Cisco supports three different modes of spanning tree operation starting with
this legacy PVST+
and now we have seen that we can support rapid PVST+ which takes all the
enhancements of rapid spanning tree and applies them to that
PVST+ architecture which creates a spanning tree topology per VLAN hence per V
LAN spanning tree.
But then there's this third option that we can support on Cisco switches which
is known as MST multiple spanning tree. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
is known as MST multiple spanning tree.
It's time for us to talk about what exactly this is and start to understand
when we might deploy it into our networks.
And this has to do with the fact that Cisco started something that the industry
wanted in on.
If we think about this from an industry perspective, what the industry had was
known as common spanning tree or CST
back when Cisco had per VLAN spanning tree plus and CST just had a single top
ology.
We couldn't leverage all of the blocked links in a CST world. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We couldn't leverage all of the blocked links in a CST world.
And so yeah, Cisco switches just had an advantage back in those days as one of
the reasons why we'd purchase Cisco switches
in addition to maybe some other features and benefits and Cisco did a lot to
enhance spanning tree including the topology concept.
So then when rapid spanning tree came out or RSTP, this was released as 802.1W
as an industry standard and yet it still had a single topology. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
as an industry standard and yet it still had a single topology.
So it was a whole lot faster but rapid PVST+ still had an advantage over rapid
spanning tree protocol.
And so now MST comes around and says, "Okay, we like rapid spanning tree
protocol but we want to have multiple topologies like Cisco has."
And so the industry finally achieved this using MST and believe it or not, they
actually were able to enhance or really make more efficient this multiple top WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
actually were able to enhance or really make more efficient this multiple top
ology concept.
So to understand that, let's talk about the concept of per VLAN spanning tree
or PVST+ and what exactly it's doing.
So recall, again, Cisco had a good thing going with PVST+ and yes, this applies
to rapid PVST+ as well because what we had here was multiple topologies.
But keep in mind what exactly it actually was. It was a topology per VLAN. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But keep in mind what exactly it actually was. It was a topology per VLAN.
And this was huge because if I had a topology like this, a triangle topology,
maybe a couple of distribution layer switches up here, we'd have to block one
of these.
And again, if I only have a single topology, this is the only topology I have.
I'm blocking this link which means it might as well not exist.
Now, if I had expensive one gig or 10 gig connections, then I paid for a bunch
of optics that are just sitting there not being used outside of a failure WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
of optics that are just sitting there not being used outside of a failure
scenario.
Now, the failure scenario was important. We need our redundancy because if this
link on the left goes down, then we fire up this link and I've got full
bandwidth capabilities.
But at the same time, we recognize that it's hard to cost justify maybe 10 gig
optics, especially if they're expensive single mode fiber types of optics in
this situation because it's just sitting there idle.
And so Cisco created the multiple topology concept. So VLAN 10, for example, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so Cisco created the multiple topology concept. So VLAN 10, for example,
could use these two links and block this one over here.
Meanwhile, VLAN 20 could use these two links and block this one over here. Now,
we're no longer blocking entirely one link, rather we're blocking one link per
VLAN, meaning that VLAN 10 has full access to all of the throughput on the left
and VLAN 20 has full throughput on the link on the right there.
Now, we can apply this to many other VLANs as well. So, for example, I could WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now, we can apply this to many other VLANs as well. So, for example, I could
take VLAN 30 and give it the same topology as VLAN 10, where it's writing on
these two links and blocking this link over here.
And likewise, then I could do that to VLAN 40 on the right and that will match
the topology that VLAN 20 has.
But herein lies the problem with per VLAN spanning tree. Each one of these many
different VLANs is getting its own topology.
In this architecture alone, we have four different topologies. And the reason WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
In this architecture alone, we have four different topologies. And the reason
for that is because we have four different VLANs.
Every single VLAN gets its own topology, which naturally means if we have 100 V
LANs, then we're going to have 100 topologies.
Why is this a big deal? Well, we keep in mind what an STP topology is. STP top
ology involves all of the aspects of spanning tree protocol. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
ology involves all of the aspects of spanning tree protocol.
So, we're sending BPDUs, for example, for every single topology. We're
maintaining a list of blocked ports for every single topology.
We have to manage TCNs for every single topology. This starts to get very
overwhelming for switches to manage this many different spanning tree protocol
topologies.
So, here's a question that we should probably try to ask at some point. How
many topologies are really needed in a given network? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
many topologies are really needed in a given network?
Now, as we've said a few times, back when spanning tree protocol was first
developed, it was really developed with more dynamic architectures in mind.
Meaning that we were just sort of, what's the word, organically? We were organ
ically mapping new switches out here.
We didn't really have an architecture in mind. But now we have network
architectures.
We have a pair of distribution switches in most campus environments. And when WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We have a pair of distribution switches in most campus environments. And when
we talk about layer two architectures, we're usually either talking about
campus or data center.
Data centers these days either run a similar architecture or they run leaf and
spine, which is a totally different architecture.
But now in our campus architectures, we're still usually running distribution
and access.
So if these are our largest layer two domains, then this is where spanning tree
protocol is going to need the most topologies.
So here we have a pair of distribution switches with access switches. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So here we have a pair of distribution switches with access switches.
Connect them all up to one another. We see that we've got triangles, a little
bit of everywhere. And so these are all individual loops.
So, let's just look at one individual loop. Let's look at this switch right
here.
What I'm going to find is we're going to split our VLANs between the two
distribution switches. This is pretty common.
So maybe 10, 20, and 30 are rooted here.
On the left switch, meanwhile, VLANs, 15, 25, and 35 are rooted here on the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
On the left switch, meanwhile, VLANs, 15, 25, and 35 are rooted here on the
right switch.
Which means that the right switch being the root and these three different VLAN
s, we're going to see a blocked link maybe over here, whereas the root on the
left is going to block it simply on that link.
So we have two different topologies in mind. And if I scale this out to 50 VLAN
s on the left and 50 VLANs on the right, I still have these exact same two top WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
s on the left and 50 VLANs on the right, I still have these exact same two top
ologies in mind.
And I can apply this to all of the other access layer switches as well.
So the topology doesn't change even if I add more VLANs because we have this
distribution and access architecture already locked in.
Meaning that usually in most scenarios, I need two topologies.
Well, how many do I get?
As many as I have VLANs, and that is the problem with PVST+ WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
As many as I have VLANs, and that is the problem with PVST+
If we actually were to get on to a catalyst 9K, and this is going to vary
depending on the platform, it only supports 128 spanning tree topologies.
And the reason for that is, like we said, each spanning tree topology adds a
whole lot of overhead to a switch.
It has to process and send BP to use, it has to do all of the things involved
with an STP topology.
So once we get past 128 of these topologies, we start to run out of resources. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So once we get past 128 of these topologies, we start to run out of resources.
And so if I enable 129 VLANs, what happens?
Well, one of these VLANs has no topology.
That is a major problem.
If we have no STP topology, we have no blocked boards, which means that we have
loops.
Yes, Cisco switches will allow us to enable more VLANs than we have topologies,
and it will allow us to form loops in that regard.
So this is a major concern in our world, and we have to be very careful about WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So this is a major concern in our world, and we have to be very careful about
deploying per VLAN spanning tree, or rapid PVST+ into our environments, where
we have that many VLANs.
Now, how often does this happen? Not terribly often, but in a very large
extensive layer two network, we might find that we have VLANs sprawl, and this
could become a concern in certain environments.
So at last, we can talk about the value of MST and what they're trying to do WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So at last, we can talk about the value of MST and what they're trying to do
with multiple topologies, again, adding in extra efficiency.
What MST wants to do is to change the relationship. So it's not just a topology
per VLAN. That's what Cisco does, but rather create a specific number of top
ologies being very intentional, and then taking, let's say we create two top
ologies here, and then we create our VLANs, and we add them into the topologies. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
ologies here, and then we create our VLANs, and we add them into the topologies.
We're changing the structure of this. We're no longer saying that every VLAN is
going to have its own topologies and its own BPDs, etc. Rather, we're going to
create individual topologies, and we can have more of these if we desire, but
we've said that in most environments, we only need two topologies.
So maybe we create two, maybe we create three, but either way, we can map our V
LANs into the topologies that we create.
These topologies have a name in the MST world, they are known as instances. We WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
These topologies have a name in the MST world, they are known as instances. We
're creating instances of MST, and again, we're applying VLANs into the
different instances.
So now as we might imagine, let's say in this topology here, we're going to
need same concept here, we're going to need BPDUs and TCNs and blocked ports
per topology, and yet at this point, we only have three.
We have three BPDUs, we have to process, we have three sets of blocked ports
that we need to manage, we have three sets of TCNs we have to manage, and yet WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that we need to manage, we have three sets of TCNs we have to manage, and yet
we could have dozens, if not hundreds of VLANs, and continue to create new VLAN
s
and map them into the existing topologies, and this stays locked into place.
So MST is going to attempt to create what Cisco has from a multiple topology
perspective, while adding efficiency, and therefore making it potentially the
right option, even in Cisco networks, where we don't necessarily have to rely WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
right option, even in Cisco networks, where we don't necessarily have to rely
on industry standards, but in this case, we might want to do so.
So.