Our conversation about the access layer wouldn't be complete if we didn't talk
about the
different topology options that we have available for us here.
And really, when we talk about options, we're really saying that we've got some
bad options
and we've got some better options.
And the modern network, fortunately, gives us some really solid ways to build
out access
layer topologies.
But in order to understand how the modern network devices help us out here, we
need to understand
the sort of the classic access layer topology. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the sort of the classic access layer topology.
And we do need to understand that a lot of this is going to tie to how spanning
tree
protocol ends up working for us and how it helps us to block our loops, but how
this can
actually cause us a little bit of grief when it comes to these topology designs
.
So how does it look from a classic perspective?
Well, when we zero in and lock in on a single access closet here, in fact, let
's make that
closet just a little bit bigger.
We know that this one closet could have many switches. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We know that this one closet could have many switches.
Let's just draw it out such that we've got four switches.
So we have four switches in this access closet.
Each one is a single rack unit, autonomous switch.
You have to configure each one.
And then we have a pair of distribution switches up here that are connected to
each other.
So here's the question.
How do I connect the distribution layer to the access layer switches? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
How do I connect the distribution layer to the access layer switches?
This is a difficult conversation to have when these four switches are each
their own fully
functioning switch because now we have to worry about what the spanning tree
topology
looks like as we connect to each one.
So we had a couple of choices.
First of all, we could do the head end design.
Let's draw this out in different colors.
So we could do the head end type of design, which means that we're going to
connect our
distribution to one of the switches called the head end switch.
And then we would connect all of these switches hanging off of that switch and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And then we would connect all of these switches hanging off of that switch and
potentially
loop this back around as well.
And what we'll find here is that we're going to spanning tree block one of
these ports
to the distribution layer to prevent this loop from happening.
But then we also have this loop down here and so we're going to need to block
something
in here, maybe a link down here.
And so we have full connectivity, no problem here except the big downside of
course is what WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
course is what
happens if the head end fails.
If this one switch goes down, then we have to worry about the fact that the
whole closet
lost access to the distribution layer.
And so this isn't a super ideal topology.
So instead what we can do is we can do a fall loop.
The fall loop looks like this.
Again, drawing in a different color here with the same drawing.
But we're going to connect into one of these switches from one distribution WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But we're going to connect into one of these switches from one distribution
switch.
We're going to connect into a different switch through the other distribution
and then we're
going to basically ride the same connections here between the two.
So it's going to look like that.
And now we have one big loop.
And yeah, we're probably blocking that link in there.
So at this point, we might say, okay, this looks a lot better.
If this switch goes down, we can still get out this way.
But this creates a little bit of a problem as well because now if this switch WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But this creates a little bit of a problem as well because now if this switch
goes down,
this switch has to hop one, two, three times to get out of the closet.
And let's say we have five or six switches in a row.
Well, then we have even more hops in here before we get out to the distribution
side.
And so this has issues with efficiency in the event of a failure.
Now by the way, it's not just in the event of a failure because look at this
switch right WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
switch right
here.
We just blocked this link that's going to the fourth switch.
So how does this switch get out to the rest of the network?
Well, even in the event of not having a failure, I guess I left the cross on
there.
It's okay.
This X out.
But either way, this switch is going to hop here and then hop here and then get
out of
the distribution switch this way.
Now we had a little bit of efficiency problem with the first head end design as
well.
And so there's really no great way to get around the efficiency issue. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so there's really no great way to get around the efficiency issue.
But either way, sometimes it felt like we were stuck between a rock in a hard
place
when it came to access closet design.
So what do we do in the modern network to get around this?
Well fortunately for us, the modern network gives us two technologies that can
help us out
in technically three technologies, although the third one doesn't necessarily
get used very
much in the data or access closet.
So our three technologies.
First of all, we have chassis switches. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
First of all, we have chassis switches.
One of all, our second technology is we have stackable switches and that third
technology
that we don't see a whole lot traditionally was VSS virtual switching system.
But these days Cisco calls this stack wise virtual.
But ultimately it's the same thing.
The difference here is this is just a single pair of switches.
So what are we talking about here?
Why do these chassis switches and stackable switches help us out? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Why do these chassis switches and stackable switches help us out?
Well all of these solutions are going to take many switches, many physical
switches and
turn them into a single logical switch.
Now the chassis switch is probably the one that is most evident with this
because we don't
even use different physical switches for a chassis.
We are going to take an empty chassis. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We are going to take an empty chassis.
We throw line cards in here.
Let's say we throw two line cards in, we've got a bunch of ports and then we
have a supervisor
that I as the admin, that's me here with my laptop.
I am going to log into the supervisor and I have just a single config for the
entire chassis
switch.
And that makes sense.
A lot of us have seen chassis switches for a long time.
We've truly been around since the 1990s.
So they've been around for a very long time. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So they've been around for a very long time.
But the advantage to having this is again it is a single configuration and it
doesn't matter
how many line cards I have.
I could install a third line card with more ports and it just adds more ports
to my single
configuration.
Well this same technology actually applies to stackable switches too.
The difference is we're no longer using line cards.
So I take individual switches, sort of like we have over here with the four
individual WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
individual
switches.
And I use a backplane connector, some kind of cable that allows for very high
throughput
backplane communications and that turns this also into a single config.
So whether it's a stackable or whether it's a chassis either way we end up with
the same
concept.
And so when it's a single configuration what we really ultimately care about WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so when it's a single configuration what we really ultimately care about
for the most
part is that it's a single STP node.
I'm just going to greatly simplify my environment in the way that I end up
deploying my access
closet topologies.
Now a quick note about the stack wise virtual concept.
So again this is for a pair of switches usually intended for core and
distribution applications.
It's not to say we couldn't use it in the access closet. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It's not to say we couldn't use it in the access closet.
It's just that it only impacts a pair of switches.
So you usually have two switches and you're not even connecting backplane
cables.
You just connect maybe a couple of fiber cables.
Maybe these are 40 gig fiber or something along those lines.
And it turns this into again a single configuration, a single logical switch.
So that's very helpful in a lot of situations outside the access closet.
Maybe not so much inside the access closet. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Maybe not so much inside the access closet.
So now that we have these more modern designed switches in mind, let's go ahead
and look at
how this can help us.
So when we're looking at now a modern design framework, we're still going to
have those
two distribution switches.
And now we can actually deploy a chassis or a stack.
I'm just going to generically draw it like this.
This is a chassis or a stack. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
This is a chassis or a stack.
And now I can bring a connection in physically to one of these switches or line
cards.
I can bring the other connection in to the other line card.
And at this point, we're good because we don't have to worry about blocking
connections between
any of these.
We don't have to worry about inefficiencies and forwarding traffic through
other logical
switches.
We simply block one of these ports from an STP perspective and I can get out WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We simply block one of these ports from an STP perspective and I can get out
the other
link.
If it goes down, then we use the backup connection.
Now what if we have a second one of these in the same access closet?
Maybe we've got a need for eight switches.
So we deploy two stacks of four, something like that.
Well, I deal world.
We do the same thing.
We connect up to the distribution layer and we block one of these ports.
But this does require notice one, two, three, four fiber connections, which WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But this does require notice one, two, three, four fiber connections, which
requires not
only extra transceivers on the distribution switches, but also potentially
infrastructure
fiber.
And we might not always have that.
And so we might also, I'll say or down here, we might also bring in, let's draw
it like
this.
So a connection from each distribution switch into each access switch and then
we connect
them like this.
And at this point, we would have to block this connection between them. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And at this point, we would have to block this connection between them.
So ultimately in steady state operation, it doesn't really matter because both
of them
previously had uplinks that weren't blocked.
And now they still have uplinks that aren't blocked.
And we have backup paths otherwise.
So either one of these topologies is actually going to work just fine.
Now I want to add one other quick note.
What about if we have this VSS or Stackwise Virtual Connection in place? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
What about if we have this VSS or Stackwise Virtual Connection in place?
What if we put it here at the data center, or a data center distribution block
at the
wrong D word in my head?
What if we made this a single logical switch?
Well, if we have a single logical switch now connecting to a logical chassis or
Stack
switch, we can actually port channel this connection because we have one device
connecting
to one device. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
to one device.
And this is the most beautiful topology of them all because now we don't have
to have
any blocked ports in the entire infrastructure.
And again, ideally if we have another one in the same closet, we do the same
thing and
we port channel.
If we have to connect between them, then it is what it is.
But you do be aware that this third option, even though it doesn't apply
directly into
the access closet, can actually seriously help us with our topology and our STP
blocking
situation if we were to deploy it into the distribution layer. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
situation if we were to deploy it into the distribution layer.