Q

Virtual LANs basics

I am trying to learn about Virtual LANs. So far I know what it is and how it works. Could you advise me about implementing one? Do I need special software to manage it? (I have 3com switches 4400 and 4924.)
Regards
Berhe
Dear Berhe,
VLAN's are another hot topic these days and one which every networking guru should try at least once!

Even though you have stated that you aware on how they work, let me point out a few details that will help you troubleshoot and identify your problem, should your switches support VLANs.

There are two types of VLANs:
a) Static VLANs
b) Dynamic VLANs.

Static VLANs are used most in today's networks and are also the most secure. With Static VLANs the VLAN membership is assigned to a port on the switch, rather than the MAC address of the device connected to the specific port.

Dynamic VLANs are more rare and less secure. The VLAN membership is assigned to the MAC address of the host or device. This means that when a host is connected to any port on a switch that's configured to support VLAN, the switch will lookup its internal table and find out which VLAN the particular MAC address is part of and automatically assign the host to the appropriate VLAN.

Now, I am not sure about the two models you are mentioning here. You should visit the 3Com Web site and confirm they actually do support VLAN's.

If they do, then you should obtain some more information from the 3Com Web site on how to configure them.

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In most cases, all switches that support VLANs will use the IEEE 802.1q method of frame tagging. Frame tagging is a way of keeping track of users and frames as they travel through the switching fabric of a switch. It's like a 'tag' that's stuck on each frame in order to identify its VLAN membership.

If you cascade two switches together (as you are most probably doing,) they will use frame tagging only through their special backbone connectors (found in stackable switches.) This 'tag' is removed before it exits the switch port to find its way to the destination pc or device.

This also means that if you tried to cascade your switches hoping that the VLAN would work for both, then it would most probably fail.

In order to cascade two switches together for a VLAN, they must support 'Trunk Links' where the above mentioned 'tags' will be sent through a port of the first switch, into the port of the second switch.
 

This was first published in June 2003
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