Thursday, July 28, 2011


Where we need VLANS?
1) When we want to set up a LAN between computers in several LANs
2) When we want to separate one LAN in to several LANs
These may be done due to security purposes, load-balancing purposes, less cost, forming virtual groups etc. Therefore, I have listed some uses of VLANs

· Geographic Focus - Traditional network designs focus on physical locations of equipment and personnel for addressing and LAN segment placement. Because of this there are a few significant drawbacks:

· Network segments for physically disjointed organizations cannot be part of the same address space. Each physical location must be addressed independently, and be part of its own broadcast domain. This can force personnel to be located in a central location, or to have additional latency or connectivity shortfalls.
· Relocations of personnel and departments can become difficult, especially if the original location retains its network segments. Relocated equipment will have to be reconfigured based on the new network configuration


Embedding VLAN tag to Ethernet frame, we get what we see below
· TPID - Tag Protocol Identifier (16 bit)
· TCI - Tag Control Information (16 bit)
  • 3 bits for priority information – based on 802.1p standard
  • 12 bits – VLAN identifier – uniquely identifies the VLAN
  • 1 bit - canonical format indicator (CFI)
As shown in above picture there is a virtual LAN tag, which is determined by TPID, and TCI, so this frame can be identified by only VLAN switches. When it goes into a normal LAN last VLAN must convert it to a normal frame.

So why we need a 12 bit identifier when we are sure that most probably there will be less than 20 VLANs (NOTE: 12 bits can represent up-to 4096 VLANs) ??.

Yes, theoretically, we need only four or five bits to represent all VLANs but this is for access controlling purposes. This allows us to come up with a schema for our VLAN tags. Ex: 3 digits for department, 3 for branch, 3 for sections and 3 for uses. This is a silly example, but it will give you some kind of an idea on how to use this 12 bits.