Anytime we're talking about a campus as it relates to the network, we're
talking about
a site where we have many users trying to access the network.
Sometimes this can involve a single building, but oftentimes it involves many
buildings.
And so certainly we might think about a university type of campus, but Cisco
and other large
organizations also have campuses around the world where they have many
different buildings
all within a single cluster of one another. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
all within a single cluster of one another.
But the reality is that this actually could be a single site as well.
Because from a network design perspective, we actually don't change the design
that much
if at all.
And so let's go ahead and dive into this.
What we're going to find is that the campus is where we deploy the concept of
the core
distribution and access layers.
Inside of the context of the larger picture, the hierarchical network model, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Inside of the context of the larger picture, the hierarchical network model,
this core
distribution and access model is usually what we find is being taught and what
we're trying
to deploy when it comes to our network designs.
But we do have to understand that this is just part of the overall hierarchical
network
model.
Because we recall that we might have a core distribution and access set of
blocks.
But meanwhile we have other blocks that are going to tie in through the core.
So that's fine to have the big picture approach, but we do need to understand WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So that's fine to have the big picture approach, but we do need to understand
what's happening
here inside of the core distribution and access layers.
So let's look at this.
First of all, when we are wanting to connect our users in, they're going to be
at the bottom
of this diagram.
And they're going to be coming in throughout the entire site or many different
sites
or it could be dozens of users.
There could be thousands of users.
It just depends on the size of the location as well as the size of the network.
And so they're going to be coming in to the access layer. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so they're going to be coming in to the access layer.
Now this access layer is going to consist of usually many different access clos
ets.
This is where we start to bridge the gap between the logical world and the
physical world.
We understand that logically I want a single access block or single access
layer.
But that doesn't always match up to what we physically have.
We physically have closets and we have physically have so many fiber
connections.
And so we're going to draw this out and recognize that we have lots of WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so we're going to draw this out and recognize that we have lots of
different access closets.
And within each access closet, we're going to have some number of access
switches.
And so again, this is where we sort of drill into the details where the
hierarchical network
model up here just represents the entire access layer as a single block.
We know that there's a whole lot more going on inside of that block.
Number one, we've got all these closets that we just drew out.
But within a closet, we might have a bunch of access switches that are WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But within a closet, we might have a bunch of access switches that are
connected to one
another.
Maybe we have them full meshed.
Maybe we have a loop.
And ultimately, this is where our users are going to connect in.
So they connect into these access switches.
And for that matter, we might also see that we have network devices like access
points
plugging in here as well.
Not to mention IP phones, as another network device that we'll connect in at
the access layer. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the access layer.
Now at the next level, this is where we're going to have our distribution block
up here.
But our distribution block, like many of our blocks, is going to consist of
multiple
switches.
In our case, we are typically going to connect a pair of switches together.
And these two switches are responsible for connecting to all of these access
switches.
Now again, we look at the model and we see that there's a single connection
here.
That's just to show which block is connecting to which block.
Physically, this is going to look a lot different. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Physically, this is going to look a lot different.
We are especially going to expect that we would have redundant connections from
each access
closet up to each distribution switch.
This is certainly in an ideal world, but there again, it might be a problem
with trying to
map our logical ideal world to what is actually reality.
For example, we know that we have physical limitations.
We might have fiber connections going to all of these access closets.
But what if we don't actually have fiber over here? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But what if we don't actually have fiber over here?
We know that we can also connect via copper, but copper has a limitation of 100
meters.
And maybe this access closet is 150 meters away from the distribution closet.
So what do we do?
Well, at this point, we might connect this access closet into the access closet
that is
nearest it.
Maybe it's 80 meters away.
And so we run copper connections from one access closet to another. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so we run copper connections from one access closet to another.
Does this defy the network model by connecting an access block to an access
block?
And kind of yes, it does.
But this is the reality that we have to deal with.
Where we actually do have a situation where the real world doesn't perfectly
align with
what we see in our textbooks.
This is one of the biggest parts of network design that can drive a lot of
engineers to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
engineers to
really dislike it because we have to find solutions to problems that arise
where it doesn't
perfectly fit what we're trying to do.
We have to stay flexible with our network designs.
We have to look at a situation like this and recognize that it might cost tens
of thousands
of dollars to pull fiber to that one network closet.
And yes, that would be a more appropriate design.
But from a cost perspective, it would make more sense for this organization to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But from a cost perspective, it would make more sense for this organization to
hang that
access closet off of another access closet.
Now is that going to be the case in every situation?
Is there never a situation where it makes sense to spend tens of thousands of
dollars to
connect that access closet?
I mean, no, this isn't going to be true everywhere.
That access closet might represent users such as the executive team.
Maybe the executive team is in their own suite on their own floor.
And we know that they might actually have problems connecting from one access WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And we know that they might actually have problems connecting from one access
closet to the other.
So we justify the cost of pulling fiber to that access closet.
Or we might find that that's a gymnasium in a high school.
And there's one switch that's going to connect to one PC, maybe a few PCs out
there for
the gym teachers.
And that's perfectly fine for them to hop through another access closet.
So this is where we do have to stay flexible in balancing what we are trying to
do with
the model versus what we have available to us again from a real world WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the model versus what we have available to us again from a real world
perspective.
Now lastly in all of this, we have the network core.
And the network core is another block that's going to be represented by a pair
of switches.
Now the core is going to be primarily operating at layer three.
Layer three is going to provide us with a whole lot more scale than layer two.
And there's very rarely a time when we need layer two connectivity from one
site through
the core to another site.
This just doesn't usually happen. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
This just doesn't usually happen.
And if it does happen that requirement exists, we're going to have to have some
conversations
about how to provide that.
But in most situations, it's not going to be a major concern.
Because the core is operating at layer three, we're going to connect in to all
of these
switches at layer three.
And we might find that we have a full mesh here.
We might find though we have a partial mesh, we're probably every none at
connections between
the core switches.
But all of this is operating at layer three, which means we actually have a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But all of this is operating at layer three, which means we actually have a
dividing line
through the distribution layer where we have layer three services upstream and
we have
layer two services downstream.
Now there is a network design that involves layer three down to the access
layer.
And we will be talking about that in a future skill.
But in most situations, we have this dividing line with layer three being
upstream, layer
two being downstream and do notice that the connections between the
distribution switches, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
distribution switches,
those are going to be at layer two.
So now I have a couple of connections pulled and we port channel them primarily
because
it layer two, we don't want to be blocking one of those ports using spanning
tree.
We want both of them to be active.
And so it's more important to bundle those than it would be to bundle the
connections between
these layer three core switches.
We don't get as much benefit out of performing port channels at layer three.
Now there are a few things to notice as we bring this conversation back to the
hierarchical WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
hierarchical
model.
First of all, we see that this core block connecting to a distribution block,
connecting to an access
block.
From a model perspective, as we've already pointed out, we just have a single
connection
here.
We don't show redundancy.
We don't show the number of switches that are in each block.
But a block can represent multiple switches.
It doesn't represent a single one in pretty much any case.
We almost always at least want to redundancy.
And here at the access layer, we might have dozens and dozens of switches WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And here at the access layer, we might have dozens and dozens of switches
depending on the
size of our network.
So many switches, usually at least two.
Furthermore, the connections here are usually going to be redundant.
Now sometimes they're going to be a full mesh.
Sometimes they're going to be a partial mesh.
But either way, we want some amount of redundancy between these blocks.
And so where I might draw my core and distribution block like this, we know WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so where I might draw my core and distribution block like this, we know
that physically it
might look like we have over here.
Where we have a full mesh of connectivity between a pair of switches in the
core block and
a pair of switches in the distribution block.
Furthermore, as we think about the scalability of this model, now we can answer
the question
of what happens if we need a second site.
We mentioned at the very start, a campus might have multiple buildings.
And so what happens if I have this building here where I maybe have the access WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so what happens if I have this building here where I maybe have the access
and distribution
and even the core block all inside my headquarters building.
But then I have this other site over here.
I do it with this second site.
And the inclination might be to believe that well, we have core distribution
and access
over here as well.
Except that thinking back to the network model, we only have a single core.
We can have multiple distribution blocks, but there's not any value or at least
there's WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
there's
potentially downside to trying to create multiple cores.
We want a single core as part of our building block design.
So instead, in this case, what we do is we have all of our access switches and
they connect
back to a distribution block or a distribution pair of switches here inside of
site 2.
And then we connect this distribution block into the core in the same way that
we connect
these two blocks even though they're inside the same physical site. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
these two blocks even though they're inside the same physical site.
This is where we can start to think about things from a logical versus physical
perspective.
Yeah, physical is going to require us to do some things that we might not want
to do like
what we talked about over here between the access closets.
But when it comes to logical connections, we are trying to represent the
hierarchical
network model in any way we can.
So if I have this third site over here with the distribution access layers,
then I can
connect that distribution into the core as well. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
connect that distribution into the core as well.
Now what happens if I don't have fiber between the headquarters and site 3.
Well, then I might have to get creative and connect a distribution block to a
distribution
block.
And I might have to make an exception to the rule that the hierarchical model
is trying
to present.
But ideally in a perfect world, I would connect this distribution block to this
core block
over at the headquarters site.
We would keep all of these switches to dedicated purposes, either your
distribution switch or WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
distribution switch or
a core switch or an access switch.
And this is where the closer we can stick to the model, the more scalability we
're going
to get.
And as we see here, I could have any number of sites here at this campus and
all of the
distribution blocks could connect back to the core block and my overall network
model
wouldn't change.