802.1Q Header

Previously we learn about switch port type i.e access port and Trunk port. We also get to know the type of trunk tagging i.e ISl and 802.1Q. Now lets deeply know about 802.1Q taggind done on trunk port.

802.1Q is open standard trunk tagging and is also called encapsulation because it can be addded anywhere inside layer 2 header. The size of header is 4 bytes and contains VLAN information. Where ISl doesnot support Native vlan, 802.1Q supports Native Vlan.
802.1Q Header

Dot1Q header consist two part TPI and TCI.

TPI : Stands for Tag Protocol identifier and it is used just to tell receiving device that tag is of dot1q. the size of TPI is 2 bytes

TCI : Stands for Tag control identifier and size of it is 2 byte which consist of COS, CFI and Vlan ID.
- COS (Class of service) is used in QOS for priotization of Traffic. the size of COS is 3 bits.
- CFI ( Canomical Format Identifier) is used for tokken ring vlan and is not in use nowdays. The size of CFI is 1 bit.
- Vlan ID is used to store Vlan information which ranges from 0-4095 and the size of VLan ID is 12 bits.

Vlan number can be upto 4095 only because the space given for Vlan tagging is 12 bits.
i.e.
(2)12 = 4096
this means ranges from 0-4095