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It is a challenge to configure,
troubleshoot, or manage

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a device if we're not
sure how it works.

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So in this Nugget,
I'd like to share

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with you the basics of how
a Layer 2 switch operates.

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So let's begin by
introducing this topology,

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and we're pretty proud of it.

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We have three PCs--

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PC-1, 2, and 3--

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and each of them are
connected physically

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with a little patch cable
that goes from their network

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interface card at their PC down
to the port on this switch.

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So PC-1 is physically
connected to port 1/1,

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PC-2 is physically connected
to port 1/2, and PC-3--

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da-da-da-da--
connected to port 1/3.

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Also, for this topology, I
have assigned each of the PCs

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a network layer address, and
that's these top numbers.

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PC-1 can be reached at
10.0.0.11, PC-2 at 10.0.0.22,

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and PC-3 at 10.0.0.33.

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I've also included the Layer
2 MAC address, or at least

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a portion of it, for
each of the computers.

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Because it is 12
digits long, I thought

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I'd just go ahead and put an x.x
for the first eight characters,

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and then write out the last four
digits, which I've done here.

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So PC-1's MAC address,
its Layer 2 address,

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ends in 6800, PC-2 ends and
6801, and PC-3 ends in 6802.

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So now that we've
been introduced

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to the topology in this
three-node network,

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let's talk about what
this switch right here

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does for a living.

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So we plug that in.

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It's powered on.

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And one of the first
things this switch is

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going to do and continue to
do is to listen to the frames

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as they come in and learn.

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And the question
might be, well, what

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exactly is it going to learn?

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Well, it's going to
learn the MAC addresses

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on the PCs or other devices
that are connected to its ports.

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And the way it does
that, it pays attention

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to the source MAC address.

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And let's use PC-1
as an example.

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Let's imagine that
PC-1 is sending

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a frame of data, bit by
bit, into the network

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through this port on the switch.

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And it just so happens I had
conveniently available a packet

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capture showing the Layer
2 information that PC-1

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would be using if it was
communicating with PC-2.

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And just to give a clear
picture of what's happening,

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PC-1 is doing a ping here.

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It's sourcing it from its
IP address of 10.0.0.11,

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and it's pinging over to
10.0.0.22, right here.

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Now, that's the Layer 3
information, but at Layer 2,

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that corresponds to the source
Layer 2 address of the MAC

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address ending in 6800--
this guy right here--

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going to the destination
MAC address of 6801,

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which is this guy right here.

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So as PC-1 sends his
frame into the switch,

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from a learning
perspective, the switch

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is looking at the source MAC
address on the incoming frames,

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because the switch
is curious and wants

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to know where all the
MAC addresses live.

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So in short, when PC-1 sends
a frame into the network

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and includes its
source MAC address,

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the switch is going
to build a table.

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It's going to say, OK,
MAC address x.x.6800,

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that MAC address, because I
saw it come in on port 1/1,

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is associated with port 1/1.

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So once these devices have
all sent at least one frame

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into the network, which includes
their source MAC address,

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the switch is going to
have learned and then

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put into a table all
their MAC addresses

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and where they can be reached.

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So we're getting PC-2.

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The MAC address is x.x.6801, and
it's associated with port 1/2.

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And for PC-3, it's
the MAC address

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that ends in 6802 on port 1/3.

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So the major concept
is that the switch

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is listening and learning
regarding the source

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MAC addresses for all the
frames that enter its ports.

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And then, secondly,
it's building a table

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so if it ever needs to
forward a frame to any

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of those MAC addresses, it
knows exactly which port it

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should send the frame out to
reach that destination MAC

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address.

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And one of the beautiful
things about a Layer 2 switch

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is that it can do
intelligent forwarding.

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Case in point.

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If PC-2 is going to
send a frame to PC-3,

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it can craft that frame with
PC-3's Layer 2 destination

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address.

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And when that frame goes into
the switch, the switch says,

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hmm, I know where
that MAC address

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lives that ends in 6802.

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It's off of port 1/3.

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And it can forward that
directly to that port

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and forward it to PC-3,
and that traffic never

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even had to bother PC-2 at all.

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It wasn't for PC-2, PC-2
didn't care about it,

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and we're not
interrupting anybody else

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except for PC-3, who's
going to receive that frame.

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And if this switch
is just powered on

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and hasn't learned where
everybody lives yet

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and it receives the frame--

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let's imagine that PC-1 sends
a frame into the network--

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the switch looks at the
destination address,

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and the switch looks at this
table it's built and says,

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oh, I don't know where
that destination lives.

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The switch is going to err
on the side of caution,

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and it's going to
forward that frame out

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to every other port where
there's a device connected

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with the hope that
whoever that frame was for

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would have a chance
to take a look at it.

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And what the switch
is not going to do,

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it's not going to bother
forwarding the frame

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out the same port
that it came in

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on because that would be silly.

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If PC-1 is sending a frame, PC-1
already knows about that frame.

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So a frame coming
into the switch

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where the switch does not know
where that destination MAC

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address lives is forwarded
to all other ports.

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And that same kind
of activity happens

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if a device sends a
broadcast into the network.

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Let's imagine that PC-3
sends a broadcast frame

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into the switch.

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A broadcast is also going to be
forwarded out all other ports

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because it's a broadcast.

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Somebody might need to see it.

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So the processing of an
unknown destination address

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and a broadcast are handled
the same on the Layer 2 switch.

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They're sent out every other
port except for the port

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that that frame came in on.

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In this Nugget,
we've taken a look

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at the basic operations
of a Layer 2 switch.

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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.