vlans
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VLANVirtual Local Area Networks
Computer Networks
Tutun JuhanaTelecommunication EngineeringSchool of Electrical Engineering & InformaticsInstitut Teknologi Bandung
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Source: Hands-On Networking, From Theory to Practice, by MARIA LUISA MERANI, MAURIZIO CASONI, ALTER CERRONI, Cambridge Uni.Press, 2009
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• Powerful and flexible solutions to build switched environments
• The evident advantage is to allow the logical grouping of stations into disjoint broadcast domains, regardless of their physical location
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VLAN classification
• Static VLANs – The network administrator assigns switch
ports to a specific VLAN– That’s why static VLANs sometimes referred
to as port-based VLANs• Dynamic VLANs
– The criterion leading the VLAN membership is the station address, either layer-2 (MAC address), or layer-3 (network address)
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Static VLANs
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Dynamic VLAN
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VLAN on a single switch
• Switch configuration encompasses two steps1. The creation of two VLANs with their respective
VLAN identifier, VID
2. The assignment of the desired switch ports to each VLAN
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VLAN on multiple switchesA switch-to-switch connection allows intra-VLAN communication
The two points worthy of mention are:1. Frames originating within both VLANs need to traverse the connection between
the two switches;2. The two switches will have to share some type of VLAN database (no matter
whether a static or dynamic VLAN configuration is adopted)
such a brute force approach would require each switch to know the assignment of the other switch ports for the shared VLAN
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Complex VLAN databases and the consequent burdensome switch forwarding operations are not the correct answer
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The tag
Tagging to the rescue
• The tag solution is partly provided by the IEEE 802.1Q standard
• Each frame carry a tag the tag contain a VID the switches exclusively rely on VIDs for inter-switch forwarding
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Inter-VLAN communications
The presence of a layer-3 device (a router) is mandatory to allow members of different
VLANs to interact
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easy to implement, it lacks scalability
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