VLANs - advantages and disadvantages?

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basically vlans are just virtual representations of a logical network. Imagine you have 3 depts, sales, finance and HR. In each you have 20 pcs. Those PCs go into 3 switches. Those switches can share VLAN info, but you have to set up trunking between them. The management decide to move everyone around rather than have each department on one floor, you have 3 departments spread randomly over three floors.

As it happens, sales do a lot of file sharing and clog up the network. You could stick all of sales into a VLAN, basically a broadcast domain, and no one elses traffic would be affected by their bandwidth hogging antics.

Finance have issues with security, so you can stick them onto a different VLAN. HR only access one other server, so you could create their VLAN to limit the amount of hops their traffic has to travel. You can set it based on switch ports or MAC addresses.

Remember VLANs being broadcast domains require routers to enable communication between VLANs. A layer 3 switch can route.

VLAN 1 always exists.

In summary, VLANs are good for bandwidth restrictions, security (Access lists), broadcast and traffic management, and people/depts with specific needs.

Hope that helps, thats all i remember from my ccna studies :)
 
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