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A more recent trend in networking is to establish the concept of a fabric

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rather than a network.

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What exactly is this concept of a fabric?

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Well, typically when we're talking about a fabric, we are talking about

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creating some form

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of overlay network, which runs on top of an underlay network.

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The mechanism that we're going to use for this is ultimately going to be by

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building tunnels.
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building tunnels.

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And the way I often describe this is to say that the tunnels are going to form

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the network

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that we want, whereas the underlay is the network that we have.

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Let's give an example of this.

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And one of the best places to look when we're talking about fabrics is inside

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of the data

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center.

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The data center is one of these network places.

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It showed up on our hierarchical network model as a block that connects into

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the core.

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Now, a data center is intended to host an organization's applications and an
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Now, a data center is intended to host an organization's applications and an

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organization's

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data.

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So typically, we're going to deploy a bunch of servers into the data center.

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We're going to deploy storage arrays into the data center.

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And this is also where we're going to see a significant amount of virtual

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ization.

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We're talking about virtual servers or VMs, which creates virtual networking

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needs.

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And that'll be a topic of conversation later on in the course.
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And that'll be a topic of conversation later on in the course.

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But the data center is a very critical network location.

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We need a very firm network model or network architecture that's going to allow

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us to number

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one, establish uptime.

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We need to stay online in our data center.

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And furthermore, we do need to continue to focus on scale.

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The problem is that a lot of our data center applications and a lot of the

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technology we
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technology we

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deployed into the data center requires layer two networking.

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So if we need layer two networking, that's great except layer two is not really

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known

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for its scaling.

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Or is it known for its uptime spanning tree loops and failures can cause

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broadcast storms

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on our network.

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And so this is a little bit of a conflict of interest between what the data

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center needs

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and what the data center ultimately has.
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and what the data center ultimately has.

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Now a traditional data center architecture is going to look a lot like a campus

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network.

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We're going to have a distribution block that connects into an access block.

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And this is where all of these servers and storage arrays are going to connect

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in.

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Except instead of calling the distribution block by the distribution name in

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order to distinguish

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us from campus networks, we're going to call this an aggregation block.
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us from campus networks, we're going to call this an aggregation block.

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The aggregation block basically is a distribution block, but it's going to have

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data center specific

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features features such as fiber channel over ethernet.

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And virtual port channeling.

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And these are the types of technologies that enable us to deploy all of the,

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again, applications

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and data hosting that's required in order to keep our organization online and

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functional.

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These days, if an organization loses access to its applications or data, the
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These days, if an organization loses access to its applications or data, the

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whole organization

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might as well be down.

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We might as well not even be trying to work because we probably can't take

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customer orders

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and a lot of us can't do our jobs without the applications that allow us to do

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our jobs.

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So this traditional data center model is really problematic because, again, it

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is going

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to, first of all, enable us to have layer two everywhere, which is good, but it

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doesn't

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particularly scale well.
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particularly scale well.

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We have scaling issues thanks to layer two.

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And just thanks to the fact that we only can have a pair of switches here in

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the aggregation

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layer in the same way we can only have a pair of switches in the distribution

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layer.

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If our access switches expand out greatly, which can happen in very large scale

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data centers,

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we could have hundreds of access layer switches.

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We might very quickly outgrow our aggregation layer.

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And then we have to deploy a new aggregation layer.
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And then we have to deploy a new aggregation layer.

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Well, where does that go?

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The new aggregation layer and new access layer.

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So a GG and a CC are going to have to connect through the network core to get

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to the other

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aggregation layer.

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Well, this is layer three and therefore we've just broken our ability to have

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layer two

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stretched between the two different access layers.

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And so this is why traditional data center architectures have been overhauled.

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So what do we do today?
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So what do we do today?

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So what we're doing in the data center is we're deploying the fabric concept.

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And so this is where we're bringing the conversation full circle to come back

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to the concept

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of the fabric.

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The idea here is I'm going to deploy a different physical network connection,

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what we call a leaf

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spine network, which can have multiple spine switches.

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Let's say we have four spine switches and pretty much as many leaf switches as

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we want.

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Certainly there are scalability limits, but we can have dozens and even
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Certainly there are scalability limits, but we can have dozens and even

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hundreds of these

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leaf switches.

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And we connect all of these devices up to one another, but we only connect

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spine switches

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to leaf switches and we only connect, well, leaf switches back to spine

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switches.

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We don't actually connect spine switches to each other or leaf switches to each

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other.

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And furthermore, all of these connections are layer three.

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So this is known as the underlay.
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So this is known as the underlay.

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It's the physical network that again represents what we have.

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What do we have?

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We have a layer three network.

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Well, what do we want?

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But we've said over and over again that we want our data center to allow for

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layer two

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connectivity everywhere.

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Meaning that if I have a host over here, that's dot one on a subnet, then the

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host over here

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that's dot 100 on the same subnet and the same broadcast domain should have
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that's dot 100 on the same subnet and the same broadcast domain should have

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access to

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one another layer two.

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So that's great and all except that's not what I have.

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I have a layer three fabric, which is great from a scaling perspective.

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By the way, this is why we're building it.

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This is for the sake of getting this scaling, but we don't want to compromise

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on the fact

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that we wanted layer two everywhere.

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And so how do we get this?

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Well, this is going to come from, or as well, maybe I should say it's going to

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define what

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we want, what we want is a layer two network.
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we want, what we want is a layer two network.

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So what are we going to do?

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Well, we're going to build an overlay.

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And in the case of a fabric like this, we're going to get our layer to via a

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protocol known

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as VXLAN.

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It's essentially saying we're going to extend our VLAN technology.

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So what VXLAN is going to do is it's going to provide layer two tunnels.
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So what VXLAN is going to do is it's going to provide layer two tunnels.

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And this is going to create the overlay network.

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So what happens is we build VXLAN tunnels among all of these different leaf

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switches.

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And there's lots and lots of these tunnels.

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Keep that in mind.

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But ultimately what we're doing is we're creating the network that we want.

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We're allowing this host on the left to communicate to this host on the right.

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And they have layer two communication, even though the network that we have is

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layer

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three.
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three.

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This is pretty cool.

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And it's a big part of software defined networking, which is a whole trend in

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networking

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these days that ultimately provides us with this same concept, the ability to

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deploy a

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network that we want over a network that we have.

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And oftentimes we're going to use a network controller in order to do that.

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That's a big part of the SDN world because building all of those tunnels and

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being dynamic

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with them can be very cumbersome for you and I to try to manage via the CLI.
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with them can be very cumbersome for you and I to try to manage via the CLI.

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So an SDN environment might say we're going to have a network controller that

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does all

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of this for us.

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But regardless of whether we're doing it manually or via a controller, either

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way this concept

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gives us the fabric that network that we want, even though we have a different

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network

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underneath it.

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Now this type of technology doesn't stop with the data center.

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Cisco has a whole campus architecture known as software defined access.
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Cisco has a whole campus architecture known as software defined access.

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And at the heart of which is their SDN controller known as catalyst center.

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We are going to be talking about SD access because it's within scope of the

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exam and this

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course.

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And it's going to be built on VXLAN but it's going to be deployed not to the

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data center,

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but ultimately to the campus environment.

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It's called SD access because it's delivering this layer to connectivity to our

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access layer

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on the campus side.
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on the campus side.

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And so understanding that fabrics can apply yes within the data center but also

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outside

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the data center is going to be a major part of bringing together our network

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designs.

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But understanding that we have these traditional architectures that allow us to

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deploy access

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layers, aggregation layers, distribution layers, those are going to be

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absolutely fine,

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especially for small and medium networks.

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But as we start to grow our networks to very, very large enterprises, we might

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want to

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consider migrating to some form of fabric.
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consider migrating to some form of fabric.

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