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As we move down the stack, we are now going to talk about the distribution

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layer.

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The distribution layer purpose is right there in the name.

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It's to distribute.

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It's to communicate out to the access layer in this case.

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We have a core block and a distribution block.

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And again, even though we like to logically connect these like so, we know that

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there's

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really a pair of switches here and we might fully mesh those.

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But when we look at the access layer and we'll be talking a lot more detail
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But when we look at the access layer and we'll be talking a lot more detail

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about the

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access layer here coming up, there are a lot of switches down here.

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And so the distribution layer needs to make sure that it's communicating with

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all of those

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access layer switches.

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So let's talk about what features are going to be required in order to make

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sure that

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a distribution switch can properly serve in the role of being in the

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distribution block.

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First and foremost, we need to make sure that we have layer three switches in

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this role.
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this role.

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The reason for that is as we described before, there's going to be a dividing

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line here,

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where we have layer three upstream and layer two downstream.

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Now this does assume layer two at the access layer and that is usually what we

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're going

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to see in the real world.

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It's very common for us to have layer two extended out there outside of layer

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three

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access layer deployments as well as fabrics like SD access.

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So we'll stay focused on layer two being pushed out to the access layer and
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So we'll stay focused on layer two being pushed out to the access layer and

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address layer

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three later on here in this course.

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So we understand that we're going to have layer three upstream layer two

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downstream.

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And so we need to make sure that we do support the proper routing protocols.

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Although interestingly, we don't actually have to support much in the way of

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routing.

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As if we think about it, the distribution layer is not sitting here in the

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middle.

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The core is going to have all of the connections going everywhere, which means
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The core is going to have all of the connections going everywhere, which means

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it needs to take

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routes in and send routes down to the distribution layer.

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So at that point, we really only have a single path up from a layer three

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perspective.

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This one path can easily be turned into either a default route or have a

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significant number

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of summarized routes.

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And so this is going to be a topic of conversation as far as how to perform

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routing.

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So the distribution, again, this is for distribution switches now from a
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So the distribution, again, this is for distribution switches now from a

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distribution switch perspective,

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we can be pretty content oftentimes by configuring it as a layer three stub.

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Now this looks different between EIG, European, OSPF.

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Stub from an EIG RP perspective is a node EIG RP.

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Here we go.

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It's a node configuration.

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But from an OSPF perspective, it needs to be an area.

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And so we need to make sure it fits into the routing topology, whether it's EIG
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And so we need to make sure it fits into the routing topology, whether it's EIG

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RP or

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OSPF.

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But yeah, we don't actually need to worry too much about a complex layer three

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configuration

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here.

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We just need to get a routing protocol spun up and be able to integrate with

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the core

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from that perspective.

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So from here, let's go ahead and talk a little bit about port count, which

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should sound familiar.

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Ultimately, port counts going to be a requirement at every layer.

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I know we just talked about it at the core layer, but it is going to matter
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I know we just talked about it at the core layer, but it is going to matter

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here as well.

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We need to make sure that we have enough connections to connect to all of the

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access layer

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switches.

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So not just port count, but also port type.

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We spent enough time talking about that in the last video that we probably get

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a sense

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of that here.

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The difference is that we probably have less bandwidth requirements and less

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throughput

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requirements on distribution switches.

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So we don't have to worry as much about bandwidth or about packets per second,

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although we do

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still need to make sure that it has enough to handle the situation.
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still need to make sure that it has enough to handle the situation.

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But this is relative to our core switching requirements.

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Now next, we need to think about what configurations go on the distribution

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switch.

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And there's going to be quite a few that we need to talk about.

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And this is where with the core switch, our conversation was very short.

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We didn't have to talk about much in the way of configuration because we didn't

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have a

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lot of concerns there.
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lot of concerns there.

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But here with our distribution switch, we're going to need to make sure that we

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configure

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our switch virtual interfaces, which is the fancy acronym for VLAN interfaces.

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These SVIs are going to be the gateways for our VLANs.

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This is a very important role of the distribution layer and the distribution

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switches because

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as our hosts down here, that's a laptop, connects up into the network, it's

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going to need to

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get off of its subnet.
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get off of its subnet.

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It's going to have to find its default gateway and that's going to be an SVI

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here that

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draws a dot on the distribution switch.

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Well, this host might be in VLAN 10 and we might have dozens of VLANs.

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We might have 20 and 30 and 35 and 41 and that's a random list of VLAN IDs.

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But that alone says that we need five different SVIs on the distribution switch

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and again,

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we might have dozens more.

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So we need to make sure that we're supporting those SVIs.
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So we need to make sure that we're supporting those SVIs.

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Now the SVIs must be redundant and the way we establish and achieve redundancy

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on VLAN

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interfaces and specifically on gateways would be via a first hop or done to see

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protocol.

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Now there are many FHRP's out there.

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We're going to be talking about those in this course as well.

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But we typically in a Cisco environment like to think about the hot standby

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router protocol

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or HSRP and then from an industry standard perspective, we might see the
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or HSRP and then from an industry standard perspective, we might see the

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virtual router.

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There we go.

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Virtual router redundancy protocol or VRRP and then we might also want to talk

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about global

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load balancing protocol or GLBP, which is another Cisco protocol.

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Now usually we're talking about one of these ones in the front.

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GLBP doesn't get used a whole lot these days.

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But regardless of which one we're talking about, we do need to support the FHRP

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's.

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Furthermore, we might have a host down here that's in VLAN 20 and we don't want
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Furthermore, we might have a host down here that's in VLAN 20 and we don't want

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those

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hosts to be able to communicate.

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Well talking about security and the ability for us to control who can talk to

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whom, this

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is the concept of access control.

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Now this can get very difficult to manage at a global scale.

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But within the context of a distribution block, we might actually be able to

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wrap our

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arms around it.

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And the way we do that is by deploying access control lists to our SVIs.
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And the way we do that is by deploying access control lists to our SVIs.

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And so this is going to give us security enforcement at the distribution level.

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Now this doesn't help us to prevent communication between let's say two hosts

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on VLAN 10.

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Because if we're staying on VLAN 10, we're not passing through an SVI and

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therefore the

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access control list never actually gets to block the traffic.

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But it can help us with VLAN to VLAN access control and that's something we've
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But it can help us with VLAN to VLAN access control and that's something we've

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been doing

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for quite a while on our networks.

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Now another important configuration is going to involve spanning tree protocol.

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Again, for better or worse, we're running layer two downstream.

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We are going to need STP involved.

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Now we should always be running some form of rapid STP these days.

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That's either going to be Cisco's flavor, rapid PVST or technically rapid PVST

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plus or

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multi instant spanning tree or multiple spanning tree.
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multi instant spanning tree or multiple spanning tree.

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That's what we call these days, which is MST.

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So we're going to want to make sure we have the proper STP version running, but

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we also

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need to consider STP roots and topologies.

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We want to make sure that we are maximizing the efficiency of our links.

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And because we have two distributions, which is we're going to want to make

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sure that we split

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the root configurations between distribution switch one and distribution switch
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the root configurations between distribution switch one and distribution switch

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two and

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establish at least a pair of topologies in the environment, which again

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increases the

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efficiency of what we want to do here.

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Now lastly, there is one more configuration that I wanted to point out and this

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actually

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has to do with our routing protocols up here.

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So we mentioned that we might be running a stub environment.

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So if we're running a stub environment, one of the best things we can do is to
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So if we're running a stub environment, one of the best things we can do is to

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perform route

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summarization.

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The idea of route summarization, if I can squeeze it in there, there we go.

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Route summarization is saying that I want to take a bunch of subnets like 10.0.

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0.0/24

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and then we'll just list off a few others here, dot 1.0, dot 2.0, dot 3.0.

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And so if all of these are slash 24's, then we could combine this all into a
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And so if all of these are slash 24's, then we could combine this all into a

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single route

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that is 10.0.0, 10.0, dot 0, there we go, slash 22.

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This summarized route is basically saying, hey, rather than advertising for

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routes, I will

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advertise a single route.

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Now how big of a difference this may because dependent on the size of the

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network.

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If we have dozens of distribution blocks and each distribution block has dozens
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If we have dozens of distribution blocks and each distribution block has dozens

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of SVIs,

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each one of these routes or subnets would belong to an individual local VLAN

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for example.

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Well, at that point we could make a massive difference in the routing table

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size on the

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core.

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We want to take the routing table and reduce it as much as possible because

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that will help

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it perform that fast layer 3 switching.

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So this is a way the distribution block can help the core block get the job
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So this is a way the distribution block can help the core block get the job

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done.

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If it has fewer routes to sift through, when it's performing its routing table

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lookups,

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then it will be a much faster switch.

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So route summarization is a very important part of distribution blocks.

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Now if our routes are very pretty like this where they go in a straight order

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and we can

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easily summarize them, then that's going to make for a better conversation than

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if we

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have routes that are all over subnets that are kind of all over the place.
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have routes that are all over subnets that are kind of all over the place.

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And so when it comes to good network design, we might want to make sure that as

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much as

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possible, we are performing the distribution of subnets in such a way that they

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can be

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summarized upstream to the network core.