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Well, we said it at the end of that video that if I have a topology that

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requires me

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to be a little bit more efficient, then I can run Pervieland Spanning Tree and

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I can

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make it more efficient.

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And we said we'd have VLAN 10 here, VLAN 10 will block one of these ports

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because this

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is the root switch on the left, and then I'm going to use this connection and

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this connection,

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whereas VLAN 20 has the root on the right, and therefore we're blocking a port

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over here,
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over here,

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and we're going to use that middle link as well as the link on the right.

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So nothing wrong with this.

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We have two VLANs and we have two Spanning Tree topologies.

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But here's the question.

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How many topologies are needed in order for me to be efficient here?

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In most network topologies where we have larger loops, we're usually talking

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about the distribution

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layer and the access layer.

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And if we think back to the start of this course, when we talked about network

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architecture,
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architecture,

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we use the usually two switches here as a pair of distribution switches, and

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then we

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can have lots of access layer switches.

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We don't scale beyond just a pair of distribution switches in most cases.

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So how many topologies do we need?

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Well, we probably need about as many topologies as we have distribution

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switches.

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If that's two, then that means we need two topologies.

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And it's great that PVST+ in other case gave us for two VLANs, two topologies.
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And it's great that PVST+ in other case gave us for two VLANs, two topologies.

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But you can probably tell where we're going with this.

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What happens when we consider all the other VLANs?

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Now what if I have VLANs 10 and 30 and 50 and 70 on the left in VLANs 20 and 40

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and

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60 and 80?

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This is just eight VLANs, let alone 20 or 50 or 100 VLANs in the environment

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that are

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not.

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Well, at this point, I'm going to have to create topologies for all eight of

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these VLANs,
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these VLANs,

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because PVST+ is always going to be a one-to-one relationship.

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And so if I have eight VLANs, that means I have eight topologies.

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Now are they different topologies?

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No, they're going to be the same topologies.

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I wrote these out for this way for a reason, probably 10, 30, 50 and 70 will

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all share

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the same topology as VLAN 10.

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Same thing over on the right, VLANs 20, 40, 60, 80.

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I basically just said, hey, if it's an even number in the 10's digit or an odd
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I basically just said, hey, if it's an even number in the 10's digit or an odd

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number

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in the 10's digit, then we'll sort it out that way.

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Now different organizations will sort their VLANs in different ways, but

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ideally we end

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up with about the same between these two sides.

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So if we've got four on this side, we've got four on that side.

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If we have 10 on one side, we have 10 on the other and so on and so forth.

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So we've largely arbitrarily picked which VLANs go on which side, but now the

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problem

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is that even though I'm going to share the topology for these three VLANs, I'm
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is that even though I'm going to share the topology for these three VLANs, I'm

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going

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to share the same topology here.

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I still need eight separate topologies.

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So in the end, I'm going to have VLAN 30 which shares the exact same path as V

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LAN 10 and

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then I could draw two other VLANs on here because here's VLAN 50 coming again,

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sharing

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the blue path here and then I could grab maybe purple and say this is VLAN 70

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and we

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see that even though there's no point, we are managing all of these different
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see that even though there's no point, we are managing all of these different

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topologies.

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I say no point because we already have the topology, we just couldn't share it

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and so

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we now have eight different topologies in play.

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I just drew a few of them.

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So what is happening under the hood?

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I mean, why is this such a big deal?

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Well, one of the reasons for this is because when we really look at a topology,

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we're looking

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at a complete spanning tree protocol operation.

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What this means is that we have to maintain the elections if there's new
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What this means is that we have to maintain the elections if there's new

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switches.

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Regardless, we have to figure out who's going to be the root switch.

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We have to identify different port types.

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We say types, we're usually talking about the different roles as well as the

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different

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states.

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So if a port, for example, let's say this port here, we see that for the top

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ology on the

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left here, this is going to be a root port.
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left here, this is going to be a root port.

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And yet I've got to maintain four different root ports for the four different V

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LANs that

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are all going across those different links.

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So we've got to have four different VLANs that are being tracked and we're root

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port

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for all four of those, for example.

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And same thing with all the other roles in the states.

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It's going to be on a per topology basis because again, a topology encompasses

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the entirety

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of STP operation.

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And this is a big one.
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And this is a big one.

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We have to worry about BP-DU propagation, which is the sending of our BP-DUs

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and the

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transmitting of them, forwarding them along, and for that matter, the

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processing.

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Every time I get a BP-DU, I have to process it.

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I have to figure out, hey, is this still the BP-DU I was expecting?

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Is the root ID the same, the cost of the root the same?

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And then I recalculate things based on what I got.

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Furthermore, BP-DUs are sent out every two seconds by default.
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Furthermore, BP-DUs are sent out every two seconds by default.

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So if BP-DU propagation and processing is part of STP operation and STP

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operation is

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going to be fully self-contained within each topology, well, that means that I

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am truly

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sending out eight different BP-DUs every two seconds.

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So we have to be a little bit careful with this in production networks.

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Now, trust me, Cisco switches, they can handle eight of these.

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They can handle eight topologies, no problem whatsoever.
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They can handle eight topologies, no problem whatsoever.

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But different hardware will be able to support different amounts of topologies

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and different

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numbers of VLANs.

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So we look at the spec sheet for a particular hardware, we might find that it

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supports,

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well, maybe it supports all 4096 VLANs.

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This is pretty common.

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Every now and again, you'll find a switch that supports 512 or 64 or something

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like

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that, especially the lower end switches.

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But let's say we have a switch that supports 4096 VLANs.
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But let's say we have a switch that supports 4096 VLANs.

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That same switch might support 1024 STP instances.

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Where an instance is really a topology.

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But it often gets referred to as an instance as well.

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So if we only support 1024, but we need one for every VLAN, what does that mean

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?

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What that means is on the 1,025th VLAN, it will have no STP topology.
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What that means is on the 1,025th VLAN, it will have no STP topology.

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If it has no STP topology, then that means we will have loops.

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We're not running spanning tree and there's no way for me to share the spanning

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tree topology

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from a different VLAN.

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So this more or less limits us to the number of active VLANs.

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Sure, I can support 4096.

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It doesn't do me a whole lot of good if I can't use it and I definitely can't

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use

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it if I'm not running spanning tree.

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This is something to be aware of.
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This is something to be aware of.

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It's really not something we need to be too worried about.

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It's very rare that a switch is going to give us a very limiting amount of

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spanning tree

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protocol instances.

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For example, we're probably not going to find a switch that only supports 16 or

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something

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really low.

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But at the same time, be aware that this is one of the consequences of Perviel

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and spanning

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tree.

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Which is again, coming back to this idea that we have a one-to-one relationship

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between

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our topologies as well as our VLAN count.
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our topologies as well as our VLAN count.

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Now the last implication in all of this comes down to the configuration in iOSX

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E.

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What we'll find is that when I'm configuring priorities, when I'm configuring

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costs, even

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when I'm configuring timers because it's part of operations, all of these are

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going

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to have to be configured per VLAN.

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There's no way for me to just say, "Hey, I just want to enable a certain timer
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There's no way for me to just say, "Hey, I just want to enable a certain timer

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or a certain

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cost on a link without either in some way impacting every VLAN," which some

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commands

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allow us to do that.

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Or otherwise, I have to specify which VLAN I want to change the configuration

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for.

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This is certainly going to be a takeaway as we get into lab configuration and

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looking at

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iOSXE configuration, we can expect a lot of our commands to require us to

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specify a VLAN

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ID.
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ID.