- Hosts in a VLAN live in their own
broadcast domain and can communicate freely.
VLANs create network partitioning and traffic
separation at Layer 2 of the OSI model.
And as I said when I told you why
we still need routers, if you want hosts
or any other IP addressable device
to communicate between VLANs you must
have a Layer 3 device to provide routing.
For this you can use a router that has
an interface for each VLAN or a router
that supports 802.1Q routing.
If you had two or three VLANs you can
get by with a router equipped with two
or three fast Ethernet or gigabit connections.
10Base-T is okay for proxying at home
but for anything else I'd highly recommend
gigabit interfaces for real power under the hood.
Each router interface is then plugged
in to an access link for each VLAN.
This means that each of the routers interface
IP addresses would then become the default
gateway address for hosts in their respective VLAN.
If you have more vlans available than router
interfaces you can configure trunking on
one or more Ethernet interfaces or
purchase a Layer 3 switch.
In this graphic we can see what a fast Ethernet
interface on a router will look like
when configured with 802.1Q trunking.
This allows all VLANs to communicate
through one interface and Cisco calls this
Router on a Stick.
I want to point out that this creates
a potential bottle neck as well
as a single point of failure.
So your host and VLAN count is limited.
To how many?
Well that depends on your traffic level.
To really make things right you'd
be better off using a higher end switch
or routing on the backplane.
But if you just happen to have a router
sitting around configuring this method is free.
However, for performance sake instead
of using an external router interface
for each VLAN or an external Router
on a Stick we can configure a logical interface
on the backplane of the Layer 3 switch
and this is called inter-VLAN routing.
And it is configured with something called
a switched virtual interface or SVI.