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I thought it would be
useful to demonstrate

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the actual process
of a switch learning

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the layer 2 addresses
of the devices

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that are connected to it.

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And although the vendors
vary and the commands

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may be slightly different
on displaying information

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about the dynamically
learned MAC addresses,

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the concept remains
the same regarding

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the layer 2 switches learning,
dynamically, the MAC addresses

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of those devices that are
connected to the switch.

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So in our topology, we have
these three PCs on these three

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ports on the switch.

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And these three PCs have
these respective IP addresses

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at layer 3.

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And they have these three
respective layer 2 addresses,

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or at least the
last four characters

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of those MAC
addresses, at layer 2.

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So when we start off, if these
PCs have been absolutely quiet,

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not one single frame, we can
look at the MAC address table

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and the switch, if these PCs
have not sent any frames,

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should not yet know about any of
their respective MAC addresses

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on any of these ports.

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However, once we
generate some traffic,

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we should be able
to go back and see

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this table is being populated.

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So we're going to
begin on the switch.

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Now this happens to be
a switch module that's

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a module inside of a
router, a Cisco router.

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And so based on even the type
of Cisco switch you have,

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the commands may be
slightly different to show

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the MAC address table.

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So on this switch
module in this router

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the command to show
the MAC address table

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is show mac-address-table.

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So we'll enter that
and press Enter.

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And at the moment, the
switch knows about itself.

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It has a management interface.

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But it hasn't
dynamically learned

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about any of the PCs that are
connected to ports 1/1, 1/2,

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or 1/3 yet.

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Let's fix that right now.

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Let's bring up a
console window for PC 1.

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And if we simply do a ping
of PC 2 and a ping of PC 3,

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if they reply back
to PC 1, that's going

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to allow the switch to
learn all three devices'

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MAC addresses because
each of the devices,

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when they respond and use their
source layer 2 MAC address,

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the switch goes, boom, gotcha.

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And it will add that to
the MAC address table.

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So here on PC 1, let's do
a ping over to 10.0.0.22.

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That's the IP address of PC 2.

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That looks good.

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And let's also do a ping
over to PC 3 at 10.0.0.33.

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Press Enter.

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That looks good as well.

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We'll let that complete.

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And then let's go
back to the switch.

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And now we're just going
to use the Up Arrow

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key to do the same command
again to look at the MAC address

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table, and press Enter.

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And we can now see that
the switch has dynamically

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learned three MAC addresses
on three respective ports.

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PC 1, ending in 6800, being
learned off port Fa1/1,

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or fast ethernet 1/1.

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PC 2's MAC address of 6801,
learned off of port 1/2.

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And PC 3's MAC address
ending in 6802, dynamically

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learned off switch port 1/3.

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And the beautiful thing now
is that if the switch needs

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to forward frames to any
of those MAC addresses,

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it knows exactly how to do it.

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And it can forward
those frames directly

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based on the learning
it did as opposed

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to just sending a frame to
all other ports on the switch.

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In this Nugget,
we've demonstrated

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how a switch can
dynamically learn the layer

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2 addresses of those
devices that are

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connected to the switch ports.

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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.