when in reality,
we really could use multiple spanning tree protocol
and group VLANs together on a common root bridge.
Group VLANs together on a common root bridge.
Half the traffic may be going through here.
Things to consider, right?
Reduce the total number of trees.
Build trees, place the VLANs in the trees you want.
So now when it comes to our bridge protocol data unit,
our BPDU and our what?
Our bridge ID or bridge ID has changed now.
Because our bridge ID, instead of it being the VLAN,
notice it says instance now instead of VLAN.
What do we mean by instance?
Well, the instance refers to the tree.
So if I have a tree that's anchored here,
and this is the root bridge,
this might be, for example, instance number one.
So the bridge priority for instance number one can be controlled.
So instance number one would be like one.
So instead of doing it on a VLAN by VLAN basis,
we're doing it on an instance by instance basis.
Think that an instance equals a tree.
And your tree is going to have whatever VLANs you want inside of it now.
So likewise, this could be the root bridge.
The root bridge for instance number two, right?
So any BPDUs for that one are going to have the bridge priority of a two in it there.
So the bridge priority number itself, zero through 65,535,
just like we saw before.
But now we have instance one, two, three, four, and so on and so forth.
And you can only build up to a maximum of,
last time I checked was 16 trees, I believe.
And that's okay.
In this case, look, we only need two, right?
But you could do up to 16 trees with multiple spanning tree protocol.
That's not something you'll be tested on.
So you don't have to worry about that.
All right.
Now, in order for you to be part of the same spanning tree protocol environment,
you have to be part of what is called the same MSTP region with the same information.
So all of these switches that are going to be participating in multiple spanning tree protocol,
they are going to have to be configured the exact same way with the exact same parameters.
So an MSTP region is a collection of switches sharing the same
view of the spanning tree protocol topology.
So they are going to partition it the same way in the logical way that it needs to be partitioned.
They are going to be able to agree on the same root bridges and all that stuff.
So if any of these values I'm about to show you are ever different on any of the switches,
then you are not part of the same region.
Therefore, you'll build your own tree totally separate of everyone else.
So it's really important that these three parameters are perfect in your configuration
on all of the switches that are part of the same spanning tree region.
So we know that this one, this one, this one, and this one,
they all need to be in the same region because we want them to build a tree together.
So therefore, the configuration name, the region name has to be the exact same on all four devices.
The revision number has to be the same on all four devices.
Now, the revision number here is totally different than VTP.
So in VTP, remember, every time you made a change to the configuration,
the revision number went up by one.
And that was used to help with synchronization.
The configuration revision number here is for you when it comes to change management.
That's what it is.
Change management.
And for you, for your configuration management as well.
So that way there, you can keep track of your changes
and you can keep track of your configuration revisions that you have done.
It's just a manual number.
So if you make a change, you're supposed to update the number on all of your devices.
And therefore, you keep it consistent.
So once you make a change, you update the number.
But the number has to be the same on all the devices.
So if any number is not updated, then what?
It's going to build a different tree.
It's not going to be part of the group.
And then lastly, the instance, the VLAN mappings, that is what?
The building of the trees.
So you are going to specify, I want this many trees.
And these are the VLANs I want in every single tree.
So let me just show you a configuration of that right here.
So you'll see DSW1.
I'm typing in spanning tree MST configuration.
And then I'm in MST configuration mode.
And from there, I put the revision number of four.
So I'm just adding a number I picked, okay?
So in this case, I'm starting with four.
You probably start with one and work your way up from there.
Every time you make a change, you update your revision number.
And you would have to do that on every single device.
So it's a manual revision number.
And it has to be the same.
So you can see, same, same, same, same, right?
Configuration name.
I named it Encore.
So same.
And then the instance to VLAN mapping.
So this is going to be tree number one.
And these are the VLANs I want in tree number one.
This is instance number two, tree number two.
So I'm just using thousand VLANs as an example here.
But in our environment, and you'll see I've done the same thing.
So look, the config has to be 100% the same.
There is no exception to this rule.
If one of these values are out of place, then it's not going to work.
You're going to have discrepancy.
So in this case, for example, we go to ASW1, type in configure terminal MST configuration.
And what I want to point out to you is that you can do the configuration
without affecting your current spanning tree topology.
That's all this is, a configuration.
A configuration that you then apply.
So I'm running RapidSpanningTree protocol right now.
I can do all this configuration.
It doesn't affect my RapidSpanningTree protocol right now.
So, for example, let's go ahead and type in revision.
Put a revision number of six here for fun.
Let's give it a name, ENCOR.
And then let's go ahead and say, like, for example, instance, instance number one.
Let's pretend that we're going to put VLANs 10 and 200 in this one.
And then instance number two, we're going to do VLANs, what is it, 20 and 50.
That's what we're going to do.
All right.
So there's my configuration.
So do show run pipe to section MST.
SpanningTree protocol.
All right.
Nope.
Not that one either.
Show run.
I forget what it is.
All right.
So there you go.
So let's go ahead now.
You can just copy and paste this configuration to all the other devices,
because they've got to be the exact same.
There you go.
OK.
So now that they all have them the same, if we take a look now,
we type in something like show SpanningTree MST configuration,
you'll see that it says right now we're not in MST mode.
So the configuration is ready to go.
We just haven't enabled it yet on all of our devices.
And you'll see now that every VLAN by default is part of instance zero.
So there's this instance zero.
So you actually have, even though we created two trees, we actually have three trees.
And instance zero is going to be the tree for all the other VLANs if they exist on your device.
And then if you have VLANs 10 through 19, no, that's not what I wanted.
How did I end up with that config?
It's not what we wanted, was it?
No.
What did I type in to cause that?
Did I accidentally hit a dash?
Oh, yes, I put a dash.
That's not what we wanted.
Let's fix that because that's not good.
That's not what we want.
So we definitely want to fix that.
Okay, so configure terminal.
So VLAN 10 comma 200.
200.
And then we want instance number two, VLAN 20 comma 50.
I did it again.
All right.
Now let's take a look at our running config.
So I'm going to have to probably delete it.
All right.
And then we'll put in this command here now.
We'll put in these two values here.
We'll type in instance one, VLAN 10 comma.
Doesn't take effect until I log out.
That's correct.
So you got to exit the mode before it actually takes effect.
So let's just do our second instance and make sure we got it.
So we said 20 and 50.
That looks a lot better.
Okay.
So we'll copy and paste that over here.
But first we're going to have to say new.
And then paste this one in.
There you go.
Now we should be good.
All right.
Oh, my gosh.
There we go.
That looks a lot better.
All right.
So with that in mind, as you can see, this is going to match on all of our devices.
And if it doesn't match on all of our devices, then it's not going to work.
So something to pay attention to on the exam is if you're ever comparing
this configuration to the configuration on another device, another device, another device
to figure out why something's not working, just come flat out and look for these things.
Is that the same on all the devices?
Is this the exact same on all the devices?
Because if it's not, that is why it's not working.
And then you're going to have to answer the multiple choice question, the drag and drop,
or the what simulation and make any corrections that are necessary in order to get it to work.
OK.
So I got an exit out here.
First for it to take effect on that one.
We'll take a peek now.
So, yeah.
So just basically you're comparing.
Are they the same?
And right now they look like they're the same.
Right.
So how do you turn it on now?
You go to every single device and you would type in spanning tree mode MST.
Enter.
Go over here, spanning tree mode, MST.
And now we're running multiple spanning tree protocol on all of the devices.
So let that cook for just a moment.
All right.
Because it can take a little bit of time to fully converge and resolve itself of any kind.
Of any conflicts that actually exist.
So we just saw that in our environment when we configured it.
Right.
To transition over to multiple spanning tree protocol, as we saw, spanning tree mode MST.
So now when you're looking at the output, you'll look at things like the actual tree itself.
So instead of looking at it on a VLAN by VLAN basis, like you did with
per VLAN spanning tree plus a rapid pre-linked VLAN spanning tree plus,
you're going to look at it on a tree by tree basis.
And so you'll see the trees that are mapped in that particular case.
So let's see if we can see that now.
Do show spanning tree MST1 as an example.
There you go.
Right.
And if we go over to something like DSW1 and we type in do show span MST1,
we'll see that we're not the root port for VLAN 10 or like we were before.
Why is that?
Because this is totally different.
So right now we're at the factory default settings when it comes to
multiple spanning tree protocol, except for the trees we have created here.
So meaning that everything we did in our previous example,
where DSW1 was the root bridge and all that stuff does not apply anymore.
But we can make it happen.
So in this case, for example, you would type in spanning tree MST1 priority zero.
And now that would make me the root bridge for this entire tree.
Right.
Right now I'm not the root bridge for the entire tree.
Why?
Because it doesn't say I'm the root bridge for the entire tree.
Not all my ports are designated ports.
So now let's take a look at it.
Now that we have a priority of zero, you'll see that now I am now the root bridge.
And you'll see that we have all designated ports.
This is in a dispute state right now because it's still converging.
So once that all sorts itself out, then everything's going to be good.
If we go to ASW1 as an example and take a peek at MST1,
notice that our port fast configuration carried over.
So port fast configs will carry over.
BPDU guard configs will carry over.
No problem there.
But you'll see here that what do we have?
45 is not the root port right now.
It's 46.
So my question is, is did our port priorities?
Yeah.
So even the port priorities carried over, as you can see.
So any of those interface adjustments we made,
so on the interface changing cost or interface changing port IDs,
all of that stuff got carried over.
Port fast gets carried over.
BPDU guard gets carried over.
But things like your root bridge doesn't get carried over.
So that might be something you'd have to adjust after that fact.
All right.
Now everything's fine.
Everything's in the forwarding state.
We don't have any disputes right now.
And notice that the cost is automatically using the long path cost.
The long path cost as opposed to the short path cost in a particular case.
And that automatically adjusts on all our devices.
So just like before, changing port cost is super simple.
But instead of changing it on a VLAN by VLAN basis, you have to do it on a what?
You're going to have to do it on a instance by instance basis in this case.
So notice it says 4 here.
I think it's because if we look at show run for interface,
gigabit ethernet 1 size 0 size 45, we had the cost set to 4.
Yeah, we did.
So as you can see, setting the cost of 4 overruled anything that was going on over there.
So let's go ahead and change our cost or just remove this command on that interface right now.
And that'll change over here shortly.
Oh, no, wait.
I only did the one interface.
So 46 as well.
All right.
So I'm just trying to show you – just walking you through some of the things that we've already done
earlier today.
And you'll see everything else that we talked about applies the same to multiple spanning
tree protocol, which is great.
There's not a big learning curve to it except for that configuration we did.
And then adjusting your – everything based on what?
MST as opposed to VLANs.
So I have some slides for that, too, if you ever want to review the slides.
So changing the priority, making DSW1 the root bridge.
Right now it's the root bridge for the entire MST tree in that case.
So if we want to do DSW2 as the root bridge now for MST2, we can go ahead and do that.
And so now, for example, VLANs 1 through 500 are using DSW1 as the root bridge in this scenario,
and 501 through 1,000 are using MST2 as the root bridge, which is awesome.
So the thing I want you to realize is that as we're exchanging BPDUs and stuff like that,
that is going to be extremely different now with multiple spanning tree protocol.
And the reason why is because we don't have to share information that is already
pre-configured on ourselves.
So when you think about something like our revision number, our revision – our configuration name,
and you think about our instance to VLAN mapping, all of that stuff – all of that stuff is what?
Configured on every single device.
So we don't have to share that information with each other.
But what we do have to do is verify that we have the same information.
So when you look here at something like DSW1, we have a digest that is created
for that configuration.
We have a digest that is created.
Notice the digest is the exact same.
Why is that important?
Because this is how they verify that they're part of the exact same region.
So whenever we exchange our BPDUs, we don't have to send the instance ID mappings in that.
We don't have to send our revision number.
We don't have to send our domain name, like things like that.
Why?
Because we just have to send the digest.
And the digest is a way that we can essentially authenticate with each other and say,
oh, yeah, we're part of the same region.
Therefore, we can exchange or I can accept this BPDU from you,
and I can go ahead and process this BPDU from you because the digest that you sent
is the same digest that I have.
And so that's one way they can verify that they are speaking to the right devices
in that particular environment.
So if somebody comes along that's rogue, that is not using the right revision number,
not using the right name, therefore, they're not using the right digest,
they're going to be ignored.
So you're only going to talk to those devices that have the same digest as you.
So as you can see, there's a little bit of a safety feature added into this as well,
unlike PVST+, or Rapid PVST+, where if you get a BPDU from anybody,
you're just going to go ahead and agree with them because there's no way to verify
that they're part of the same group as you.
Whereas in this case, you are part of the same group.
Now, you see this configuration information that we talked about earlier?
Right, or all of this stuff here, I should say, let's just go out to here.
Do show spanning tree MST configuration.
All right, what I want you to realize is all of this information,
you don't have to configure on every device.
You don't.
Why?
Because there's a way that you can auto-propagate this information to every other device.
What service or protocol that we've already talked about, folks,
do you think has the ability to allow you to configure it once
and have it automatically propagated to every other switch in the environment?
VTP version three, that's right.
If you're using VTP version three,
then VTP version three, you can set it up.
So, anything you do with MSTP automatically gets sent to the other devices.
So, you won't have to log into every other device and configure them yourself.
All right, and that's why this message digest is really good.
So, that way, when you are exchanging those messages dynamically
and you're changing BPDUs dynamically, this helps as well, right?
So, another thing to keep in mind, just for the real world,
it may come up on the exam or not, but you do not send a BPDU for every single instance.
So, even though I have three instances right now, I have zero, one, and two,
we only send one BPDU, and we send one BPDU for instance zero.
That's what we're sending, and we don't send a VLAN list or anything like that.
Why?
Because we send the digest instead, as I was talking about.
So, we have instance zero.
By default, all VLANs are mapped to instance zero, as we saw.
So, if I just get out of the way here for a second,
and you'll see that all the VLANs are originally mapped to instance zero,
and then we have VLANs one and two.
And I think we can see that if I scroll back up.
When I was having those challenges,
oh, I'm sure I typed it in, and we saw that they were all part of instance zero.
I think it was on DSW two.
Yeah, here is an example.
So, this was before our config was finalized.
You see how they're all mapped to instance zero by default.
So, if you just wanted everything in one tree,
you could just leave it in instance zero, and you're done.
But if you want to get creative,
then that's when you start breaking them out into other instances.
So, when we send a BPDU, we're only sending it for IST.
The IST, also known as instance zero, or MST instance zero.
The name could be many different things, as you can see in my documentation here.
And the IST, so this IST zero, this BPDU for the IST,
carries all the Spanish-related information for itself.
And it's just standard RST, but it's a standard RST.
So, it's a standard RST.
So, for itself, and it's just standard RSTP information.
So, it's Rapid Spanish Protocol information.
And then it has additional special fields
that are going to be utilized as well to carry other information.
For example, like we are going to have M records.
So, for every other instance you have,
you're going to have an M record associated with it.
So, you have all the instance zero information,
and then behind that, you have an M record for what?
Instance one.
You have an M record for instance two.
So, for example, if you have DSW1 as the root bridge for instance one,
then that M record would have that priority information in it.
So, the bridge ID would be part of the M record for instance one.
The bridge ID would be part of the M record for instance number two.
So, those manipulations and changes you make to the specific instances,
those are carried in the M records themselves,
as well as other information that is very specific for those instances.
Other than that, if there's nothing specific for those instances,
then the information from instance zero applies.
So, this ensures that you only have one BPDU going out every single interface,
as opposed to one BPDU for each and every single tree,
or as in our rapid Spanish tree protocol case,
and Spanish tree protocol case, one for every single VLAN.
So, you see the difference now in performance as well?
There's a lot less pressure on our switches
when we use multiple Spanish tree protocol
because there's less BPDUs being sent,
less BPDUs being received, and therefore processing is a lot better.
Now, you have to be super careful here with multiple Spanish tree protocol,
the last thing we're going to talk about for Encore.
And what is that?
It's the fact that you are now going to have to be careful
with your trunk filtering and with your access ports.
So, for example, let's say you have two switches,
and this is VLAN 10 and this is VLAN 20.
You have this set up for VLAN 10 and you have this set up for VLAN 20.
So, access ports, this is not going to work for you.
Why is this not going to work for you?
Because Spanish tree protocol is looking at your instances
when it's deciding whether to block or allow.
So, if VLAN 10 is in instance number one, for example,
and let's say VLAN 20 is also in instance number one.
Say they're both in instance number one.
Well, we have a loop here for instance number one, don't we?
We do.
Okay, let's block.
Notice how VLAN 20 now is completely cut off on the network.
So, what does that mean?
It means you really can't use access ports between your switches at all
when you're using MSTP.
So, but when it comes to trunk ports now, so if you're using trunks,
keep in mind that if you decide, oh, I'm going to filter,
set up a VLAN filter in a trunk,
and you're like, let's allow VLAN 10 here and let's allow VLAN 20 here.
Well, again, you see the problem?
Since they're both in instance one, if we block that trunk link,
then you've just blocked VLAN 20 traffic.
So, the moral of the story is that when it comes to filtering,
you're going to have to make sure that your filtering
for your VLANs is going to be equivalent to the VLANs that are in that instance.
So, if you want to allow the VLANs, then you have to do allow 10, 20.
Allow 10, 20.
So, that way when this is blocked, you see everything works out perfectly fine.
So, that's something to keep in mind when it comes to these.
So, on the exam, if all of a sudden it's an MSTP example,
and they have VLAN filtering on for trunks,
and it's doing something like this, well, that's why you have a problem.
That's why VLAN 20 is being cut off.
It's because you have to remember that we're blocking on an instance by instance basis now,
not a VLAN by VLAN basis now.
So, a lot of good information there.
The MSTP question shouldn't be very difficult on the certification exam.
You're going to get more heavily tested on the other stuff we talked about today,
but you know enough about MST to ace any question that shows up on the certification exam.
Thank you.