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Defining VLAN

By ggoerke ·
Question on the definition of a VLAN.

Any collision domain I define with a layer2 switched port, whether I have a hub or single workstation plugged into the port, becomes a VLAN. Is this correct? To go a step further, on some layer2 switches I could configure 2 ports as a single collision domain - and thus a VLAN. Is this correct?

Does this mean that by definition, when I replace the hubs in the wiring closet with layer 2 switches, each of my workstations/devices will have their own VLAN?

Thank you in advance for answering.

Gary Goerke

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Well, not quite

by zk In reply to Defining VLAN

A hub is a broadcast domain and collision domain. If two workstations on the same hub send data at the same time, collision happens.

Every single port on a layer 2 switch is a seperate collision domain. So even if two workstations on the switch send data frames at the same time, there'd be no collisions (theoretically). If the data are about to collide then the switch will store it temporarily until the other traffic is cleared. Thus it is also called store and forward bridging (used interchangably with switching). There are other methods as well that I can't recall.

Now all the above are about collision. And they have nothing to do with VLAN. Below is something about broadcast.

Both layer 1 hub (repeater) and layer 2 switch are all single broadcast domain by default, that is, if data is sent to a computer it will send it through all its ports. On a hub you can't do nothing about it. On a layer 2 switch, you can seperate them into multiple broadcast domains using VLANs.

So the answer is VLANs are broadcast domains (not collision domains) defined using software (cisco IOS for example). So you can say like port 1-12 are one VLAN (one broadcast domain). 12-24 are different VLAN. So if an ethernet frame is (multicast/broadcast) sent to computers on port 1-4, only computer 1-12 will feel the traffic. But not 12-24. That is the power of VLAN.

VLAN doesn't happen by default. You have to configure it using software like (cisco ios for example). These softwares on switches make them smarter. You can define a VLAN across two switches even. Say port 12-24 on switch 1 + port 1-12 on switch 2 can be a VLAN.

In short, by replacing the hubs with layer 2 switches each of your workstation/devices will be on differentcollision domain (yes) but not on different broadcast domain (VLAN), until you configure it.

HTH & GL

Somebody plz correct me if I'm way off, as it's been a while ;-)

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