- I know I keep telling you this,
but so you never forget it
here I go one last time.
By default switches break up collision domains
and routers break up broadcast domains.
In contrast to the networks of yesterday
that were based on collapsed backbones,
today's network design is characterized by
a flatter architecture
thanks to switches.
So now what?
By creating Virtual Local Area Networks or VLANs
we break up a broadcast domain
in a pure switched internetwork.
A VLAN is a logical grouping
of network users and resources connected
to administratively defined ports on a switch.
A VLAN is treated like its own subnet or broadcast domain.
Meaning that frames broadcasted onto the network
are only switched between the ports
logically grouped within the same VLAN.
When you create VLANs you're given the ability
to create smaller broadcast domains
within a layer 2 switched internetwork
by assigning different ports on the switch
to different subnetworks.
Does this mean we no longer need routers?
It really depends on what your
particular networking needs and goals are.
By default hosting a specific VLAN
can't communicate with hosts
that are members of another VLAN.
So if you want inter-VLAN communication
the answer is that you still need a router,
or inter-VLAN routing
referred to as IVR.
This graphic shows the flat network architecture
that used to be so typical
for layer 2 switch networks.
With this configuration every broadcast packet transmitted
is seen by every device on the network.
Regardless on whether the device needs
to receive the data or not.
By default routers only allow broadcast
to occur within the originating network
while switches forward broadcasts to all segments.
Oh and by the way the reason it's called a flat network
is because it's one broadcast domain,
not because the actual design is physically flat.
In this graphic Host A sends a broadcast out on all ports
except for the port that originally received it.
Compare that to this network.
It pictures a switch network and shows Host A
sending a frame with Host D as its destination.
Clearly the important factor here
is that the frame is only forwarded
at the port where Host A is located.
This is a huge improvement over the old hub networks
unless having one collision domain by default
is what you really want.
The biggest benefit gained
by having a layer 2 switch network
is that it creates individual collision domains
for segments for each device
plugged into a port on the switch.
This scenario frees us from the Ethernet density constraints
on older networks and allows us to build larger networks.
But too often each new advance comes with new issues.
For instance the more users and devices
that populate and use a network
the more broadcasts and packets each switch must handle.
But wait.
There's hope
if you create virtual LANs.
You can solve many of the problems associated
with layer 2 switching with virtual LANs.