- [Instructor] We know that with traditional
spanning tree protocol,
the IEE 802.1d standard,
we had what was called a CST,
a Common Spanning Tree,
meaning that all VLANS share
the same spanning tree topology
they had the same root bridge,
although the switch is used the same at root ports,
all the links had the same designated ports,
all the same ports were blocking on our switches
for every single VLAN,
and then Cisco recognized that same topology
might not be optimal for every VLAN we have.
One VLAN might benefit from having one switch as the root,
and another VLAN might benefit from having
a different switch as the root,
just based on the traffic flow patterns,
and Cisco came up with PVST+,
Per VLAN Spanning Tree Protocol Plus,
where each VLAN could get its own spanning tree instance.
However, we might have lots of VLANS
configured on our switches,
if we had a couple of thousand VLANS,
we could run a couple of thousand instances
of spanning tree,
one for each VLAN,
but another option is to use MSTP,
and that's Multiple Spanning Trees Protocol,
or you might also see this written as
Multiple Spanning Tree Protocol, MST,
and there is an IEEE standard for this,
it's IEE 802.1s,
and here's the idea,
yes, different VLANS might benefit from different
spanning tree topologies,
however, a lot of VLANS might share
the same spanning tree topology
as their optimal topology.
For example, let's say that in this very simple topology,
switch A would be the best root
based on the traffic patterns
for VLANS one, two, three, and four,
and switch B would be the optimal root bridge
for VLANS five, six, seven and eight,
well instead of running eight different instances
of spanning tree, as we would do with PVST+,
what if we did this.
What if we said, if we break it down,
there's really only two spanning tree topologies
that we need,
one that has switch A as the root,
and one that has switch B as the root,
so instead of creating eight different
spanning tree instances,
let's just create two instances,
and that's what we've done here.
We said in instance one, switch A is the root,
in instance two of Multiple Spanning Trees Protocol,
switch B is the root,
and we'll assign VLANS to the appropriate instance,
we assign them to the instance
that has their optimal spanning tree,
so this is really the best of both worlds, isn't it?
We're minimizing the number of spanning tree instances
that we have to run
while making sure every VLAN
has an optimal spanning tree for that VLANS traffic flow,
that's a review of the theory
of Multiple Spanning Trees Protocol,
in our next video,
we're going to take a look at its configuration.