Let's turn our attention to the access layer. A lot of our focus with two
tier and three tier models is on the core and distribution side of things, but
the
access layer needs some attention as well. And we didn't spend a little bit of
time in the last skill talking about our different topologies. But now we want
to
talk about how exactly we connect our distribution switches down to our
access switches. Because even though we can come up with topologies for within WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
access switches. Because even though we can come up with topologies for within
the access layer, this connection is kind of interesting. Now in a traditional
network architecture, we are going to create these as layer two links. And if
we
just had a single layer to link, we'd lack redundancy, but it would be very
clean and very easy to deploy. For better or worse, we know that our topologies
are a lot more complex than that. For example, when we really draw this out, we
have a pair of distribution switches connected at layer two, by the way. And WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
have a pair of distribution switches connected at layer two, by the way. And
then down here, we have an access closet with many switches. And potentially we
use those technologies we talked about our chassies or our stacks, but either
way, even if we do stack all these together, we know that we are deploying this
with spanning tree protocol at play. We're going to have to block one of these
links, ideally on a pervlan basis. And ultimately, we are creating a very large
layer two domain. Now this has some amount of benefits. Because if I were to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
layer two domain. Now this has some amount of benefits. Because if I were to
draw
this out, going to another access closet, so this is closet number one, and
this
is closet number two. And we're going into another maybe a switchback here, or
a
set of switches. And yeah, we're going to block one of these links from a
spanning tree perspective. But ultimately, what we have here is we have a
extended layer two environment. So now I can have a host over here, a user that
's
connected in to the access closet on the right. And I can have an user over WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
connected in to the access closet on the right. And I can have an user over
here
that's connected to the access closet on the left. And these can be part of the
same VLAN and the same broadcast domain and the same subnet. So this user over
here could be 10.100.1.5. And this device over here could be 10.100.1.7. And
they
can communicate at layer two. This gives us a maximum amount of flexibility. So WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
can communicate at layer two. This gives us a maximum amount of flexibility. So
we're talking about the pros, the benefits of this design. We have flexibility
because I might need to deploy something who knows what over here, a device
that needs to be on the same subnet as this device over here. If that's the
case and that's what I need, then this design is going to support that. And I
might
not need this today, but I might need it in the future. And that's one of the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
not need this today, but I might need it in the future. And that's one of the
concerns of this layer to design is just making sure again from a flexibility
perspective that I can do whatever I need to do from a from a deployment
perspective in the future, because I've built my network out with layer two.
But
here's the thing about this in the modern network. This need is less and less
because most of my users are not trying to communicate with other devices on WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
because most of my users are not trying to communicate with other devices on
the
local network. Rather, they're going out to my data center to communicate with
my
applications. Or increasingly more and more these days, I'm going out to the
internet where I also have applications. Cloud applications, software, whatever
it
is that I'm connecting to. But this is more what we'd refer to as northbound
traffic. If I look at my drawing over on the left, we call it northbound WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
traffic. If I look at my drawing over on the left, we call it northbound
traffic
because I'm sending it through the distribution layer to whatever lies beyond
that. Maybe it's a dedicated core, maybe it's a collapsed core, but I'm going
out to
the internet or to my data center. But regardless, it's northbound traffic. I'm
not necessarily doing this east west traffic type of network flow anymore,
going from one user to another or maybe a printer of some kind over here that
used to require layer two communications and connectivity. It just doesn't WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
used to require layer two communications and connectivity. It just doesn't
happen
as often in our modern networks. Another aspect of traditional network
architectures
where this was more required was wireless roaming because when I look at my
wireless roaming, we'll say Wi-Fi roaming here. When I look at my wireless
roaming, I always want to roam at layer two. If I perform a layer three
roam, I have to change IP addresses and that gets extremely complex and usually
results in dropped sessions. So I want to stay at layer two. And the reality is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
results in dropped sessions. So I want to stay at layer two. And the reality is
that a wireless user here. So I guess this is an old cell phone with an antenna
on
it. I don't know. But either way, we have some kind of mobile device here. And
we
might start by communicating to an access point here connected to Closet 2 on
the right. And I walk this way. And now I'm connected to an access point that's
hanging off of Closet 1. So this transition, this wireless roam, will be at WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
hanging off of Closet 1. So this transition, this wireless roam, will be at
layer
two these days, which is or with this design. And that's really good. We still
want layer two roaming on the modern network. But in the modern network,
wireless roaming is determined oftentimes by wireless controllers. And if we
have wireless controllers in place in the network, the layer two happens
regardless of what the underlying infrastructure looks like, thanks to our WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
regardless of what the underlying infrastructure looks like, thanks to our
tunneling mechanism. So the fact that we have tunnels means that we almost like
that fabric concept where we're sending our layer two traffic over whatever
the underlay is. And at that point, we're good to go. We are performing our
roaming
via layer two, even if the underlying architecture isn't layer two. And so yeah
,
we've got this flexible nature of layer two, but we do have to consider that we WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
we've got this flexible nature of layer two, but we do have to consider that we
don't necessarily need it in a lot of our environments. But at the same time,
this is also a fairly simple architecture. And that's really good from a
supportability perspective. So those are the benefits of a layer two access
layer.
But what are some of the downsides to this? Well, the biggest downside and it
just is what it is. Anytime we have spanning tree at play, it is a downside. We WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
just is what it is. Anytime we have spanning tree at play, it is a downside. We
have to deal with spanning tree. This involves the potential for loops. And at
a
minimum, it involves us trying to manage our blocked ports. And that's going to
require some amount of effort in order to properly block the right
interfaces and make sure we're being efficient that way. For example, over here
in
our diagram, we're going to need to make, you know, maybe we make the the root
of
the network on the left here for some of our VLANs. And that would probably WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the network on the left here for some of our VLANs. And that would probably
result in these links being blocked. But then we would want to create the root
over here for other VLANs, which would block these links. It's a very common
part
of our network design to allow us to have multiple topologies so that our
orange
VLANs here can use this type of topology. We see it's loop free. But then our
purple pink, whatever color this is, are this topology here. Also is loop free. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
purple pink, whatever color this is, are this topology here. Also is loop free.
And so half of VLANs go on this topology and half the VLANs go on the other top
ology.
But again, that requires extra configuration and complexity and makes it more
difficult to troubleshoot issues when they happen. And so not only do we have
to deal with spanning tree, but we have to worry about potentially broadcast
storms or just flat out broadcast domains being large. So regardless of whether WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
storms or just flat out broadcast domains being large. So regardless of whether
a burst of traffic a storm is happening, regardless if we have dozens of
access closets, we have more and more access closets here. The larger my layer
to domain is going to be and the larger my layer to domain is the more I have
to
break it up into multiple subnets and still deal with broadcast traffic going
to
all of my closets where VLANs are enabled. It just can be very cumbersome to
try to
rein that in. So in the end, having this layer to access layer design is very WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
rein that in. So in the end, having this layer to access layer design is very
much
how we traditionally do it. I would say even in the modern network, the vast
majority of our organizations are deploying layer to access layer switches. But
we
do have to understand that while that does have some amount of benefits, some
of
those benefits just aren't as relevant anymore. And it does come with a bunch
of downsides, which might lead us to want to talk about a different solution,
which
can lead us to either layer three access layer designs or even fabrics being WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
can lead us to either layer three access layer designs or even fabrics being
deployed that will change the way we extend our connectivity out to the access
layer.