CCNA™: VLAN - Virtual Local Area Networks
Some basic knowledge of LANs,
different topologies, and working of Local Area Networks is required to proceed
What is a
To refresh your memory,
a Local Area Network (LAN) is a set of connected devices like computers, hubs, and
switches sharing the same pool of logical address space. Normally, a router is
required to route packets from one LAN to another LAN. Traditionally, all
packets within a LAN are broadcast to all other devices connected to that
As a result, a traditional LAN has several disadvantages as
Usable bandwidth is shared among all the devices connected to the LAN
ALL devices connected within a LAN can hear ALL the packets irrespective
of whether the packet is meant for that device or not. It is possible for
some unscrupulous node listening to data packets not meant for that.
Suppose, your organization has different departments. Using a traditional
LAN, when any changes take place within the organization, physical cables
and devices need to be moved to reorganize the LAN infrastructure.
A LAN cannot extend beyond its physical boundary across a WAN as in
If you are looking for a simple networked solution for a small office, it may
be a good idea to have a traditional LAN setup with a few hubs or switches.
However, if you are planning for a large building or campus wide LAN for several
individual departments, a VLAN is almost essential.
(VLANs) can be considered as an intelligent LAN consisting of different
physical LAN segments enabling them to communicate with each other as if they were all on the same physical LAN segment.
VLAN: Several of the disadvantages of traditional LANs can be
eliminated with the implementation of VLANs.
1. Improved Performance: In a traditional LAN, all the hosts within the LAN
receive broadcasts, and contend for available bandwidth. As a result, the
bandwidth is shared among all the connected devices within the LAN segment. If
you are running high-bandwidth consumption applications such as groupware or
server forms, a threshold point may easily be reached. After a threshold, the
users may find the LAN too slow or un-responsive. With the use of VLAN, you can
divide the big LAN into several smaller VLANs. For example, if there are two
file servers, each operating at 100Mbps, in a traditional LAN both the servers
have to share the LAN bandwidth of 100Mbps. If you put both the servers in
separate VLANs, then both have an available bandwidth of 100Mbps each. Here the
available bandwidth has been doubled.
2. Functional separation of an institute or a company: It is often required
to separate the functional groups within a company or institute. For example, it
might be necessary to separate HR department LAN from that of Production LAN.
Traditionally, it requires a router to separate two physical LANs. However, you
can set up two VLANs, one for Finance, and the other for Production without a
router. A switch can route frames from one VLAN to another VLAN. With VLAN's it
is easier to place a workgroup together eventhough they are physically in
different buildings. In this case Finance VLAN does not forward packets to
Production VLAN, providing additional security.
3. Ease of Network Maintenance:
Network maintenance include addition, removal, and changing the network
users. With traditional LANs, when ever a User moves, it may be necessary to
re-configure the user work station, router, and the servers. Some times, it may
also be necessary to lay the cable, or reconfigure hubs and switches. If
you are using VLANs, many of these reconfiguration tasks become unnecessary. For
example, you can avoid network address configuration on the work station and the
corresponding router if you use VLAN. This is because, routing traffic within
VLANs doesn't require a router.
However, VLAN's add some administrative complexity, since the administration
needs to manage virtual workgroups using VLANs.
4. Reduced Cost
VLANs minimize the network administration by way of reduced maintenance on
account of workstation addition/deletion/changes. This in turn reduce the costs
associated with LAN maintenance.
Using a LAN, all work stations within the LAN get the frames meant for all
other work stations within the broadcast domain. Since a VLAN splits the
broadcast domain into two or more, it is possible to put work stations sharing
sensitive data in one VLAN, and other work station in another VLAN. Of course,
if two VLANs are not sufficient, you can split the work stations into as many
VLANs as required. VLAN's can also be used to set up firewalls, restrict access,
and send any intrusion alerts to the administrator.
Question: Your network has 100 nodes on a single broadcast domain. You
have implemented VLANs and segmented the network to have 2 VLANs of 50 nodes
each. The resulting broadcast traffic effectively:
A. Increases two fold
B. Remains same
C. Decreases by half
D. Increases 4 fold
Explanation: By implementing
VLANs, the effective broadcast traffic
decreases, since VLANs do not forward the broadcast traffic from one VLAN to