Cisco Systems RJ-45-to-AUX manual Why Use VLANs?

the network.

The processors in each node handle this task, which takes away from the processing power needed for other tasks and application—thus causing a slowdown that the users discover and complain about. Most network administrators pass off this slowness as a problem with the PCs, and the most vital PCs are rebuilt or replaced. When the companies finally decide to upgrade to a switched network, they can typically do so over a weekend. When the network users leave on Friday, their high−powered Pentiums stacked with RAM have the speed of 386s. When they return Monday morning, nothing is more exciting than hearing comments all over the office about how their computers boot up more quickly and run so much faster, and how they like the faster network. But did the users get a faster network? In one sense, the network did get an upgrade; but this upgrade merely eliminated the problems of a flat topology network by segmenting the network into smaller collision and broadcast domains.

How did they do this? By replacing the hubs (which send data they receive out every single port, forcing every node attached to them to process the data whether the node is meant to receive the data or not) with switches. In terms of per−port costs, replacing your hubs with switches is a solution at a quarter of the cost of upgrading the network cabling. So, what segments the network? VLANs.

Note	Sometimes, if you have a 10BaseT network with Category 3 or 4 cabling, the best solution is
	to fix the immediate problems by upgrading to Category 5 cabling and implementing a Fast
	Ethernet network in conjunction with installing switches. However, most network users do
	not need more than true 10Mbps from the Access layer switches to their desktops even if
	they are using high−bandwidth applications. After all, before they had switches, the users
	were getting along with only 3Mbps or 4Mbps on their 10Mbps link, due to broadcasts,
	collisions, and network utilization.

Why Use VLANs?

VLANs are used to segment the network into smaller broadcast domains or segments. The primary reason to segment your network is to relieve network congestion and increase bandwidth. Segmentation is often necessary to satisfy the bandwidth requirements of a new application or a type of information the network needs to be able to support, such as multimedia or graphical design applications. Other times, you may need to segment the network due to the increased traffic on the segment or subnet.

Be careful not to oversegment. Placing each port in an individual VLAN is like placing a router to stop broadcasts between each individual VLAN. Routers are like bug poison—they kill broadcasts dead. Broadcasts can’t escape through routers and they can’t escape a VLAN, either. Each VLAN becomes its own individual broadcast domain. When a network node or workstation sends out an advertisement or broadcast to the other nodes on a segment, only the nodes assigned to the VLAN to which the node sending the broadcast is assigned will receive that broadcast.

Another definition of a VLAN is a logical grouping of network users and resources connected administratively to defined ports on a switch. By creating VLANs, you are able to create smaller broadcast domains within a switch by assigning different ports on the switch to different subnetworks. Ports assigned to a VLAN are treated like their own subnet or broadcast domain. As a result, frames broadcast are only switched between ports in the same VLAN at Layer 2.

Using virtual LANs, you’re no longer confined to physical locations. VLANs can be organized by location, function, department, or even the application or protocol used, regardless of where the resources or users are located. In a flat network topology, your broadcast domain consists of all the interfaces in your segment or subnet. If no devices—such as switches or routers—divide your network, you have only one broadcast domain. On some switches, an almost limitless number of broadcast domains or VLANs can be configured.

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