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So let's return one more time to the hierarchical model.

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The goal of the hierarchical model is to allow me to expand to as many

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different network

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locations as needed with the core of the network here in the middle.

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But the thing about it, as we've learned, is that we might actually have more

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than one

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switch in each network block.

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And ideally, we're always going to have at least two switches just for the sake

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of redundancy.

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So if I want to connect to a particular building, maybe a site here on my
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So if I want to connect to a particular building, maybe a site here on my

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campus, I would connect

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into a distribution block.

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I'll put the D here, and then I'm going to have a pair of switches, and we'll

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find that

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even though I might just from an architectural perspective, draw a single line

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and say,

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"Hey, the core is going to connect to the distribution."

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We know that it's going to have a full mesh, or for that matter, maybe just has

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a couple

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of connections like this, where we have two fiber connections between the core

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and distribution.
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and distribution.

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So we take advantage of that, we do what we can here.

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So we connect our course switches together, we connect our distributions

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switches together,

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then we have the access layer down here, and this could consist of any number

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of access

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switches.

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And maybe they all have dedicated connections up to the distribution, or maybe

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we're going

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to have to hang some switches off of other switches.

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We'll actually be talking a lot about access layer design coming up here as we

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progress

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through the network design conversation.
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through the network design conversation.

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But while this can represent the campus architecture, one that a lot of us have

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probably

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seen a lot of information on, it's pretty common to study core and distribution

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and access

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layers, well, now we've expanded our horizons and realized that we have a lot

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of other network

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blocks out there.

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For example, it might just be something as simple as saying I've got another

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distribution

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block out here.

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So I've got another pair of distribution switches, I'm going to connect those

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up to the network
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up to the network

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core in some fashion, I've got another access block down here, and now I've got

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connectivity

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out to this building, which is itself its own little campus environment.

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But from here, we've got all kinds of other network locations to be aware of as

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well.

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For example, we might have a WAN block, wide area networking, and this WAN

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block is probably

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going to have a couple of routers in it.

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So these routers can connect back to the core as well, and maybe we do full
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So these routers can connect back to the core as well, and maybe we do full

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mesh these, however

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we want as fine as all layer three connectivity to the core after all.

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You can have as many connections as we want and not have to worry about

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spanning tree blocked

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ports.

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But the WAN block is going to allow us to connect out to remote sites that must

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connect via

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wide area network technologies.

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In the case of an overall campus, this distribution block that we drew here is

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probably connecting

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via some form of fiber, probably single mode fiber to extend the distance that
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via some form of fiber, probably single mode fiber to extend the distance that

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we need to

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in order to reach the network core.

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But for many of our sites, they'll have to come in via MPLS or some other

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technology like

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that.

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And we're going to bring those connections into these routers and forward them

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onto the

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core.

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And this will allow my users out here at the remote sites to be able to connect

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into the

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rest of the network.

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But it doesn't stop there either.

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We might find that we have an internet block.
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We might find that we have an internet block.

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And the internet block is where things start to get a little bit more complex

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as we need

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redundant internet designs.

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If I just have a single internet circuit coming in, I probably have a firewall

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and that firewall

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might be redundant.

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But ultimately, I'm just going to pass that off to the network core.

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But what if I need redundant inbound and outbound internet circuits, where I

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have multiple

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ISPs that are coming into my organization?

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Well, at this point, I might need a pair of routers running at the edge.
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Well, at this point, I might need a pair of routers running at the edge.

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I might even be running BGP to share routes with my ISPs.

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Maybe there are single connections into each router.

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Maybe we do have redundancy in there in some fashion.

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But ultimately, these routers are going to come in.

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And well, in a lot of situations, they'll actually run through firewalls still.

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So, my eyes will get those on the drawing.

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So maybe we have redundant firewalls here.

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And so now we can bring those connections into the network core.
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And so now we can bring those connections into the network core.

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But for larger organizations that have these more complex internet needs, hey,

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this internet

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block is going to be a really good idea.

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And for that matter, then at the end, we can still talk about our data center

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infrastructures.

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And we might find that one of our data centers is running the traditional

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aggregation and

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access model at which point our aggregation switches are just going to connect

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into our

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core switches, probably redundantly at layer three.
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core switches, probably redundantly at layer three.

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Redundancy matters even more inside the data center after all.

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Or we might find that we have a data center fabric.

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And that data center fabric is going to run spine and leaf architectures.

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And this will look a little bit different.

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As our core switches actually connect in via the leaf switches in this case.

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And that's because of how the fabric comes together.

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And so either way, whether we're running a fabric or a traditional aggregation

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access

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model, we're going to find that our data centers are going to connect in
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model, we're going to find that our data centers are going to connect in

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through the core.

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And on top of all of that, we might have even more distribution blocks or we

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might find

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that we need additional wind blocks for scaling.

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And regardless of which blocks come next, ultimately, I can still connect that

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into the network

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core there in the middle.

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And I can continue to scale my network out to greater and greater sizes.

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It's all thanks to mapping back to this hierarchical network model.
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It's all thanks to mapping back to this hierarchical network model.