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So what if I want all of the benefits of layer two and the benefits of layer

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three?

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Well, for the most part, traditionally speaking, we'd say, well, tough luck.

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You're going to have to choose one or the other.

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But modern networking gives us an option that might actually achieve this.

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And this is known as an access layer fabric.

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The idea here is that we're going to adopt that fabric concept we talked about

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earlier,

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especially within the context of the data center, but now we're applying it to
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especially within the context of the data center, but now we're applying it to

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our campuses.

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So the concept of a fabric is saying that I've got some devices here.

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And I have an underlay network.

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And remember, we say that the underlay is what I have.

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And so I configure maybe EIGRP routing here.

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This is all layer three.

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Life is good there.

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Like now, I'm going to create an overlay network, usually via tunnel mechanisms
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Like now, I'm going to create an overlay network, usually via tunnel mechanisms

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.

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And this is going to give me the network that I want.

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Furthermore, the fact that I can use VXLAN to get me layer two, even over layer

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three underlays,

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means that I could actually build out a layer two network, whoops, layer two,

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that allows

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me to have devices on all of these switches that are on the same subnet, not

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one over on

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the left, not two on the right.

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And so this is going to be kind of interesting because we're actually getting
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And so this is going to be kind of interesting because we're actually getting

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layer three underlay,

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layer three physical connections, and yet we're getting the benefits of layer

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two.

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So when we consider the pros and cons of this solution, the idea here is that

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we have

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layer three scaling, which is absolutely wonderful.

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We don't have to worry about spanning tree protocol anymore.

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That's a huge benefit of layer three.

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And yet, despite having this as sort of a layer three access layer type of
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And yet, despite having this as sort of a layer three access layer type of

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deployment,

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we get layer two flexibility.

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So I don't need to worry if in the future, or for that matter, if today I need

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devices

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to be able to communicate it layer two, even if they're in different network

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closets.

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Now there are still downsides to this.

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First and foremost, we are going to need layer three switches, and specifically

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layer three

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switches that are capable of running the XLAN.
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switches that are capable of running the XLAN.

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And so these are probably still going to be a little bit pricey, priceier than

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just standard

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layer two switches.

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And furthermore, this does introduce complexity.

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Because we're going to need to understand fabrics, underlays, overlays, VX

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lands.

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It's going to be a little require expertise that today our team might not have.

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But there is an upside to this, which is that oftentimes we'll deploy a network

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controller

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to do a lot of this for us.
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to do a lot of this for us.

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This leads us to a conversation where Cisco has created a pre-packaged solution

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that

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gives us all of this. And this is known as software defined access, oftentimes

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abbreviated

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as SDA. SDA access is going to use a controller that is going to allow us to

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just deploy switches,

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plug them into other switches, and our configurations just get automatically
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plug them into other switches, and our configurations just get automatically

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updated from this controller.

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The controller is known as catalyst center, and it's going to be responsible

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for managing

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this network for us, and it does a lot of it without us having to intervene.

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Now there's still complexity in setting up the controller and setting up the

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environment,

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but it does reduce the overall complexity.

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So how does this directly map to access layers and distribution layers?

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Well it changes the nature of our mapping a little bit.
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Well it changes the nature of our mapping a little bit.

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We are going to put the distribution switches here in the middle.

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And again, these are going to be extending layer three out to our edge.

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So if I have an access closet over here with a stack, access closet over here

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with a switch

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stack, and maybe even a pair of switch decks by the way, and then access closet

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up here

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with a switch deck.

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What I'm going to do is I'm going to build a layer three network out to each

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one of these.
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one of these.

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And so here, this is my layer three underlay.

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So far, this looks just like a layer three access model.

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But the difference is now I'm going to deploy VXLAN over the top, and this is

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going to give

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me my overlay.

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So what this functionally looks like is VXLAN tunnels are going to be built

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among all of

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these other switches.

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We're going to get a full mesh of the XLAN tunnels.
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We're going to get a full mesh of the XLAN tunnels.

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And I believe I've got them all.

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Yeah, I think so.

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So all of these switches have the XLAN tunnels between them, which creates that

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giant cloud,

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where we can say maybe we'll just wrap it all up in there.

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So we can say, you know what, we don't care about what happens inside of this

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cloud.

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Instead, all I know is that my host here and my host up here that are

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connecting in at

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different access closets can all be part of the same VLAN, and they can all be
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different access closets can all be part of the same VLAN, and they can all be

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part of

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the same subnet.

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So this is why we come back to this idea that we get pretty much all the

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benefits of layer

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two and layer three.

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And then we just have to compare it to the downsides of managing a fabric and

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consider

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those and decide if that's maybe where we want to go with our access layer

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design.

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Now we're going to be talking a lot more about Cisco's software defined access

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later on

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in the course, and we're going to have an even better sense at that point of

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all of the

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pros and cons of deploying such a solution.
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pros and cons of deploying such a solution.