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Anytime we're talking about a campus as it relates to the network, we're

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talking about

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a site where we have many users trying to access the network.

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Sometimes this can involve a single building, but oftentimes it involves many

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buildings.

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And so certainly we might think about a university type of campus, but Cisco

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and other large

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organizations also have campuses around the world where they have many

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different buildings

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all within a single cluster of one another.
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all within a single cluster of one another.

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But the reality is that this actually could be a single site as well.

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Because from a network design perspective, we actually don't change the design

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that much

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if at all.

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And so let's go ahead and dive into this.

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What we're going to find is that the campus is where we deploy the concept of

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the core

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distribution and access layers.

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Inside of the context of the larger picture, the hierarchical network model,
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Inside of the context of the larger picture, the hierarchical network model,

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this core

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distribution and access model is usually what we find is being taught and what

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we're trying

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to deploy when it comes to our network designs.

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But we do have to understand that this is just part of the overall hierarchical

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network

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model.

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Because we recall that we might have a core distribution and access set of

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blocks.

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But meanwhile we have other blocks that are going to tie in through the core.

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So that's fine to have the big picture approach, but we do need to understand
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So that's fine to have the big picture approach, but we do need to understand

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what's happening

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here inside of the core distribution and access layers.

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So let's look at this.

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First of all, when we are wanting to connect our users in, they're going to be

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at the bottom

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of this diagram.

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And they're going to be coming in throughout the entire site or many different

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sites

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or it could be dozens of users.

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There could be thousands of users.

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It just depends on the size of the location as well as the size of the network.

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And so they're going to be coming in to the access layer.
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And so they're going to be coming in to the access layer.

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Now this access layer is going to consist of usually many different access clos

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ets.

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This is where we start to bridge the gap between the logical world and the

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physical world.

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We understand that logically I want a single access block or single access

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layer.

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But that doesn't always match up to what we physically have.

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We physically have closets and we have physically have so many fiber

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connections.

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And so we're going to draw this out and recognize that we have lots of
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And so we're going to draw this out and recognize that we have lots of

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different access closets.

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And within each access closet, we're going to have some number of access

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switches.

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And so again, this is where we sort of drill into the details where the

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hierarchical network

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model up here just represents the entire access layer as a single block.

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We know that there's a whole lot more going on inside of that block.

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Number one, we've got all these closets that we just drew out.

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But within a closet, we might have a bunch of access switches that are
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But within a closet, we might have a bunch of access switches that are

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connected to one

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another.

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Maybe we have them full meshed.

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Maybe we have a loop.

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And ultimately, this is where our users are going to connect in.

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So they connect into these access switches.

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And for that matter, we might also see that we have network devices like access

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points

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plugging in here as well.

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Not to mention IP phones, as another network device that we'll connect in at

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the access layer.
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the access layer.

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Now at the next level, this is where we're going to have our distribution block

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up here.

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But our distribution block, like many of our blocks, is going to consist of

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multiple

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switches.

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In our case, we are typically going to connect a pair of switches together.

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And these two switches are responsible for connecting to all of these access

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switches.

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Now again, we look at the model and we see that there's a single connection

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here.

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That's just to show which block is connecting to which block.

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Physically, this is going to look a lot different.
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Physically, this is going to look a lot different.

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We are especially going to expect that we would have redundant connections from

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each access

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closet up to each distribution switch.

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This is certainly in an ideal world, but there again, it might be a problem

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with trying to

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map our logical ideal world to what is actually reality.

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For example, we know that we have physical limitations.

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We might have fiber connections going to all of these access closets.

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But what if we don't actually have fiber over here?
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But what if we don't actually have fiber over here?

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We know that we can also connect via copper, but copper has a limitation of 100

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meters.

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And maybe this access closet is 150 meters away from the distribution closet.

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So what do we do?

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Well, at this point, we might connect this access closet into the access closet

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that is

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nearest it.

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Maybe it's 80 meters away.

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And so we run copper connections from one access closet to another.
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And so we run copper connections from one access closet to another.

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Does this defy the network model by connecting an access block to an access

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block?

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And kind of yes, it does.

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But this is the reality that we have to deal with.

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Where we actually do have a situation where the real world doesn't perfectly

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align with

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what we see in our textbooks.

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This is one of the biggest parts of network design that can drive a lot of

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engineers to
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engineers to

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really dislike it because we have to find solutions to problems that arise

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where it doesn't

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perfectly fit what we're trying to do.

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We have to stay flexible with our network designs.

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We have to look at a situation like this and recognize that it might cost tens

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of thousands

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of dollars to pull fiber to that one network closet.

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And yes, that would be a more appropriate design.

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But from a cost perspective, it would make more sense for this organization to
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But from a cost perspective, it would make more sense for this organization to

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hang that

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access closet off of another access closet.

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Now is that going to be the case in every situation?

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Is there never a situation where it makes sense to spend tens of thousands of

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dollars to

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connect that access closet?

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I mean, no, this isn't going to be true everywhere.

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That access closet might represent users such as the executive team.

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Maybe the executive team is in their own suite on their own floor.

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And we know that they might actually have problems connecting from one access
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And we know that they might actually have problems connecting from one access

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closet to the other.

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So we justify the cost of pulling fiber to that access closet.

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Or we might find that that's a gymnasium in a high school.

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And there's one switch that's going to connect to one PC, maybe a few PCs out

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there for

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the gym teachers.

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And that's perfectly fine for them to hop through another access closet.

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So this is where we do have to stay flexible in balancing what we are trying to

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do with

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the model versus what we have available to us again from a real world
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the model versus what we have available to us again from a real world

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perspective.

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Now lastly in all of this, we have the network core.

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And the network core is another block that's going to be represented by a pair

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of switches.

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Now the core is going to be primarily operating at layer three.

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Layer three is going to provide us with a whole lot more scale than layer two.

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And there's very rarely a time when we need layer two connectivity from one

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site through

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the core to another site.

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This just doesn't usually happen.
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This just doesn't usually happen.

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And if it does happen that requirement exists, we're going to have to have some

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conversations

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about how to provide that.

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But in most situations, it's not going to be a major concern.

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Because the core is operating at layer three, we're going to connect in to all

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of these

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switches at layer three.

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And we might find that we have a full mesh here.

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We might find though we have a partial mesh, we're probably every none at

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connections between

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the core switches.

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But all of this is operating at layer three, which means we actually have a
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But all of this is operating at layer three, which means we actually have a

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dividing line

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through the distribution layer where we have layer three services upstream and

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we have

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layer two services downstream.

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Now there is a network design that involves layer three down to the access

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layer.

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And we will be talking about that in a future skill.

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But in most situations, we have this dividing line with layer three being

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upstream, layer

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two being downstream and do notice that the connections between the

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distribution switches,
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distribution switches,

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those are going to be at layer two.

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So now I have a couple of connections pulled and we port channel them primarily

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because

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it layer two, we don't want to be blocking one of those ports using spanning

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tree.

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We want both of them to be active.

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And so it's more important to bundle those than it would be to bundle the

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connections between

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these layer three core switches.

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We don't get as much benefit out of performing port channels at layer three.

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Now there are a few things to notice as we bring this conversation back to the

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hierarchical
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hierarchical

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model.

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First of all, we see that this core block connecting to a distribution block,

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connecting to an access

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block.

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From a model perspective, as we've already pointed out, we just have a single

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connection

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here.

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We don't show redundancy.

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We don't show the number of switches that are in each block.

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But a block can represent multiple switches.

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It doesn't represent a single one in pretty much any case.

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We almost always at least want to redundancy.

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And here at the access layer, we might have dozens and dozens of switches
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And here at the access layer, we might have dozens and dozens of switches

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depending on the

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size of our network.

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So many switches, usually at least two.

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Furthermore, the connections here are usually going to be redundant.

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Now sometimes they're going to be a full mesh.

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Sometimes they're going to be a partial mesh.

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But either way, we want some amount of redundancy between these blocks.

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And so where I might draw my core and distribution block like this, we know
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And so where I might draw my core and distribution block like this, we know

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that physically it

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might look like we have over here.

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Where we have a full mesh of connectivity between a pair of switches in the

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core block and

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a pair of switches in the distribution block.

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Furthermore, as we think about the scalability of this model, now we can answer

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the question

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of what happens if we need a second site.

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We mentioned at the very start, a campus might have multiple buildings.

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And so what happens if I have this building here where I maybe have the access
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And so what happens if I have this building here where I maybe have the access

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and distribution

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and even the core block all inside my headquarters building.

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But then I have this other site over here.

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I do it with this second site.

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And the inclination might be to believe that well, we have core distribution

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and access

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over here as well.

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Except that thinking back to the network model, we only have a single core.

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We can have multiple distribution blocks, but there's not any value or at least

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there's
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there's

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potentially downside to trying to create multiple cores.

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We want a single core as part of our building block design.

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So instead, in this case, what we do is we have all of our access switches and

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they connect

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back to a distribution block or a distribution pair of switches here inside of

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site 2.

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And then we connect this distribution block into the core in the same way that

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we connect

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these two blocks even though they're inside the same physical site.
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these two blocks even though they're inside the same physical site.

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This is where we can start to think about things from a logical versus physical

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perspective.

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Yeah, physical is going to require us to do some things that we might not want

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to do like

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what we talked about over here between the access closets.

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But when it comes to logical connections, we are trying to represent the

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hierarchical

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network model in any way we can.

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So if I have this third site over here with the distribution access layers,

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then I can

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connect that distribution into the core as well.
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connect that distribution into the core as well.

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Now what happens if I don't have fiber between the headquarters and site 3.

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Well, then I might have to get creative and connect a distribution block to a

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distribution

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block.

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And I might have to make an exception to the rule that the hierarchical model

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is trying

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to present.

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But ideally in a perfect world, I would connect this distribution block to this

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core block

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over at the headquarters site.

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We would keep all of these switches to dedicated purposes, either your

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distribution switch or
WEBVTT
X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000

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distribution switch or

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a core switch or an access switch.

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And this is where the closer we can stick to the model, the more scalability we

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're going

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to get.

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And as we see here, I could have any number of sites here at this campus and

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all of the

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distribution blocks could connect back to the core block and my overall network

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model

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wouldn't change.