- [Man] In this video, we want to consider
a three-tier architecture
and a couple of two-tier architectures.
First, let's consider
Cisco's traditional three-tier architecture
where we have an access layer,
a distribution layer,
and a core layer.
Down at the access layer,
that's where we're connecting out
to end devices like PCs and Laptops,
and printers and wireless access points.
And it's common in a building
that has different floors
that we've got to set up access layer switches
on each floor of that building.
Maybe there's a wiring closet on each floor
and all of our devices on that floor,
they hone back to that wiring closet
and they plug in to access layer switches.
But within the building,
we might have those wiring closets stacked.
That's a common way of doing it.
And we've got riser cable running
from one floor to another floor
to another floor.
Well all those floors
all those switches on the access layers
they hone back to a building distribution layer switch.
And at the distribution layer,
we're interconnecting the access layer switches.
And we're also providing the pathway
to get out to the rest of the world.
Maybe other buildings in a campus environment.
Maybe out to the internet.
And if we do have more than one building
let's say in a campus environment:
a business campus or university campus,
we could interconnect our different buildings.
Specifically, we can interconnect the different
building distribution layers with a core layer.
The core layer's primary concern is speed.
It wants to get traffic as quickly as possible
from one building distribution layer switch
to another building distribution layer switch
that might be in a different building.
And it also can get us out to the internet
or maybe over the IP WAN.
But that's Cisco's traditional three layer model.
And if we take a look
at the actual topologies involved here,
we can see we've got a mixture.
Think about it.
We've got a star topology.
Down at the access layer,
we're starring out to all those different devices.
For redundancy we're interconnecting
our switches between our layers.
That would be a partial mesh topology.
And since we've got both a star
and a partial mesh topology
showing up here,
we can call this a hybrid topology.
It's a mixture of different topologies.
But, again, this is the traditional
three-tier architecture.
However, sometimes we don't have enough buildings
to justify having separate core
and distribution layers.
Maybe we only have a single building.
Maybe it's a SOHO environment:
a small office, home office environment.
In a case like that,
we can take the responsibilities
of the core and distribution layers
and we can consolidate them
into a collapsed core architecture.
With a collapsed core,
we combine the functions
of the core and distribution layer switches.
Specifically, this collapsed core
it can connect out to our access layer switches
and it can also get us out to the internet.
And it can interconnect our different
access layer switches.
That's a collapsed core
or a two-tier design.
Another two-tier design
I would want you to know about
is often seen in data centers.
It's a spine-leaf design.
And here we've got switches
maybe within a rack.
And those switches might be connecting
to servers in that rack.
And those servers are running
some sort of hypervisor
and they're running virtual machines.
We'll say that the servers are the nodes here
connecting into the leaf switches.
And notice the redundancy.
each server connects into more than one
of the leaf switches.
And we might have, in this data center environment,
we might have different racks.
And we can have what's called a TOR switch.
A top of rack switch
that interconnects all the switches within that rack.
That's an example of spine switch.
A spine switch
is going to interconnect our leaf switches.
Now notice what's happening here.
Every leaf switch
connects to every spine switch.
That means if I want to go
from one leaf switch to another leaf switch,
maybe to go between servers
I only have to go through
one spine switch, maximum.
Because I'm connected to every spine switch.
And if we think about this
two-tier architecture,
the spine-leaf design,
logically, this is like having a single switch.
You could think of that collection
of spine switches
as the backplane of our logical switch.
And our leaf switches are much like
the ports on our logical switch
that connect out to our end devices.
And again, this is commonly seen
in data center environments
to give us optimal traffic flow
and lots of redundancy
between our servers.