VLANs

• In a typical shared LAN...
– Users are grouped physically based on the hub they are plugged into
– Routers segment the LAN and provide broadcast firewalls
• In VLANs...
– you can group users logically by function, department or application in use
– configuration is done through proprietary software
– VLANs can logically segment users into different subnets (broadcast
domains)
– Broadcast frames are only switched between ports on the switch or
switches with the same VLAN ID.
– Users can be logically group via software based on:
• port number
• MAC address
• protocol being used

• application being used
The above figure shows the difference between a LAN and a VLAN
• VLANs...
– work at Layer 2 & 3
– control network broadcasts
– allow users to be assigned by net admin.
– provide tighter network security
The figure shows the formation of a VLAN
• A router provides connection between different VLANs
• For example, you have VLAN1 and VLAN2.
– Within the switch, users on separate VLANs cannot talk to each other
(benefit of a VLAN!)
– However, users on VLAN1 can email users on VLAN2 but they need a
router to do it.
• Switches make filtering and forwarding decisions based on data in the frame.
• There are two techniques used.
– Frame Filtering--examines particular information about each frame (MAC
address or layer 3 protocol type)
– Frame Tagging--places a unique identifier in the header of each frame as it
is forwarded throughout the network backbone.
• Three methods for implementing VLANs
– Port-Centric
– Static
– Dynamic
• Each switched port can be assigned to a VLAN. This...
– ensures ports that do not share the same VLAN do not share broadcasts.
– ensures ports that do share the same VLAN will share broadcasts.

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