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One interesting note
about enterprise

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and corporate networks
is that they generally

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have more features, and
bells, and whistles,

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and security than a
small office home office.

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So from a physical
equipment perspective,

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one of the big
differences is going

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to be in the
switching environment.

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So in a small office
home office we

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may have a simple 8-port switch.

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However, in a
corporate environment

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they're probably going to have
racks and racks of switches.

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And they're also going
to have different ones

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based on where they are.

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They'll have some in the
wiring closets, referred

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to as access switches.

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And those access
switches can then

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connect down to bigger switches
at or near the data center.

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Those are often referred to
as distribution switches.

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That's also a likely
location to get connectivity

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between the corporation
and other sites, which

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could include connectivity
out to the internet

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or connectivity up to a remote
office or branch office.

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And in the main distribution
area near the data center,

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there's going to be
a lot of traffic that

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flows through that
point in the network.

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And to help facilitate
moving all that data,

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the higher end
switches use a feature

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known as distributed switching.

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And if we have a
Layer 2 frame that

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needs to be switched
when the frame comes in,

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if the switch has to take a
moment and think about that--

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OK, that frame came in here.

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And it needs to
go out this port.

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If that decision has to
go to a centralized brain

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every single time
a frame comes in,

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that could be a
bottleneck if we have

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hundreds or thousands of ports.

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And that's where distributed
switching can help.

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With distributed switching
the forwarding information

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is distributed out closer
to the ports, or at least

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the module on that larger
switch where those ports exist.

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Then as a result, when a frame
comes in on this port that

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needs be sent out that
port, because that learning

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and forwarding logic
is distorted down

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at this level,
this module doesn't

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have to check in with
the centralized brain

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to ask it, OK, what do I do?

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That information
about forwarding

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is cached and distributed.

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So the switch can make
that forwarding decision

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without having to bother the
central unit every single time.

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Also, what we'll typically
find in corporate environments

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is that they'll have
some switches that

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also incorporate routing
functionality in one box.

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So the question
might come up, well,

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what do you call a box with
a whole bunch of ports that

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can do Layer 2 forwarding,
based on Layer 2

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information the frames, as
well as make Layer 3 routing

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decisions.

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Is it a Layer 2 switch?

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Or is it a Layer 3 router?

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And the answer is
it's kind of both

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if it has both those
functionalities.

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And I've seen this
often referred to

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by many different vendors
as a multi-layer switch.

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So if you see the term or hear
the concept multi-layer switch,

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it can be referring to either
Layer 3 routing of IP packets

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or forwarding of Layer 2 frames,
all built into the same box,

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referred to as a
multi-layer switch.

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Back in the 80s when I had my
first official corporate job

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in the world of
IT and networking,

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we had dedicated leased lines
between our location, which

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was in Southern California
and a branch office

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or remote site in
northern California.

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And the lease line
was fairly expensive.

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It was a dedicated circuit
between us and our other site.

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And because it was
really expensive,

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we had some fairly
intelligent gear

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on both ends that
could do things

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like time division multiplexing
and other kind of tricks.

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And that way we could utilize
that dedicated circuit

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as much as possible
between our two locations.

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And a dedicated circuit,
a dedicated leased line

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is an example of
circuit switching, where

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we're moving data frames
between point A and point

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B over a single path every time.

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So all the traffic would go over
that leased line from point A

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to point B. And then back
from point B to point A.

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Now, we loved the fact
that we had connectivity.

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But we didn't love the cost.

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So in the late
80s and early 90s,

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the big rage was a new
technology called frame relay.

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And effectively what
it did was this.

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We had a service provider.

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I'll put SP for short.

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Instead of having a
dedicated leased line,

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we would go ahead and
rent a logical circuit.

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So the central site would
connect to the cloud.

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The remote site would
connect to the cloud.

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And then inside the
service provider cloud

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we'd have a logical
connection between the two.

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But it wasn't a physical
dedicated circuit.

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And in this cloud
the service provider

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had a lot of different paths.

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So we might have some of
our packets go over one path

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and other packets go
over a different path.

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The key is it's all packetized.

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So we're not using
the same exact path

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as that traffic is moved,
or forwarded, or switched,

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if you will, over that
service provider network.

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And that concept of
individual packets going over

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a network and perhaps
not all of them

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taking the exact same
path, is referred to

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as packet switching.

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And from a customer
perspective, we really

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don't care if all the
packets take the same path

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as long as they get to the
other side in a timely fashion.

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And in the right order it
would be helpful as well.

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Another useful idea-- and
it's up and coming and will be

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for a long time to come--

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is the concept of software
defined networking,

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or SDN for short.

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Because think about a large
network with lots of gear.

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If we had to administer each one
of these by connecting to them

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and then configuring them,
whether there are switches,

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or routers, or firewalls,
or access points,

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connecting to each one of
those is a pain in the puzuski.

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So at software
defined networking,

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the concept is this.

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We're not going to
have the humans touch

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the actual infrastructure.

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Instead, we're going
to have humans interact

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with applications.

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And that application
could go both ways.

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It could give us
information as a human.

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And we could also make changes.

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But we're doing it
through the application.

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And those apps are
communicating to controllers.

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So there's control.

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Looks like we have a
controller right here

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on the network that's connected
to our network infrastructure.

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Our applications
would interoperate

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with those controllers.

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And the controllers
would then interoperate

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with the actual infrastructure.

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I hope this has been
informative for you.

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And I'd like to thank
you for viewing.

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