- In this video,
we want to take a look at the configuration of
MST, Multiple Spanning Tree protocol.
And we talked in our previous video about
the theory of MST and we said
that at it's essence, it's trying to minimize
the number of spanning tree incidences we have
while giving us an optimal spanning tree for
each of our VLAN's.
'Cuz different VLAN's, they might have
different traffic flows!
But instead of having a separate instance
of spanning tree for every single VLAN,
we might have several VLANs that would
benefit from the same spanning tree
with the same root bridge.
That's what we're going to set up in this video.
And here I've configured four non-default VLANs.
Let's do a show VLAN reef command.
And you see that we have VLANs 100, 200, 300, and 400.
Let's assign VLANs 100 and 300 to the same instance.
And let's assign VLAN 200 to a different instance.
And we're not going to assign the default VLAN of one
or VLAN 400 to anything, and we'll see
what happens to those VLANs.
But let's get started by defining our two instances.
We'll go into global configuration mode
and I'll say spanning hyphen tree MST configuration.
Now, I'll say instance, give an instance number
we'll say for instance one we're going to create
a spanning tree for VLANs 100 and 300.
For instance two, we're going to create
a spanning tree for VLAN 200.
And we need to give that same configuration
on all of our switches in this topology.
So let's go over to switch two
and give exactly the same set of commands.
I'll say spanning hyphen tree MST configuration
instance one VLAN 100,
300,
instance two, VLAN 200.
One switch to go, let's go over to switch three.
And we'll go into global configuration mode.
And we'll say spanning hyphen tree MST configuration
instance one is going to be for VLANs
100 and 300.
Instance two is going to be for VLAN 200.
Now that I've defined my instances,
we want to go in and say which bridge is the root bridge
for our different instances.
So let's say that switch SW1 is going to be the
primary root bridge for instance one
and it's going to be a secondary root bridge
for instance two.
Here's how we configure that.
We got to global configuration mode
and we say spanning hyphen tree MST
we give the instance number of one
and I'll say root primary.
Now, for MST instance two I'll say
I want to be the secondary root.
To do that, I'll say root secondary.
And let's imagine that switch SW3 is going to
be the root for instance two and it's going to
be the secondary root for instance one.
So let's go over to SW3 and give those commands.
And we'll say spanning hyphen tree MST
instance two, I want to be the primary root.
For instance one, it'll be the secondary root.
And the final thing we need to do is set the STP
mode on all our switches to MST.
So let's go through all of our switches and do that.
On switch SW1, we'll say spanning hyphen tree mode
context sensitive help shows that
we could use the default of PBST.
We could do rapid PBST which we'll
talk about in an upcoming video.
Or we can do MST. We're going to do MST.
And we're going to do that on all of our switches.
Let's go over to switch SW2.
And we'll say spanning hyphen tree mode MST
One switch to go.
Over on switch SW3,
we'll say spanning hyphen tree mode MST.
And now that we've configured multiple
spanning tree protocol, let's verify it.
We'll just stay here on switch SW3 and do a show
spanning hyphen tree summary command.
And this tell us that this switch is in MST mode.
And it tells us that we are the root bridge
for MST instance two.
But notice it shows us three MST instances.
We have three instances and one we did not define.
This is MST instance zero.
Well zero is going to contain all the VLANs
that we did not assign to another instance.
So in our case, it's going to be VLAN one and VLAN 400.
They'll belong to that instance.
Now let's verify that from the perspective
of switch SW3 we really are the root for instance two.
We'll go take a look at the instance two VLAN of 200.
And we'll see that we're the secondary root for VLAN 100
which is a member of instance one.
And we'll see what's up with instance zero as well.
But first of all, let's take a look at one of the VLANs
in instance one, VLAN 100.
I'll do a show spanning hyphen tree VLAN 100.
We said we wanted to be the secondary root for instance one
and we see that we're not the root.
The root has this mac address and you can see
from the topology on-screen that's switch SW1.
That's what we set as the root for instance one.
But notice that our priority is not the default priority
of 32768, it's 28672.
And that's because we said we wanted
to be the secondary root.
What about VLAN 200?
That's a member of instance two.
We should be the root for this VLAN and we aren't
it says this bridge is the root.
And if we take a look at VLAN one,
which we did not assign to an instance,
and therefore it became a member of instance zero,
we see that the root is,
looking at the mac address it appears to be SW2.
And the reason is not because of any configuration we gave,
it was because it had the lowest mac address.
That was the default tie-breaker if we don't
manipulate the bridge priority.
The lowest mac address is going to become the root.
And in this case, for VLAN one, it's going to be switch SW2.
And notice that for each of these instances
there's a different topology for our spanning tree.
For instance zero, switch two's the root.
For instance one, switch SW1 is the root.
And for instance two, we're the root on SW3.
And that's a look at how to configure MST.