Well, we said it at the end of that video that if I have a topology that
requires me
to be a little bit more efficient, then I can run Pervieland Spanning Tree and
I can
make it more efficient.
And we said we'd have VLAN 10 here, VLAN 10 will block one of these ports
because this
is the root switch on the left, and then I'm going to use this connection and
this connection,
whereas VLAN 20 has the root on the right, and therefore we're blocking a port
over here, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
over here,
and we're going to use that middle link as well as the link on the right.
So nothing wrong with this.
We have two VLANs and we have two Spanning Tree topologies.
But here's the question.
How many topologies are needed in order for me to be efficient here?
In most network topologies where we have larger loops, we're usually talking
about the distribution
layer and the access layer.
And if we think back to the start of this course, when we talked about network
architecture, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
architecture,
we use the usually two switches here as a pair of distribution switches, and
then we
can have lots of access layer switches.
We don't scale beyond just a pair of distribution switches in most cases.
So how many topologies do we need?
Well, we probably need about as many topologies as we have distribution
switches.
If that's two, then that means we need two topologies.
And it's great that PVST+ in other case gave us for two VLANs, two topologies. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And it's great that PVST+ in other case gave us for two VLANs, two topologies.
But you can probably tell where we're going with this.
What happens when we consider all the other VLANs?
Now what if I have VLANs 10 and 30 and 50 and 70 on the left in VLANs 20 and 40
and
60 and 80?
This is just eight VLANs, let alone 20 or 50 or 100 VLANs in the environment
that are
not.
Well, at this point, I'm going to have to create topologies for all eight of
these VLANs, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
these VLANs,
because PVST+ is always going to be a one-to-one relationship.
And so if I have eight VLANs, that means I have eight topologies.
Now are they different topologies?
No, they're going to be the same topologies.
I wrote these out for this way for a reason, probably 10, 30, 50 and 70 will
all share
the same topology as VLAN 10.
Same thing over on the right, VLANs 20, 40, 60, 80.
I basically just said, hey, if it's an even number in the 10's digit or an odd WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I basically just said, hey, if it's an even number in the 10's digit or an odd
number
in the 10's digit, then we'll sort it out that way.
Now different organizations will sort their VLANs in different ways, but
ideally we end
up with about the same between these two sides.
So if we've got four on this side, we've got four on that side.
If we have 10 on one side, we have 10 on the other and so on and so forth.
So we've largely arbitrarily picked which VLANs go on which side, but now the
problem
is that even though I'm going to share the topology for these three VLANs, I'm WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
is that even though I'm going to share the topology for these three VLANs, I'm
going
to share the same topology here.
I still need eight separate topologies.
So in the end, I'm going to have VLAN 30 which shares the exact same path as V
LAN 10 and
then I could draw two other VLANs on here because here's VLAN 50 coming again,
sharing
the blue path here and then I could grab maybe purple and say this is VLAN 70
and we
see that even though there's no point, we are managing all of these different WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
see that even though there's no point, we are managing all of these different
topologies.
I say no point because we already have the topology, we just couldn't share it
and so
we now have eight different topologies in play.
I just drew a few of them.
So what is happening under the hood?
I mean, why is this such a big deal?
Well, one of the reasons for this is because when we really look at a topology,
we're looking
at a complete spanning tree protocol operation.
What this means is that we have to maintain the elections if there's new WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
What this means is that we have to maintain the elections if there's new
switches.
Regardless, we have to figure out who's going to be the root switch.
We have to identify different port types.
We say types, we're usually talking about the different roles as well as the
different
states.
So if a port, for example, let's say this port here, we see that for the top
ology on the
left here, this is going to be a root port. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
left here, this is going to be a root port.
And yet I've got to maintain four different root ports for the four different V
LANs that
are all going across those different links.
So we've got to have four different VLANs that are being tracked and we're root
port
for all four of those, for example.
And same thing with all the other roles in the states.
It's going to be on a per topology basis because again, a topology encompasses
the entirety
of STP operation.
And this is a big one. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And this is a big one.
We have to worry about BP-DU propagation, which is the sending of our BP-DUs
and the
transmitting of them, forwarding them along, and for that matter, the
processing.
Every time I get a BP-DU, I have to process it.
I have to figure out, hey, is this still the BP-DU I was expecting?
Is the root ID the same, the cost of the root the same?
And then I recalculate things based on what I got.
Furthermore, BP-DUs are sent out every two seconds by default. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Furthermore, BP-DUs are sent out every two seconds by default.
So if BP-DU propagation and processing is part of STP operation and STP
operation is
going to be fully self-contained within each topology, well, that means that I
am truly
sending out eight different BP-DUs every two seconds.
So we have to be a little bit careful with this in production networks.
Now, trust me, Cisco switches, they can handle eight of these.
They can handle eight topologies, no problem whatsoever. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
They can handle eight topologies, no problem whatsoever.
But different hardware will be able to support different amounts of topologies
and different
numbers of VLANs.
So we look at the spec sheet for a particular hardware, we might find that it
supports,
well, maybe it supports all 4096 VLANs.
This is pretty common.
Every now and again, you'll find a switch that supports 512 or 64 or something
like
that, especially the lower end switches.
But let's say we have a switch that supports 4096 VLANs. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But let's say we have a switch that supports 4096 VLANs.
That same switch might support 1024 STP instances.
Where an instance is really a topology.
But it often gets referred to as an instance as well.
So if we only support 1024, but we need one for every VLAN, what does that mean
?
What that means is on the 1,025th VLAN, it will have no STP topology. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
What that means is on the 1,025th VLAN, it will have no STP topology.
If it has no STP topology, then that means we will have loops.
We're not running spanning tree and there's no way for me to share the spanning
tree topology
from a different VLAN.
So this more or less limits us to the number of active VLANs.
Sure, I can support 4096.
It doesn't do me a whole lot of good if I can't use it and I definitely can't
use
it if I'm not running spanning tree.
This is something to be aware of. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
This is something to be aware of.
It's really not something we need to be too worried about.
It's very rare that a switch is going to give us a very limiting amount of
spanning tree
protocol instances.
For example, we're probably not going to find a switch that only supports 16 or
something
really low.
But at the same time, be aware that this is one of the consequences of Perviel
and spanning
tree.
Which is again, coming back to this idea that we have a one-to-one relationship
between
our topologies as well as our VLAN count. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
our topologies as well as our VLAN count.
Now the last implication in all of this comes down to the configuration in iOSX
E.
What we'll find is that when I'm configuring priorities, when I'm configuring
costs, even
when I'm configuring timers because it's part of operations, all of these are
going
to have to be configured per VLAN.
There's no way for me to just say, "Hey, I just want to enable a certain timer WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
There's no way for me to just say, "Hey, I just want to enable a certain timer
or a certain
cost on a link without either in some way impacting every VLAN," which some
commands
allow us to do that.
Or otherwise, I have to specify which VLAN I want to change the configuration
for.
This is certainly going to be a takeaway as we get into lab configuration and
looking at
iOSXE configuration, we can expect a lot of our commands to require us to
specify a VLAN
ID. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
ID.