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So this is the format that we're going to use for our quiz questions.

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I'm going to display a question like this on the screen and you're going to

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have the

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opportunity to again pause the video just to make sure that you have time to

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answer.

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And then we'll roll right into the explanation, which is why you need the pause

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button.

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So either way, make sure that you're taking the time to answer these questions

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as we go.

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So first question here.

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And I believe we have seven or eight of these.

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Now I'm trying to remember either way.

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Well, we've got about that range.

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So first question, what description best describes the hierarchical network
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So first question, what description best describes the hierarchical network

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model?

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Take a moment to review these answer choices and give your best answer.

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Now the answer that I would give for this is going to be right here, D, the

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principles

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that provide structure and scale.

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But let's go ahead and go through each one of these because I think that we

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could probably

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convince ourselves that any one of these answers choices is true.

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For example, when we say vendor specific, we might be thinking, okay, is this a
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For example, when we say vendor specific, we might be thinking, okay, is this a

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Cisco set

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of principles or does this apply broadly to everyone?

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And it does apply broadly to everyone.

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This is not just something that Cisco proposes and pushes out there.

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It's something that all network vendors agree on.

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And this is how we should build our networks out regardless of whether we're

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using Cisco

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hardware or not.

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Now next, this one might have caught some people.

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It's a template for scaling to very large networks.

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It absolutely scales very large networks, but not exclusively.
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It absolutely scales very large networks, but not exclusively.

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It also scales for very small networks, meaning that I can collapse my layers.

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Remember, we could have a collapsed core and distribution layer.

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In fact, we could have a collapsed core distribution and access layer.

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If indeed we have users coming into the same block and then we connect out to

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other blocks,

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those are the functions of core and access and then maybe distributions in

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there as well.

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So remember, it's not just about getting to very large networks.
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So remember, it's not just about getting to very large networks.

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It can actually scale down as well.

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See a set of rules that should never be broken.

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The phrase allergy here is kind of intentional.

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You should be very careful about question answers that have always or never in

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there.

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It's not that we should plan to break the rules, but again, collapsing core

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distribution

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access together or if we have a physical limitation, if we need to connect a

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distribution switch

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on one building to a distribution switch in another, which then connects to a
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on one building to a distribution switch in another, which then connects to a

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core.

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Is this allowed by the model?

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Technically not.

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It's not meant to allow us to just sort of daisy chain all of these network

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blocks together,

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but at the same time, does the real world require this?

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Yeah, the real world does require this.

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Maybe I need to spend a bunch of money digging fiber to get this distribution

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switch to the

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core switch, whereas this connection here by and large works fine.

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Is it perfect?
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Is it perfect?

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No.

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Is it what we probably have to expect in the real world?

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Yes.

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And then lastly down here, three tier core distribution access.

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A lot of us might have come into this architectural conversation, expecting to

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only talk about

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these three network layers because these are the ones that commonly get cited,

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especially

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when we're first getting introduced into networking.

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But the reality is that, hey, we've got a lot more layers and a lot more

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network blocks

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to talk about now, and so it goes well beyond these three tiers.
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to talk about now, and so it goes well beyond these three tiers.

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So D is the right answer.

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It provides structure and it provides scale, whether that's scaling up or

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scaling down,

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but either way, it gives us a foundational set of principles that allow us to

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build our

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networks in a way that we won't regret down the road.

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We don't want to organically grow our networks in such a way that it creates a

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massive headache

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later when we're trying to do whatever phase comes next.

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So here we have an open-ended question.

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It's what three layers are used in the campus architecture.
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It's what three layers are used in the campus architecture.

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The answer here is probably, well, we kind of just talked about it in the last

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question,

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but it's probably something we're all somewhat familiar with coming into this.

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It's the core distribution and access layer.

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So when we talk about the hierarchical network model, it has a core block in

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the middle, and

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then we attach all of these other blocks to the core block.

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And so we can be very flexible with how we scale out our architecture, but
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And so we can be very flexible with how we scale out our architecture, but

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within a site,

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let's say over here, this is going to be a campus site.

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This campus architecture concept is where we get the core distribution and

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access, otherwise

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known as the three-tier model.

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And so we're going to have a distribution block, which is usually just a pair

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of switches

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that connect up to the core.

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And then we have all of the access switches connecting to those distribution

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switches.

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Ideally, in a perfect world, we have separate hardware for each one of these
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Ideally, in a perfect world, we have separate hardware for each one of these

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layers, the core,

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the distribution, and the access.

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But sometimes we do have to collapse those layers as we've talked about and

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even mentioned

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in the last quiz question.

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Now something to keep in mind in all of this is what happens if I have another

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site over

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here.

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So this would be another campus site.

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It might be all one campus to be clear.

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I should probably call this a site over here as well.

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We might have one campus with many different buildings, and each building is a

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distribution,

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type of switch, and we might find that we have to hang some buildings off of
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type of switch, and we might find that we have to hang some buildings off of

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other buildings.

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But ideally, we do go through some kind of campus core.

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And at this point, we see that this three-tier model still applies, but notice

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that the

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core is shared.

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So my three-tier model over here exists, and my three-tier model exists right

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here.

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And again, that core block is going to be the same core block for both of those

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sites.

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So just some things to keep in mind as we evolve our conversation beyond the

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three-tier architecture
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three-tier architecture

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or the three-tier model and move more to that higher level global view of what

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a network

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should look like using the hierarchical network model.

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So now we're going to talk about a fun concept and something that's going to

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come up absolutely

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later on in this course when we get to Cisco Software Defined Access and Cat

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alyst Center.

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But either way, we're going to be talking about access fabrics.

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And that is a technology that is, in my opinion, very exciting and a lot of fun
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And that is a technology that is, in my opinion, very exciting and a lot of fun

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.

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Either way, what is the value of a fabric architecture when compared to a

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classic design?

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Let's look forward to separate benefits that we can use to justify the

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deployment of a

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campus fabric architecture.

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So generally speaking, what we're going to be looking at here is the ability to

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remove

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the need for spanning tree protocol and the fact that it scales out to larger

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network

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sizes.

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Unfortunately, the cost of a campus fabric requires layer three switches and
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Unfortunately, the cost of a campus fabric requires layer three switches and

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there, well, I should

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say the architecture requires layer three switches and therefore the cost is

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going to be

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more.

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So we can actually expect it to be a more expensive solution.

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It also doesn't reduce complexity.

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I would love for it to be a less complex model that gives us all the other

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benefits.

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But the reality is that deploying standard spanning tree-based switches, that's

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pretty simple

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even though we don't like spanning tree.

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And it is far more complex to run routing protocols and VXLAN.
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And it is far more complex to run routing protocols and VXLAN.

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And then lastly, eliminating the distribution block, this is false.

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We actually do still use the distribution block from a physical perspective.

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It's just not going to run the same protocols that it was using or that it was

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running in

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a standard environment.

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So just as a quick reminder of what exactly a fabric architecture is, the idea

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here is

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that we're going to take our classic campus architecture where we have a

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distribution

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and access layer like this and we have to decide between layer two or possibly
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and access layer like this and we have to decide between layer two or possibly

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deploying

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layer three here.

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Usually we deploy layer two and this is where we need spanning tree protocol

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and all of

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these things.

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So this is to be our traditional architecture.

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But now instead we're going to create a fabric that sort of creates this cloud

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that carries

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our layer two traffic across layer three.

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So now we could draw our access layer switches more in a circle.
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So now we could draw our access layer switches more in a circle.

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At least that's how I like to do it.

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And then you've got this kind of cloud concept because they're all connecting

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to an underlay,

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which is ultimately still the distribution switches here in the middle.

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Or distributions, which is do still exist.

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It is layer three out to the edge and we're going to run some kind of routing

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protocol

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in there to handle this.

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And now though we're forming a set of the X land tunnels that carries our layer

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two traffic

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everywhere.

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And so this still allows my laptops here as I draw them out and maybe my
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And so this still allows my laptops here as I draw them out and maybe my

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printers, etc.

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All of these devices can continue to extend layer two to one another as if it

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was this architecture

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that was extending layer two natively.

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So a little bit different and certainly something we're going to be diving into

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in a lot more

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detail later on.

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Again, when we get to software defined access.

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But for now, what we really need to understand is just sort of the value of a
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But for now, what we really need to understand is just sort of the value of a

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fabric architecture

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at the access layer and what exactly goes into making it special when compared

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to the classic

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design.

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So this is question number four.

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And I did just confirm that there are indeed seven questions.

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So just to give us an update, this is sort of the middle question of this group

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.

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But I do encourage taking a moment to pause the video because what we want to

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do here is

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we want to map each one of these configurations or features.

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And ultimately what we might call functions to either the access distribution
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And ultimately what we might call functions to either the access distribution

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or core

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distribution or access block.

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And what this is really asking us to do is make sure that we know where these

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functions

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go because we don't necessarily want our core switch providing POE to end users

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because

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that's not the purpose of the network core.

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So just to give away one of the answers kind of, I guess.

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And either way, go ahead and pause the video and make sure that you can map

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each one of these

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features and functions to one of these blocks.
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features and functions to one of these blocks.

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So as we go through this together, what we want to do is recognize sort of the

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purpose

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of each one of these blocks because that's going to help us remember what

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functions need

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to go aware.

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So access, the most important part about the access block is this is where our

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users connect.

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So anything involving user connectivity should go on the access layer,

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distribution is going

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to be all about servicing the downstream from a layer three perspective and a
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to be all about servicing the downstream from a layer three perspective and a

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layer two

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perspective.

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So we're talking about spanning tree protocol and FHRP's, that's the kind of

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thing we can

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expect on the distribution layer.

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And this can be the tricky one because sometimes it's hard to know exactly

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where a function

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goes.

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So as long as the distribution layer almost feels like it could go into the

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core or access.

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Now the core is focused on switching traffic among all of the different blocks.

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So we've got other distribution blocks out there.
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So we've got other distribution blocks out there.

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We might have a data center aggregation block.

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We might have a WAN block or an internet block.

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And the core is designed to connect us to all of those.

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So now let's go and go through each one of these functions.

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So when we talk about authentication, this is endpoint authentication.

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And again, if it has to do with users, this is going to involve the access

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layer.

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So that's an access layer technology.

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Stub routing.

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So in a traditional environment, the access layer doesn't do layer two.
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So in a traditional environment, the access layer doesn't do layer two.

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And as a result, the routing is going to go up here between the core and the

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distribution.

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So stub routing is used to make our routing table smaller and more efficient.

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And so when we're advertising routes up to the core, we want these routes to be

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simplified.

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And therefore, stub routing is going to be a distribution concept.

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And by the way, so is route summarization, since we're on the topic.

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Actually, I'll write this over here.
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Actually, I'll write this over here.

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So route summarization is also going to be for distribution.

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It allows me to again, reduce the routing table size of my environment.

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QoS tagging has to be at the very edge.

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As user traffic comes in, we want to identify what type of traffic it is and

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tag it.

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So that's going to be access layer.

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FHRP's we've just said go in the distribution layer.

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That's going to be what allows us to route, not route, but make sure that we
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That's going to be what allows us to route, not route, but make sure that we

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have redundancy

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among the gateways, which are going to ride on SVIs.

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And those SVIs therefore reside on the distribution layer.

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So we'll mark that here.

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Access layer enforcement access layer access control, ECL access control list.

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Got to get my terminology right.

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ACL enforcement is going to happen on the SVIs.

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And therefore again, because the SVIs are at the distribution block, that's

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going to happen
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going to happen

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at the distribution layer.

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Full routing is a function of the core.

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Again, we want to simplify our routing operation from the distribution layers

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perspective, but

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the core is responsible for all of this routing and layer three switching.

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And so not only is the core doing full routing, but it's also doing that fast

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layer three switching

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over here on the right.

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So SVIs already addressed POE is providing power to end devices.

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So that belongs to the access layer.

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The ELAN assignment, just like QOS tagging, is going to happen at the edge.
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The ELAN assignment, just like QOS tagging, is going to happen at the edge.

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We're going to apply a VLAN tag based on which port a device comes in on.

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And same thing with the QOS trust boundary, that is an access layer function.

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Meaning that if a packet comes in and it has a QOS tag of let's say maybe DSCP

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EF, that's

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one of the highest, if not the highest priority that we can assign a packet.

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Well the question is, do I trust it?
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Well the question is, do I trust it?

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Because if it's just a PC trying to get prioritized access, we don't want to

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trust it.

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But if it's a phone, an IP phone, trying to say, hey, I'm carrying voice

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traffic, then

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we absolutely want to trust it.

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So we define the trust boundary at the edge.

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And once a packet hits the distribution layer, we shouldn't be asking about

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whether we

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trust it or not.

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We should absolutely be trusting the packets because the access layer already

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performed

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that filtering for us.
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that filtering for us.

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So this is a very large list of functions.

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And if we're able to go through this and identify where all of these functions

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go, that

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means we have a very firm grasp of the core distribution and access

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functionality.

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So sticking with the topic of the network architecture and the hierarchical

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model, we want

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to look at this diagram and identify the functions served by each block.

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Now when we say functions here, I don't mean the functions on the previous

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slide that

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would take a long time.
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would take a long time.

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I just mean hey, is this performing the function of an access block, a

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distribution block,

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or a core block, or this is a little bit of a hint, or is it multiple, hence

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the plurality

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there.

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So anyways, let's go ahead and pause the video and start to assign these block

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functions.

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So anywhere that users are connecting, we can expect that to be an access layer

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functionality.
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functionality.

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This means that these switches here out at the edge are going to be access

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blocks, but

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the users are also connecting up here, which means that we have access layer

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functionality

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here at this site.

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Now anything that's connecting down to the access layer is going to perform

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distribution.

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So we are going to perform distribution functions on these switches.

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So here at site 2, we see that we have a distribution block connecting to an

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access block and over
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access block and over

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here same thing, we have distribution that's going to be needed to connect down

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to the access

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layer.

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Now here's the interesting thing.

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We have two distribution blocks connecting to one another.

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And so is either of these our core block?

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Well if we had just said that, hey, maybe we've got an internet block up here,

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it's coming

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in over here, that would have clarified things.

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But ultimately one of these is going to have to serve as the network core.

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And right now, as it's drawn, we could have picked either one.
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And right now, as it's drawn, we could have picked either one.

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So yes, that means we could have picked site 1 to say that, hey, this is the

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core block,

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which means that when I do have another block coming in, it's going to connect

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over here.

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And that does mean that all three functions exist on one set of switch hardware

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.

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Meanwhile if you had said that the core is going to be over here, that's fine.

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It just means that future blocks have to connect here.

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So a little bit of an open-ended aspect to the core function, but it is

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something for us

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to recognize that we're going to want to identify that function based on future
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to recognize that we're going to want to identify that function based on future

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connectivity.

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All right, question number six.

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And the following benefits describe a layer 2 or layer 3 access layer

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deployment.

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So when we're looking at these benefits, lower cost infrastructure and point

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flexibility,

372
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seamless wireless roaming, this is absolutely the qualities we'd expect out of

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layer 2.

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Layer 2 comes with downsides, especially spanning tree protocol.

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But there's a reason why we see most organizations deploying layer 2.
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But there's a reason why we see most organizations deploying layer 2.

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It does make it so we don't have to deploy layer 3 switches, which is more

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expensive, so it

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creates lower cost.

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It maximizes our endpoint flexibility because now we can ensure that all of our

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devices

381
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can be on different subnet, regardless of if they connect in at different

382
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access layer

383
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closets.

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That requirement doesn't come up a lot these days, but if it does come up, at

385
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least we're

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prepared for it.

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And it doesn't show seamless wireless roaming.
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We've mentioned that wireless laying controllers can enable seamless roaming

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when we have layer

390
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3 access closet design.

391
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But if we don't have wireless laying controllers, then layer 3 access layer

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closets, that's going

393
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to give us a bad time, ultimately, because when I roam from one access point to

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another,

395
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and we're connected to access switches that connect in to a layer 3

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distribution, at

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this point, I'm conducting a layer 3 roam.
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this point, I'm conducting a layer 3 roam.

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And that's going to be very painful because I'm going to have to get a new IP

399
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address,

400
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unless the wireless system has some very fancy backend systems to try to tunnel

401
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traffic

402
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around.

403
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Wireless laying controllers kind of fall into that category.

404
19:14.960 --> 19:19.230
It creates tunnels that allows us to do layer 2 roaming across layer 3

405
19:19.230 --> 19:20.200
boundaries.

406
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But ultimately, if I want to be sure that this is a layer 2 roam, I should go

407
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with layer

408
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2 access layer deployments.

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Layer 3 has the benefit of better scale, and it makes it so that I can
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Layer 3 has the benefit of better scale, and it makes it so that I can

410
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ultimately eliminate

411
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spanning tree.

412
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That's a huge benefit of layer 3, but it does come with some downsides as well,

413
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meaning

414
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that there is no perfect solution to when choosing between layer 2 and layer 3

415
19:47.780 --> 19:48.800
designs.

416
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All right.

417
19:50.960 --> 19:52.800
We have reached the last question.

418
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Question number 7.

419
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What protocol is at the heart of access layer fabrics by enabling layer 2 over

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layer 3?
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layer 3?

421
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Well the answer here is straightforward.

422
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It is the XLAN.

423
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And one of those questions where we either know it or we don't.

424
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But when we look at tunneling protocols, a lot of them are going to carry layer

425
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3 over

426
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layer 3.

427
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I take a layer 2 frame that has an Ethernet header, let's say, and I strip it

428
20:21.990 --> 20:22.520
off and

429
20:22.520 --> 20:27.360
I take this layer 3 packet that remains, and I put another layer 3 header in

430
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front of

431
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it, along with a new layer 2 header, and then we forward it across, and we
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it, along with a new layer 2 header, and then we forward it across, and we

432
20:32.740 --> 20:33.520
strip these

433
20:33.520 --> 20:38.390
off, we extract the original layer 3, but notice we have lost the layer 2

434
20:38.390 --> 20:39.560
information.

435
20:39.560 --> 20:42.040
Well the XLAN is going to keep that.

436
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It's going to keep the layer 2 Ethernet header information, it keeps the entire

437
20:47.560 --> 20:48.200
frame,

438
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and then it puts the encapsulation here, we send it across, and once we get rid

439
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of the encapsulation

440
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by de-incapsulating, we're left with the original frame and its entirety.
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by de-incapsulating, we're left with the original frame and its entirety.

441
21:00.880 --> 21:05.400
So the XLAN enables layer 2 over layer 3, and we're going to see it not only in

442
21:05.400 --> 21:05.720
data

443
21:05.720 --> 21:09.510
center, spine and leaf fabrics, but even in campus architectures when we're

444
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looking at

445
21:10.160 --> 21:15.760
fabric access technology, such as Cisco's software defined access.

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