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Our conversation about the access layer wouldn't be complete if we didn't talk

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about the

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different topology options that we have available for us here.

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And really, when we talk about options, we're really saying that we've got some

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bad options

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and we've got some better options.

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And the modern network, fortunately, gives us some really solid ways to build

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out access

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layer topologies.

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But in order to understand how the modern network devices help us out here, we

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need to understand

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the sort of the classic access layer topology.
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the sort of the classic access layer topology.

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And we do need to understand that a lot of this is going to tie to how spanning

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tree

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protocol ends up working for us and how it helps us to block our loops, but how

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this can

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actually cause us a little bit of grief when it comes to these topology designs

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.

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So how does it look from a classic perspective?

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Well, when we zero in and lock in on a single access closet here, in fact, let

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's make that

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closet just a little bit bigger.

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We know that this one closet could have many switches.
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We know that this one closet could have many switches.

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Let's just draw it out such that we've got four switches.

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So we have four switches in this access closet.

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Each one is a single rack unit, autonomous switch.

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You have to configure each one.

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And then we have a pair of distribution switches up here that are connected to

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each other.

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So here's the question.

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How do I connect the distribution layer to the access layer switches?
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How do I connect the distribution layer to the access layer switches?

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This is a difficult conversation to have when these four switches are each

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their own fully

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functioning switch because now we have to worry about what the spanning tree

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topology

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looks like as we connect to each one.

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So we had a couple of choices.

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First of all, we could do the head end design.

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Let's draw this out in different colors.

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So we could do the head end type of design, which means that we're going to

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connect our

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distribution to one of the switches called the head end switch.

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And then we would connect all of these switches hanging off of that switch and
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And then we would connect all of these switches hanging off of that switch and

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potentially

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loop this back around as well.

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And what we'll find here is that we're going to spanning tree block one of

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these ports

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to the distribution layer to prevent this loop from happening.

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But then we also have this loop down here and so we're going to need to block

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something

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in here, maybe a link down here.

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And so we have full connectivity, no problem here except the big downside of

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course is what
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course is what

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happens if the head end fails.

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If this one switch goes down, then we have to worry about the fact that the

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whole closet

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lost access to the distribution layer.

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And so this isn't a super ideal topology.

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So instead what we can do is we can do a fall loop.

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The fall loop looks like this.

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Again, drawing in a different color here with the same drawing.

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But we're going to connect into one of these switches from one distribution
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But we're going to connect into one of these switches from one distribution

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switch.

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We're going to connect into a different switch through the other distribution

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and then we're

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going to basically ride the same connections here between the two.

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So it's going to look like that.

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And now we have one big loop.

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And yeah, we're probably blocking that link in there.

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So at this point, we might say, okay, this looks a lot better.

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If this switch goes down, we can still get out this way.

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But this creates a little bit of a problem as well because now if this switch
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But this creates a little bit of a problem as well because now if this switch

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goes down,

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this switch has to hop one, two, three times to get out of the closet.

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And let's say we have five or six switches in a row.

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Well, then we have even more hops in here before we get out to the distribution

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side.

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And so this has issues with efficiency in the event of a failure.

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Now by the way, it's not just in the event of a failure because look at this

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switch right
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switch right

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here.

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We just blocked this link that's going to the fourth switch.

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So how does this switch get out to the rest of the network?

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Well, even in the event of not having a failure, I guess I left the cross on

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there.

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It's okay.

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This X out.

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But either way, this switch is going to hop here and then hop here and then get

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out of

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the distribution switch this way.

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Now we had a little bit of efficiency problem with the first head end design as

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well.

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And so there's really no great way to get around the efficiency issue.
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And so there's really no great way to get around the efficiency issue.

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But either way, sometimes it felt like we were stuck between a rock in a hard

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place

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when it came to access closet design.

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So what do we do in the modern network to get around this?

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Well fortunately for us, the modern network gives us two technologies that can

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help us out

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in technically three technologies, although the third one doesn't necessarily

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get used very

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much in the data or access closet.

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So our three technologies.

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First of all, we have chassis switches.
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First of all, we have chassis switches.

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One of all, our second technology is we have stackable switches and that third

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technology

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that we don't see a whole lot traditionally was VSS virtual switching system.

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But these days Cisco calls this stack wise virtual.

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But ultimately it's the same thing.

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The difference here is this is just a single pair of switches.

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So what are we talking about here?

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Why do these chassis switches and stackable switches help us out?
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Why do these chassis switches and stackable switches help us out?

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Well all of these solutions are going to take many switches, many physical

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switches and

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turn them into a single logical switch.

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Now the chassis switch is probably the one that is most evident with this

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because we don't

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even use different physical switches for a chassis.

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We are going to take an empty chassis.
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We are going to take an empty chassis.

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We throw line cards in here.

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Let's say we throw two line cards in, we've got a bunch of ports and then we

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have a supervisor

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that I as the admin, that's me here with my laptop.

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I am going to log into the supervisor and I have just a single config for the

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entire chassis

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switch.

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And that makes sense.

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A lot of us have seen chassis switches for a long time.

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We've truly been around since the 1990s.

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So they've been around for a very long time.
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So they've been around for a very long time.

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But the advantage to having this is again it is a single configuration and it

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doesn't matter

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how many line cards I have.

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I could install a third line card with more ports and it just adds more ports

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to my single

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configuration.

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Well this same technology actually applies to stackable switches too.

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The difference is we're no longer using line cards.

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So I take individual switches, sort of like we have over here with the four

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individual
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individual

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switches.

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And I use a backplane connector, some kind of cable that allows for very high

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throughput

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backplane communications and that turns this also into a single config.

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So whether it's a stackable or whether it's a chassis either way we end up with

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the same

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concept.

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And so when it's a single configuration what we really ultimately care about
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And so when it's a single configuration what we really ultimately care about

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for the most

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part is that it's a single STP node.

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I'm just going to greatly simplify my environment in the way that I end up

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deploying my access

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closet topologies.

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Now a quick note about the stack wise virtual concept.

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So again this is for a pair of switches usually intended for core and

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distribution applications.

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It's not to say we couldn't use it in the access closet.
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It's not to say we couldn't use it in the access closet.

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It's just that it only impacts a pair of switches.

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So you usually have two switches and you're not even connecting backplane

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cables.

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You just connect maybe a couple of fiber cables.

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Maybe these are 40 gig fiber or something along those lines.

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And it turns this into again a single configuration, a single logical switch.

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So that's very helpful in a lot of situations outside the access closet.

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Maybe not so much inside the access closet.
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Maybe not so much inside the access closet.

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So now that we have these more modern designed switches in mind, let's go ahead

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and look at

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how this can help us.

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So when we're looking at now a modern design framework, we're still going to

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have those

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two distribution switches.

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And now we can actually deploy a chassis or a stack.

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I'm just going to generically draw it like this.

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This is a chassis or a stack.
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This is a chassis or a stack.

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And now I can bring a connection in physically to one of these switches or line

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cards.

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I can bring the other connection in to the other line card.

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And at this point, we're good because we don't have to worry about blocking

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connections between

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any of these.

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We don't have to worry about inefficiencies and forwarding traffic through

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other logical

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switches.

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We simply block one of these ports from an STP perspective and I can get out
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We simply block one of these ports from an STP perspective and I can get out

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the other

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link.

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If it goes down, then we use the backup connection.

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Now what if we have a second one of these in the same access closet?

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Maybe we've got a need for eight switches.

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So we deploy two stacks of four, something like that.

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Well, I deal world.

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We do the same thing.

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We connect up to the distribution layer and we block one of these ports.

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But this does require notice one, two, three, four fiber connections, which
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But this does require notice one, two, three, four fiber connections, which

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requires not

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only extra transceivers on the distribution switches, but also potentially

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infrastructure

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fiber.

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And we might not always have that.

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And so we might also, I'll say or down here, we might also bring in, let's draw

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it like

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this.

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So a connection from each distribution switch into each access switch and then

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we connect

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them like this.

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And at this point, we would have to block this connection between them.
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And at this point, we would have to block this connection between them.

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So ultimately in steady state operation, it doesn't really matter because both

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of them

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previously had uplinks that weren't blocked.

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And now they still have uplinks that aren't blocked.

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And we have backup paths otherwise.

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So either one of these topologies is actually going to work just fine.

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Now I want to add one other quick note.

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What about if we have this VSS or Stackwise Virtual Connection in place?
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What about if we have this VSS or Stackwise Virtual Connection in place?

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What if we put it here at the data center, or a data center distribution block

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at the

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wrong D word in my head?

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What if we made this a single logical switch?

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Well, if we have a single logical switch now connecting to a logical chassis or

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Stack

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switch, we can actually port channel this connection because we have one device

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connecting

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to one device.
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to one device.

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And this is the most beautiful topology of them all because now we don't have

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to have

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any blocked ports in the entire infrastructure.

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And again, ideally if we have another one in the same closet, we do the same

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thing and

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we port channel.

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If we have to connect between them, then it is what it is.

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But you do be aware that this third option, even though it doesn't apply

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directly into

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the access closet, can actually seriously help us with our topology and our STP

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blocking

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situation if we were to deploy it into the distribution layer.
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situation if we were to deploy it into the distribution layer.