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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