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I saw a picture of a network-structure with VLANs in it:

enter image description here

Why would somebody structure their network like this? Does this even make sense? I mean, they are in a different LAN, why then mapping a VLAN to each one?

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Using VLANs is like breaking a switch into multiple unconnected switches. On a single switch, no hosts on a VLAN will ever see any traffic for a different VLAN. Traffic must pass through a router, where it can be controlled, to get from one VLAN to a different VLAN.

There are many reasons to use VLANs. One is that switches will flood unknown unicast traffic, and broadcast and multicast traffic is sent to all the other interfaces. Someone with bad intentions could be snooping on that traffic for other networks if the switch wasn't broken into VLANs. With VLANs, that type of traffic is restricted to interfaces configured for the VLAN.

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  • So you mean, when each of those 3 sepperate LANs wouldn't be in a different VLAN, the switch would for instance still flood broadcasts from one LAN to the other. That makes sense...
    – watchme
    Jun 1, 2018 at 10:45
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    Exactly. A switch will flood unknown unicasts, broadcast, and multicast to every switch interface in the VLAN. If there is only one VLAN, no matter how many networks, then every interface still gets all that type of traffic. Switches know nothing about network (layer-3) addressing, and VLANs separate the traffic by layer-2, which the switch understands.
    – Ron Maupin ♦
    Jun 1, 2018 at 10:54
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    How is this mapped!? I mean, when a frames which a specific MAC and VLAN arrives at the switch, how does the Switch then know where to send it? Or when a frame with a broadcast-adress arrives? Is there a VLAN-to-Port mapping? is this the vlan.dat-file?
    – watchme
    Jun 1, 2018 at 11:47
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    Yes, there is a mapping. You can run the command "show vlans" on a switch to see the port/vlans association.
    – pHeoz
    Jun 1, 2018 at 12:25
  • @watchme, each access interface is assigned a VLAN in its configuration (the default is VLAN 1), so a switch knows that any traffic received on a particular access interface belongs to a particular VLAN. Trunk interfaces can carry multiple VLANs because they add VLAN tags to the frames in order for the switch on the other end of the trunk to be able to separate the frames back to the different VLANs.
    – Ron Maupin ♦
    Jun 1, 2018 at 19:13
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Generally, network virtualization allows your network's logical structure to become largely independent from its physical structure.

VLANs allow you to segment your network in any reasonable way, no matter where the nodes are physically connected. In your diagram, there is a functional segmentation that is completely independent from the physical connectivity - absolutely valid and reasonable when the separation is required.

Instead of using dedicated segment switches on each floor and running each uplink separately, VLANs allow you to aggregate (trunk) the uplinks and use a single switch (or fewer switches) for horizontal floor distribution.

With additional meshing features like MSTP or RPVST this becomes even more attractive and resource-saving.

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