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Let's talk a little bit more about the concept of the multiple spanning tree

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instance or

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the MSTI, which is at the very heart of MST.

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So like we've already diagrammed out, let's just go ahead and get this out here

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.

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We create our instances and then we can map our VLANs such as 10, 20, and 30,

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like we

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did in the previous example, into one of these instances and then VLANs 15, and

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25, and 35,

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and any other VLANs we desire would go into the other instance.

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Now these instances are given an ID.
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Now these instances are given an ID.

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So our ID can range from 0 to 65, and as we might imagine, that range is going

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to be actually

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not the full range, but up to 40, 94.

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So we're not going to get 40, 95, however, 0 will be used, this is known as the

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default

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instance ID.

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Now when we assign these, let's just say that on the left we have instance 0 is

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on

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the left, and then on the right we'll do instance 1.
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the left, and then on the right we'll do instance 1.

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So what we're going to do now is we're going to find that these instances,

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again, each

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get their own topology drawn out in the same way that we would have done on a

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per VLAN

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basis with rapid PVSD+.

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So maybe again, VLANs 10, 20, and 30, we'd say that these are three different

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topologies

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where one of them is one of these links will be blocked for each one of those V

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LANs.

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However, with MST now, I have an instance 0 topology, and so that's going to
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However, with MST now, I have an instance 0 topology, and so that's going to

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use this

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link and this link will make this switch on the left the root for instance 0.

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Again, the root is no longer assigned per VLAN, it's assigned per instance, and

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then

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I'll block this link over here.

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Meanwhile, I'll create instance 1, I will make this switch the root for

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instance 1, it's

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going to use this link and this link and block the link on the left.

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And that's it, we are done.

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We have exactly two topologies, instance 1 on the right there, instance 0 on
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We have exactly two topologies, instance 1 on the right there, instance 0 on

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the left,

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and that is the magic of MST.

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We don't have to worry about creating a topology per VLAN.

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And furthermore, if I were to add VLAN, let's just say 40, into instance 0, the

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topologies

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don't change.

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Everything changes about spanning tree just because I decided to add a VLAN

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into the environment.

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So I can add as many VLANs as I want in here.

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I could even create a third topology if needed, however, typically in most

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environments and
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environments and

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production, this is where we're going to land with our configuration.

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Now as a side effect of this, all of our configurations are going to be mapped

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per instance now.

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All of the exact same concepts apply that we're used to with per VLAN spanning

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tree,

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it's just now per instance.

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So for example, if I want to configure priorities, well, I would normally get

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onto a Cisco switch

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and say, hey, for VLAN 1, we have a priority of 4K, 4096.
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and say, hey, for VLAN 1, we have a priority of 4K, 4096.

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And then for a VLAN 2, I have a priority of 8192.

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I'm going to do the same concept here with MST, it's just going to be per

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instance.

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So down here, we said this is the root on the left for instance zero.

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So it could have a 4K priority for instance zero, but it could have an 8K

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priority for

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instance one, meaning that if this switch on the right goes down, I take over

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as root

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for instance one in the same way I would have done so for the other VLANs.

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So we do have that going for us.
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So we do have that going for us.

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We also can configure our costs on a per instance basis.

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So I could get onto a link and say, hey, your cost of 1000 or whatever, but

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only for instance

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zero.

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Now, one thing we'll notice with MST when we're configuring it is it uses the

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long path cost

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only.

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So we're not going to be able to use the old short path costs if we're used to

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gig link

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being a cost of four and we like that too bad with MST, we're going to have to

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use the

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long path costs, which is a lot better.
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long path costs, which is a lot better.

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And as we've said, should be used anytime we have links of 10 gig or greater.

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This will also apply to our port priorities, as we might imagine.

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And from here, we might say, well, what about timers?

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Well, interestingly, timers are global, meaning that I can't configure timers

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on a per instance

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basis.

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And this is different from PVST plus.

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I was able to apply timers to per VLAN configuration on a Cisco switch running
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I was able to apply timers to per VLAN configuration on a Cisco switch running

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rapid PVST plus.

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However, now they are global.

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I can't put them or use different timers in different instances with MST, which

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is perfectly

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fine because usually we keep the timers the same and all of our instances and

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topologies

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anyways.

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So it doesn't really matter.

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It's more just a little nuance to our configuration to be aware of.

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Now very importantly, as we indicated in the last video, MST is built on rapid
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Now very importantly, as we indicated in the last video, MST is built on rapid

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STP.

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So this means that we have all of those same negotiations.

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So we're going to be able to very quickly bring ports online in the event of

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failures.

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We're going to have to worry about our edge configuration.

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And yes, Cisco still uses port fast for us to apply to our interfaces, even if

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we're

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running MST.

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So we're going to have to make sure that we get our edge configurations in

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place.

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And we also are going to use all of the same TCN parameters that we talked
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And we also are going to use all of the same TCN parameters that we talked

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about.

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So flushing the MAC address table, all of that's going to apply.

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The rapid spanning tree concepts that we covered previously, those are all the

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foundation of

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MST.

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MST just adds the instances and the topologies on top of all of those same

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mechanisms.

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So here's the question we have to ask ourselves, which is, which should we

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deploy?

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Should we deploy MST or should we deploy rapid PVST plus?

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Both of them apply the rapid concepts.

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And so we don't have to worry about that.

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That's like saying, hey, they both offer that as a solution.
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That's like saying, hey, they both offer that as a solution.

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So looking at things, MST is objectively a more efficient version of PVST plus.

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So we might think, hey, we should always run MST in our environments.

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We should never run rapid PVST plus.

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It also prevents that issue where we might accidentally create too many top

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ologies depending

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on the Cisco platform.

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And therefore it reduces the risk.

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Here's the problem with MST.

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And it's not really a technical problem.

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Because technically it's very sound.
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Because technically it's very sound.

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The problem is that a lot of Cisco engineers aren't used to it.

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And therefore, supportability in the event of an issue can be a little bit more

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challenging.

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And so there's no right or wrong answer per se to deploying rapid PVST plus

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over MST.

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In fact, in my career as a consultant, I only saw MST probably once or twice in

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Cisco networks.

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Probably 99% of Cisco networks I worked on had PVST plus rapid PVST plus,
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Probably 99% of Cisco networks I worked on had PVST plus rapid PVST plus,

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hopefully,

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running in those environments.

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And so that's typically what we like to do simply because it is the default

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configuration.

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Yes, we have to enable rapid, but Cisco devices come with PVST plus enabled.

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And it's what most of us are used to, especially those of us who have been in

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the industry

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for a very long time and who came directly from the Cisco space as opposed to

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configuring

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non-Cisco networks first.

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So long story short, no right or wrong answer here.
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So long story short, no right or wrong answer here.

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Take the information that we have, but also always consider supportability and

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making sure

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that our networking team is comfortable with MST.

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Learning it is painful.

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It's going to require network outages because we're going to have to deploy it

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and have

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ports going to blocking and it's going to be very difficult to do a migration.

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But do understand that there are a lot of benefits to MST if we do make the

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transition

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and if we decide that our team is able to support it.