R
outers break up broadcast domains. Layer 2 switches are
used to break up collision domains. If you connect all your switches together,
they will be in one broadcast domain. You can break up broadcast domains
in layer 2 switched networks by creating virtual LANs (VLANs). However,
the hosts within a VLAN can communicate only within the same VLAN by
default.
For devices in one VLAN to communicate with devices in a different VLAN,
they must be routed through a layer 3 device. This is called
inter-VLAN
routing.
You can perform inter-VLAN routing with internal route processors in a
layer 2 switch or with an external router called an
external route processor
.
In this chapter, we will cover both internal route processors, known as
Route Switch Modules (RSMs), and external route processors and how to
configure them for inter-VLAN configuration.
Routing between VLANs
A
VLAN's main job is to keep local traffic local, which it does very
well. We have already mentioned in this book that you cannot communicate
between VLANs without a router (layer 3 device), so understanding the con-
figuration of VLANs and understanding routing go hand in hand.
Route processors provide the communication that hosts need between
VLANs. However, if you are using local VLANs (see Chapter 3 for a thor-
ough explanation), you want to design your networks so at least 80 percent
of the users' traffic does not cross over into another VLAN. Therefore, you
should design the network so that the users have access to local servers and
other needed resources to prevent excessive packets from crossing the route
processor.
VLANs should be configured one for one with IP subnet designs. What
this means is that you need to create a subnet mask for your network, then
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