Let's talk a little bit more about the concept of the multiple spanning tree
instance or
the MSTI, which is at the very heart of MST.
So like we've already diagrammed out, let's just go ahead and get this out here
.
We create our instances and then we can map our VLANs such as 10, 20, and 30,
like we
did in the previous example, into one of these instances and then VLANs 15, and
25, and 35,
and any other VLANs we desire would go into the other instance.
Now these instances are given an ID. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now these instances are given an ID.
So our ID can range from 0 to 65, and as we might imagine, that range is going
to be actually
not the full range, but up to 40, 94.
So we're not going to get 40, 95, however, 0 will be used, this is known as the
default
instance ID.
Now when we assign these, let's just say that on the left we have instance 0 is
on
the left, and then on the right we'll do instance 1. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the left, and then on the right we'll do instance 1.
So what we're going to do now is we're going to find that these instances,
again, each
get their own topology drawn out in the same way that we would have done on a
per VLAN
basis with rapid PVSD+.
So maybe again, VLANs 10, 20, and 30, we'd say that these are three different
topologies
where one of them is one of these links will be blocked for each one of those V
LANs.
However, with MST now, I have an instance 0 topology, and so that's going to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
However, with MST now, I have an instance 0 topology, and so that's going to
use this
link and this link will make this switch on the left the root for instance 0.
Again, the root is no longer assigned per VLAN, it's assigned per instance, and
then
I'll block this link over here.
Meanwhile, I'll create instance 1, I will make this switch the root for
instance 1, it's
going to use this link and this link and block the link on the left.
And that's it, we are done.
We have exactly two topologies, instance 1 on the right there, instance 0 on WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We have exactly two topologies, instance 1 on the right there, instance 0 on
the left,
and that is the magic of MST.
We don't have to worry about creating a topology per VLAN.
And furthermore, if I were to add VLAN, let's just say 40, into instance 0, the
topologies
don't change.
Everything changes about spanning tree just because I decided to add a VLAN
into the environment.
So I can add as many VLANs as I want in here.
I could even create a third topology if needed, however, typically in most
environments and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
environments and
production, this is where we're going to land with our configuration.
Now as a side effect of this, all of our configurations are going to be mapped
per instance now.
All of the exact same concepts apply that we're used to with per VLAN spanning
tree,
it's just now per instance.
So for example, if I want to configure priorities, well, I would normally get
onto a Cisco switch
and say, hey, for VLAN 1, we have a priority of 4K, 4096. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and say, hey, for VLAN 1, we have a priority of 4K, 4096.
And then for a VLAN 2, I have a priority of 8192.
I'm going to do the same concept here with MST, it's just going to be per
instance.
So down here, we said this is the root on the left for instance zero.
So it could have a 4K priority for instance zero, but it could have an 8K
priority for
instance one, meaning that if this switch on the right goes down, I take over
as root
for instance one in the same way I would have done so for the other VLANs.
So we do have that going for us. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So we do have that going for us.
We also can configure our costs on a per instance basis.
So I could get onto a link and say, hey, your cost of 1000 or whatever, but
only for instance
zero.
Now, one thing we'll notice with MST when we're configuring it is it uses the
long path cost
only.
So we're not going to be able to use the old short path costs if we're used to
gig link
being a cost of four and we like that too bad with MST, we're going to have to
use the
long path costs, which is a lot better. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
long path costs, which is a lot better.
And as we've said, should be used anytime we have links of 10 gig or greater.
This will also apply to our port priorities, as we might imagine.
And from here, we might say, well, what about timers?
Well, interestingly, timers are global, meaning that I can't configure timers
on a per instance
basis.
And this is different from PVST plus.
I was able to apply timers to per VLAN configuration on a Cisco switch running WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I was able to apply timers to per VLAN configuration on a Cisco switch running
rapid PVST plus.
However, now they are global.
I can't put them or use different timers in different instances with MST, which
is perfectly
fine because usually we keep the timers the same and all of our instances and
topologies
anyways.
So it doesn't really matter.
It's more just a little nuance to our configuration to be aware of.
Now very importantly, as we indicated in the last video, MST is built on rapid WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now very importantly, as we indicated in the last video, MST is built on rapid
STP.
So this means that we have all of those same negotiations.
So we're going to be able to very quickly bring ports online in the event of
failures.
We're going to have to worry about our edge configuration.
And yes, Cisco still uses port fast for us to apply to our interfaces, even if
we're
running MST.
So we're going to have to make sure that we get our edge configurations in
place.
And we also are going to use all of the same TCN parameters that we talked WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And we also are going to use all of the same TCN parameters that we talked
about.
So flushing the MAC address table, all of that's going to apply.
The rapid spanning tree concepts that we covered previously, those are all the
foundation of
MST.
MST just adds the instances and the topologies on top of all of those same
mechanisms.
So here's the question we have to ask ourselves, which is, which should we
deploy?
Should we deploy MST or should we deploy rapid PVST plus?
Both of them apply the rapid concepts.
And so we don't have to worry about that.
That's like saying, hey, they both offer that as a solution. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
That's like saying, hey, they both offer that as a solution.
So looking at things, MST is objectively a more efficient version of PVST plus.
So we might think, hey, we should always run MST in our environments.
We should never run rapid PVST plus.
It also prevents that issue where we might accidentally create too many top
ologies depending
on the Cisco platform.
And therefore it reduces the risk.
Here's the problem with MST.
And it's not really a technical problem.
Because technically it's very sound. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Because technically it's very sound.
The problem is that a lot of Cisco engineers aren't used to it.
And therefore, supportability in the event of an issue can be a little bit more
challenging.
And so there's no right or wrong answer per se to deploying rapid PVST plus
over MST.
In fact, in my career as a consultant, I only saw MST probably once or twice in
Cisco networks.
Probably 99% of Cisco networks I worked on had PVST plus rapid PVST plus, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Probably 99% of Cisco networks I worked on had PVST plus rapid PVST plus,
hopefully,
running in those environments.
And so that's typically what we like to do simply because it is the default
configuration.
Yes, we have to enable rapid, but Cisco devices come with PVST plus enabled.
And it's what most of us are used to, especially those of us who have been in
the industry
for a very long time and who came directly from the Cisco space as opposed to
configuring
non-Cisco networks first.
So long story short, no right or wrong answer here. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So long story short, no right or wrong answer here.
Take the information that we have, but also always consider supportability and
making sure
that our networking team is comfortable with MST.
Learning it is painful.
It's going to require network outages because we're going to have to deploy it
and have
ports going to blocking and it's going to be very difficult to do a migration.
But do understand that there are a lot of benefits to MST if we do make the
transition
and if we decide that our team is able to support it.