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Chapter 3

VLANs

It is actually better to create your broadcast domain groups (VLANs),

then create a subnet mask that fits the need. That is not always possible, and
you usually have to create VLANs around an already configured network.

VLANs should not extend past the distribution switch on to the core.

Defining VLAN Boundaries

When building the switch block, you need to understand two basic methods
for defining the VLAN boundaries:

End-to-end VLANs

Local VLANs

End-to-End VLANs

End-to-end VLANs are VLANs that span the switch-fabric from end to end;
all switches in end-to-end VLANs understand about all configured VLANs.
End-to-end VLANs are configured to allow membership based on function,
project, department, and so on.

The best feature of end-to-end VLANs is that users can be placed in a

VLAN regardless of their physical location. The administrator defines the
port the user is connected to as a VLAN member. If the user moves, the
administrator defines their new port as a member of their existing VLAN. In
accordance with the 80/20 rule, the goal of an administrator in defining end-
to-end VLANs is to maintain 80 percent of the network traffic as local, or
within the VLAN. Only 20 percent or less should extend outside the VLAN.

Local VLANs

Local VLANs are configured by physical location and not by function,
project, department, and so on as with end-to-end VLANs. Local VLANs are
used in corporations that have centralized server and mainframe blocks
because end-to-end VLANs are difficult to maintain in this situation.
In other words, when the 80/20 rule becomes the 20/80 rule, end-to-
end VLANs are more difficult to maintain, and so you will want to use a
local VLAN.

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