In a legacy network, administrators
assign users to networks based on geography. The administrator attaches
the user's workstation to the nearest network cable. If the user belongs
to the engineering department and sits next to someone from the
accounting department, they both belong to the same network because they
attach to the same cable. This creates some interesting network issues
and highlights some of the reasons for using VLANs. VLANs help to
resolve many of the problems associated with legacy network designs.
Network managers can logically group
networks that span all major topologies, including high-speed
technologies such as ATM, Fiber Distributed Data Interface (FDDI), and
Fast Ethernet. By creating VLANs, system and network administrators can
control traffic patterns, react quickly to relocations, and keep up with
constant changes in the network due to moving requirements and node
relocation. VLANs provide the flexibility to carry out these actions. The network administrator simply changes the VLAN member list in the switch configuration.
The
administrator can add, remove, or move devices or make other changes to
the network configuration using software. The sections that follow
examine the five issues listed in the Figure that warrant implementation
of a VLAN.