Network Switching Overview – CompTIA Network+ N10-007 – 1.3

One of the most common network
devices we use every day is our switch. Our switch is responsible
for either forwarding or dropping frames
on our network, based on the destination
MAC address that’s inside of our Ethernet communication. If the switch sees traffic
from a source MAC address, it makes a note of where
it saw that traffic coming from so that later
on, if any traffic was destined to that
device, it would know exactly where to send it. And because we
connect many switches to each other in
our larger networks, the switches are also
responsible for making sure that there are no
loops on the network. It does that by using
a protocol called STP, or Spanning Tree Protocol. Here’s a common
network configuration. We’ve got a switch A in
the middle of the network, and this switch has a number of
fast Ethernet interfaces on it, and the fast Ethernet
interfaces use the letter F to designate those interfaces. There’s also on this
switch, a particular card where all of these
interfaces are located. It’s card zero, and so you’ll
see each one of these start with F0. And then, each individual
interface on this card will be numbered. So we have fast Ethernet
on slot zero interface one. Slot zero, interface two. Interface three. Interface four. And interface five. We have five devices
plugged into this device– Sam, Jack, Daniel, Teal,
and the SGC server. And of course, each
one of those devices has its own unique
MAC address associated with each individual device. Inside of the switch is a
table that contains information that this switch has
gathered over time. This table has a list of all
of the MAC addresses that it has seen. And it has a list of
all of the interfaces that are associated with
those MAC addresses. This means that when Sam wants
to send information to the SGC server, Sam will put a
frame on the network that says, I’d like to send
information to the destination MAC address, 1000.5555.5555. That information is
sent down to the switch. The switch then looks
through its MAC address table and tries to find
any place where it happens to match that
destination MAC address. And of course, we have
a match, and that match is associated with
an output interface, a fast Ethernet on slot
zero, interface number five. So this switch knows
to send that traffic down that particular interface
where ultimately, it will end up at the destination address. If this is a larger network, it
might have multiple switches. For instance, a switch
A and a switch B. We not only have fast
Ethernet interfaces, some of these interfaces
are gigabit communication that start with the letter G. There are two separate
MAC address tables– one for switch A and one
for switch B. Switch A doesn’t know what’s inside the
switch B MAC address table. And switch B doesn’t know
what’s inside the switch A MAC address table. They all work independently
from each other. Let’s perform the
same function, where Sam is sending information
over to the SGC server. Sam will put a
frame on the network that has the destination MAC
address of 1000.5555.5555. And it goes to switch A.
In all of these situations, the switch only knows where
the next step is to be able to forward this frame. Switch A will look through
its MAC address table. Find a match for that
destination MAC address. And see that the
output interfaces on the gigabit network,
slot A, port number two. And it simply sends
that time frame out to that particular interface. At this point,
switch A has no idea what the next step
is going to be, but that’s OK because
switch B has now received that time
frame and switch B will look through its own set of
MAC addresses to find a match. And in this case,
it’s found a match that says, it should send
this particular frame to fast Ethernet slot zero,
interface number five, to be able to have it
reach the SGC server. Being able to build this
interface of MAC addresses and interface names is
an important function for any switch. The switch is
going to constantly be examining the incoming
information into the switch. And it makes a note of the
source MAC address associated with that traffic. When it sees that
source MAC address, it can then, start
filling in the MAC address table inside the switch. So if Sam is going to send that
information to the SGC server, let’s say, that we just
turned on the switch and there’s no
information that’s currently contained in the MAC
address table of this switch. When Sam sends that
information to the switch, the switch is going to look at
the source MAC address, which is 1000.1111.1111,
and it will recognize that it doesn’t currently have
that address in its table. So it will add that
address, and it will put the output interface
up fast Ethernet zero slash one because that was the
interface where it received that particular frame. The same thing would occur
if the SGC server was to send information
because that source MAC address of 1000.5555.5555
also does not exist in the MAC address table. The switch makes a note
of that new MAC address. And sees that it was coming
from fast Ethernet zero slash five, and adds that to
be related to the MAC address inside of its MAC address table. In that previous example,
the MAC address table did not have a list of the
source or destination MAC addresses. So what happens if a MAC address
destination comes into a switch and the switch has
nowhere to send it? In that particular case,
it floods the network with that particular traffic. Let’s take the example of
Sam sending that information to the SGC server. Sam puts traffic
onto the network that will be destined
for the SGC server. Since that MAC address
table is currently empty, the first thing the
switch is going to do is add that source MAC
address to the interface where that information
was received. But notice that this particular
MAC address table doesn’t have the destination MAC address. And in that case,
the switch just decided the only
way to make sure that traffic gets
to the end station is to send it to
all of the stations on this particular switch. And hopefully, it
will be able to find its way once it’s
received by all of the devices on the network. On this network, this
frame is received by Teal and Daniel and Jack, but
notice that the destination MAC address doesn’t match either
of their workstations. In that case, the Ethernet
adapter on those workstations will simply drop this frame. The only frame that
makes it through is the one that’s destined
for the SGC server, where the destination MAC address and
the MAC address of this device are the same. Now, the traffic’s going
to go the other direction. The SGC server will send
information back to Sam. The source MAC address is
going to be the SGC server and the destination
MAC address is going to match Sam’s MAC address. When the SGC server sends
that frame to the switch, the switch also determines that
that particular MAC address does not exist
inside of its table so we’ll add the SGC server. And of course, it already
knows what the MAC address is of Sam’s device because
it made that entry when the traffic was going to the
SGC server from the beginning. In that particular
case, the frame can be directed to
Sam without having to flood all of the other
interfaces on the network. Your work station has
a similar function that it uses to be able
to find the MAC addresses of those devices
across the network. It does this by sending an
Address Resolution Protocol broadcast, asking for the MAC
address of a particular IP that happens to be on the network. These ARP requests
occur behind the scenes, but if you have a
Packet Analyzer, you can capture
these ARP requests. And you can see exactly the
sender’s IP request, the target IP request, and you can
see what MAC address is being sent as the response
to that particular ARP. You can also look at the
command line on your computer, to see the ARPs that may be
already existing in the ARP cache for your computer. To be able to view
those, you would use the command
ARP-A. If we perform that ARP-A on my
computer, you can see there are a
number of IP addresses that I have communicated
with on this network. And you can see the
MAC address that’s associated with every single
one of those IP addresses.

12 thoughts on “Network Switching Overview – CompTIA Network+ N10-007 – 1.3

  1. It would have been better if IP addresses were in the standard form instead of 1000.1111.1111, just a suggestion. Thanks

  2. What sort of situation(s) would a switch be worried about sending specific data frames to a specific client on the network? Isn't this usually achieved in cloud-based computing or through email?

  3. So ARP is used every time a packet needs to reach a node located on a different switch that is not recorded in the MAC table? I thought the broadcast frame of the source is able to detect the MAC of the node connected on the different switch?

  4. Wouldn't it be more efficient to send an ICMP request to all connected devices and wait for as response than to send the frame to all connected devices? Is it possibly a security risk to send frames to all devices if it was possible for someone to modify the adapter to not drop frames?
    Great Video 🙂

  5. Your videos are incredibly helpful and educational. I keep finding myself having questions halfway through, and yet you answer them if I just listen through. I haven't gotten my a+ or network+ yet but these videos are my top source of knowledge, the fact that they're free is incredible. I've paid hundreds of dollars for college classes that taught me less than your videos do. Thanks so much

  6. If we assume it was a TCP packet that was transmitted through the switch, wouldn't it send a confirmation back to the sender thus adding the receivers MAC to the list? If yes is it safe to assume that this wouldn't work for a UDP packet since it does not send a confirmation back?

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