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One rule about Layer 2 Broadcast
is that the broadcast needs

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to be forwarded to all other
devices in that same Layer 2

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domain--

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that same Broadcast domain.

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Well, what if we have two
different switches that

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are connected together, and we
have a Broadcast in VLAN 100,

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for example, on one switch?

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How does that switch, when it
forwards that broadcast over

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to the second
device, how does it

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let the second switch know
that this is a Broadcast

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4 just VLAN 100 and not,
for example, VLAN 200?

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Well, the answer to that
question is solved with

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a technique called trunking
and adding a little tag called

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an 802.1Q tag to the
frames over trunks.

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And one of the analogies
that I learned decades

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ago and is still
pretty effective

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is the one about airlines.

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And I've got seven kids,
and I care a great deal

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about every one of them.

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And when they were
young, if I would ever

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have to send one of those
kids on an airplane,

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we'd want to make sure
that they get on the plane

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and were delivered correctly
to the correct party

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on the other side.

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So what the airlines
do is they create a tag

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with all the information for
the child who's traveling alone,

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and they put it on that child
for the duration of their trip.

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And then, when they
land on the other side,

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they use that tag that
the child has on them

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to correctly deliver that
frame to the right VLAN--

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or, in the example of the
plane, to deliver that child

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to the correct
person who's going

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to take care of that child
on the receiving side.

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So in our topology, we've
got three computers connected

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to the first switch, we have
a connection between the two

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switches, and then
three more computers

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connected to the second switch.

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Also, the individual ports
on their respective switches

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are carved out
into various VLANs.

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The first two ports are
in VLAN 100 on switch-1,

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and the third port
has been configured

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to be part of VLAN
200 here on switch-1.

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And we have similar
configurations

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over here on switch-2, with
port 1/1 being in VLAN 100

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and ports gig 1/2 and
1/3 being in VLAN 200.

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Also, to make things
functional and work,

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we've also configured
in this topology--

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all the computers
there in VLAN 100,

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their IP address
starts with 10.100.0.

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And all the devices in VLAN 200
have an IP address that begins

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with 10.200.0.

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And the actual
VLAN assignment is

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done based on the ports that
these devices are connected to,

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and the IP addressing
can either be

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done manually or through some
dynamic process like DHCP,

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which we'll cover in
a different Nugget.

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But for now, this is how
this network is set up.

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So if PC-1 was going
to do a ping request--

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so PC 1 is right
here at 10.100.0.1--

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if it did a ping request over
to PC-4 over here at 10.100.0.4,

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when that frame goes in
on gig 0/1, the switch--

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because gig 0/1 is configured
to be a port in VLAN 100--

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the switch knows that that
frame that's coming in

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is associated with VLAN 100.

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But here's the problem.

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When switch-1 forwards that
frame over to switch-2 so it

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can get to PC-4, it needs to
add some information-- a tag,

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if you will--

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along with that frame
to indicate, hey, buddy,

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it says to switch
number 2, this frame

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is associated with VLAN 100.

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And that additional little
information that it's adding is

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referred to as an 802.1Q.

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That's the name of the
protocol that does this,

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and it's referred to as a tag.

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We're adding that information
before we send that frame over

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to the other switch.

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And that way, when
switch-2 receives it,

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whether it's a unicast frame, or
whether it's a broadcast frame,

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switch-2 will know right when it
receives it, based on the tag,

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that this is a frame associated
just with that specific VLAN--

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VLAN 100.

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And then as switch-2 forwards
that frame up to PC-4,

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it's going to remove the tag
because it's no longer needed

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as it forwards the
frame up to PC-4,

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where that frame needs to go.

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So the play by play would be,
no tag here on the access port

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between PC-1 and the switch.

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We'd have the tag added on this
interconnection between the two

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switches, which is also
called a trunk, by the way.

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And then when switch-2 forwards
that frame out the port where

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PC-4 is connected--
that access port--

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there would be no benefit or
need to include that 802.1Q

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tag, because all PC-4 cares
about is the actual frame

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itself.

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And one of the big questions
is, to tag or not to tag?

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That is the question.

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And the primary
method that switches

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know whether to include a
tag or not include a tag

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is the type of port that
that frame is going across.

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If it's an access port,
which implies that we're

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connected to end user devices--

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devices that are
not other switches

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or don't need to have that
tagging information-- those are

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referred to as access ports.

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And that's why, when
we configure VLANs,

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we configure both
the type of port--

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whether it's an access
port or a trunk port.

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And then if it's an
access port, we also

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assign what VLAN that
access port is a member of.

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So that way, when the
switch is forwarding

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a frame over a trunk,
it's going to add the tag.

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And when it's forwarding that
frame over an access port,

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it won't bother
including the tag.

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And to reinforce this and
have some fun with this,

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here's a quick little snippet
from Wireshark packet capture

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of the frame as it went over the
trunk when PC-1 at 10.100.0.1

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was doing a ping request
over to PC-4 at 10.100.0.4.

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And once again, the capture
was taken over the trunk link.

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And as a result, here we
have the Layer 2 information

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with the source and
destination Layer 2 addresses.

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And then it wedged in here
this next little piece--

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this additional little piece--

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which is the 802.1Q tag.

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And then that tag, it includes
the ID, which is the VLAN ID.

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Effectively, that
says, this frame

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is associated with VLAN 100.

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So if switches had a voice,
maybe they'd sound like that.

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So any frames associate with
VLAN 100 that are going across

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this trunk link between
switch-1 and switch-2,

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whether they're unicast
frames for a specific host,

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or whether they're a broadcast
for all devices in that same

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VLAN, as those frames
cross the trunk,

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they're going to get this
additional little 802.1Q tag

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so that the receiving switch
knows exactly which VLAN that

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frame belongs to.

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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.