- [Narrator] Another topic we've looked at earlier
is layer 2 and layer 3 roaming capabilities.
First, let's take a look at layer 2 roaming.
When a client moves throughout a physical building,
their wireless device will associate
with different access points
which is referred to as roaming.
This can happen in a couple of different ways,
either intra controller roaming
or inter controller roaming.
Anytime a client roams between two access points
which are connected to the same wireless controller.
This is called intra controller roaming.
This is the most simplistic form of roaming
and it's also the most common.
If you have an enterprise infrastructure,
with a single wireless LAN controller.
This is what you'll see in your environment.
The roaming occurs entirely within the controller
because the access points are bound to the same controller.
With inter controller roaming,
this takes place when the client roams
from an access point in one area
that's connected to a specific wireless controller,
to an access point in another area,
so those access points are connected
into two different wireless LAN controllers.
In a large environment,
maybe a multi floor hospital for example,
you might need a multi controller setup.
This inter controller roam,
is what we refer to as layer 2 roaming.
Here we can see that there are two controllers,
in this topology.
We see WLC1,
and WLC2.
Both are connected into the main network infrastructure
and both have access to VLAN 100
as we would expect to see,
there are CAPWAP tunnels,
established between the APs and the controllers
and those are used to carry data between the devices.
We've talked about CAPWAP tunnels already
in a previous video.
We can also see that although these two access points
are connected to different controllers,
they both share the 10.1.1.0/24 network.
Our wireless laptop is currently associated
with the first access point AP1
which is interconnected to WLC1
and the laptop has an IP address
of 10.1.1.50.
If the laptop user moves to a different part of the building
and their laptop associates with WLC2 and that access point,
instead of the initial access point,
layer 2 roaming has taken place.
When this happens,
the two controllers exchange mobility messages
with one another
and the client database entry,
is transferred from the original controller to the new one.
In this case that's transferred from WLC1 to WLC2.
If there are no issues,
the client will retain the same IP address
and the entire roaming process
will be completely transparent to the user.
This is also a very fast operation,
on average it takes less than 20 milliseconds
so there's no noticeable drop in service,
during the association handover.
It's also possible,
for this inter controller roaming to take place between,
two access points
which do not lie within the same sub-net and VLAN.
We can see in this topology a slightly different scenario.
We have WLC1 using the LAN 100
in the 10.1.1.0/24 network
while WLC2,
uses VLAN 200
and that's in the 20.1.1.0/24 network.
You can see right away that this might cause an issue.
If our laptop
at 10.1.1.50
if that roams over to WLC2 using a different sub-net
and a different VLAN,
how can we possibly keep our same IP address.
and if we don't, if we have to renew our IP address?
with each one of these roams,
that's definitely going to create some noticeable issues
for the user and some drops in service.
The way this is solved,
is through an additional CAPWAP tunnel
that is created between the wireless controllers themselves.
Here you can see that we do still have CAPWAP tunnels
between the access points
and the controllers just as you would expect,
but we also have an additional CAPWAP tunnel
between,
WLC1,
and WLC2.
Our wireless client currently has an IP address of 10.1.1.50
and let's say they roam over to AP2
in the 20.1.1.0/24 network
where we have VLAN 200 instead of VLAN 100.
What happens?
Well in this case we have a layer 3 roam taking place.
These two controllers will work together,
to build that additional CAPWAP tunnel between themselves
and this tunnel is specifically used,
to carry the client traffic between the new controller WLC2,
back to the original controller WLC1.
This allows the client to retain the same IP address
of 10.1.1.50 even though it is now associated
with the 20.1.1.0/24 network.
In this layer 3 intra controller roam,
we refer to the original controller, WLC1 in this case
as the anchor controller.
This is the controller
where the clients will remain anchored,
no matter where they roam.
The newly associated controller WLC2
is referred to as the foreign controller.
As with layer 2 roaming,
layer 3 roaming is completely transparent to the client
and it allows them to remain attached,
to their original IP sub-net without any interruption.
Without layer 3 roaming,
the client would have to request a new IP address
through DHCP in the new 20.1.1.0/24 sub-net.
In order to allow this roam to take place seamlessly,
it's ideal to organize your controllers
into what are called mobility groups.
And mobility groups are very simply,
just groups of controllers
that we designate as working together
to share access point and client information.
This can also help set boundaries
by creating multiple mobility groups.
If your infrastructure
is large enough to require that many controllers.
Clients can roam seamlessly within a mobility group.
So between WLC1, 2 and 3 for example,
they can roam seamlessly as we've already looked at,
but they can also roam over to mobility Group 2
if we choose to allow that,
we can also choose to disallow that.
So that allows us to set some boundaries.
Now, clients that roam between mobility groups,
this is not a seamless roam.
They will need to re authenticate
with the new mobility group.
So that's a look at the difference
in layer 2 and layer 3 roaming
and where we would see each of those
in use in a wireless network.