>> We talked about hubs.
In the last section,
>> we had talked about routers
>> and said routers are going to isolate
>> our network into broadcast domains.
They can help subnet my network
so I have different quality of service segments
>> and privacy and security segments.
>> A router really does a lot of good things
>> for my network but a router is expensive.
>> I'm not really taking a Linksys or a NETGEAR router.
I'm talking about grown-up routers
>> like you'd use in production, a real router.
>> When we have these
>> and I have the situation of my folks on the left
>> and the sales team
>> and my HR folks in the middle,
and the VoIP network over on the right,
I still want to create
>> the separate segments or subnets,
>> but I want to do it cheaper.
That's when I bring back my switch
>> and I enable a feature called VLANs, virtual LANs.
>> A virtual LAN is a function of
the operating system on a switch.
Not every switch has VLAN capability.
It's not required for a switch to support VLANs,
but to tell you the truth,
>> just about all of them today do.
>> Typically a switch is a layer to a device.
We said switches are layered too,
they use MAC addresses to direct traffic.
But when you implement a VLAN on the switch,
you start to script the waters
>> into meeting a Layer 3 switch,
>> which is the next layer up.
With VLANs, you assign certain ports
>> on the switch to a particular VLAN.
>> That VLAN all the hosts on that VLAN
>> will share the same network ID,
>> so all the hosts connected,
>> say into ports 2, 3, and 8.
Whatever ports I want will be part of the sales VLAN.
The HR VLAN assigned
>> to whatever ports I'm plugging into
>> and the same with VoIP.
>> Broadcast isolation is done through these VLANs.
The sales VLAN broadcasts stay
within the ports assigned to the sales VLAN,
same thing for HR and same thing for VoIP.
However, if you're trying to create VLANs on
a Layer 2 switch because a Layer 2 switch does
not understand IP addresses
even though these VLANs
>> can be created and kept separate,
>> the VLANs cannot communicate.
The reason for that is that a switch can't say,
"An IP address at 172.16 goes out these ports,"
>> because the switch
>> doesn't understand the IP addresses.
What happens if you set up a VLAN
>> with a Layer 2 switch,
>> is that you have three separate subnets
because they can't communicate with each other.
There are some cases
>> where that might actually be desirable.
>> You might, for security purposes,
truly want three totally isolated networks,
but you probably want your networks
>> to be able to communicate.
>> If you want your VLANs to be able
>> to communicate you need a device
>> that functions at Layer 3
>> and understands IP addresses.
>> We can go back to the router
>> or we can upgrade our Layer 2 switch
>> and instead have a Layer 3 switch.
>> Going back to the OSI model,
that Layer 3 switch is where IP addressing is used.
Your Layer 3 switches use IP addresses.
Wrapping up with our network connectivity devices,
we talked about hubs,
>> which sends all data out,
>> all ports all the time.
Then we looked at switches that use MAC
addressing because they are Layer 2 devices.
They isolate traffic into collision domains,
that way we can use all
>> but eliminated collisions on our Ethernet networks.
>> Then we looked at routers that isolate traffic
>> and broadcast domains
>> and they're able to interconnect different networks
>> based on IP addresses,
>> but routers are expensive.
We talked about using VLANs to
create that same broadcast isolation.
If we create our VLANs on a Layer 3 switch,
not only do we get the same isolation,
but we also get inter-VLAN communication.