LANs are increasingly being divided into workgroups connected through common
backbones to form virtual LAN (VLAN) topologies. VLANs enable efficient traffic
separation, provide better bandwidth utilization, and alleviate scaling issues
by logically segmenting the physical local-area network (LAN) infrastructure
into different subnets so that packets are switched only between ports within
the same VLAN. When combined with central configuration management support,
VLANs facilitate workgroups and client/server additions and changes. Some
common reasons why a company might have VLANs:
-
Security – Separating systems that have sensitive data from the rest
of the network decreases the chances that people will gain access to
information they are not authorized to see.
-
Departments/Specific job types – Companies may want VLANs set up for
departments that are heavy network users (such as multimedia or engineering),
or a VLAN across departments that is dedicated to specific types of employees
(such as managers or sales people).
-
Broadcasts/Traffic flow – Since a principle element of a VLAN is the
fact that it does not pass broadcast traffic to nodes that are not part of the
VLAN, it automatically reduces broadcasts. Access lists (ACL) provide the
network administrator with a way to control who sees what network traffic.
The Cisco Aironet APs only support the 802.1Q Trunking protocol
standard. Cisco switches and routers can support both the pre-standard
Inter-Switch Link (ISL) protocol and 802.1Q, depending on the model and IOS
image. Switches will not allow different VLANs to talk to one another. A router
will be needed to allow different VLANs to communicate to each other. The Cisco
Aironet APs can be configured with 16 different VLANs for system design
flexibility.
WLANs can now fit nicely into the larger network because
VLANs have been enabled on the APs. This allows WLAN users to roam from access
point to access point maintaining connectivity to the proper VLAN. Figure
–
shows a sample
topology using the VLAN features.