VLAN Tagging

• On trunk ports VLAN tagging occurs.
• VLAN tagging does not occur in access ports.
• There are two techniques for VLAN tagging:
  1. DOT1q (open standard : IEEE 802.1q)
     • DOT1q adds only a 4 byte field on the data with the help of which it helps identify the VLAN to which the data belongs to.
     • A 4 byte tag is added within the Ethernet header.  (between SMAC and Ethertype)
     • DOT1q tag – 4 byte:
       • VLAN ID (12 bit field)
       • TPID  (Tag Protocol Identifier) (2 byte)
       • Priority (3 bit)
       • CFI (Canonical Frame Identifier) (1 byte)
     • Native VLAN concept is there in DOT1q
     • Native VLANs data goes untagged via trunk ports.
  2. ISL (cisco proprietary  : Inter Switch Link)
     • ISL adds 30 bytes on the data with the help of which it does the same job that DOT1q does.
     • Even cisco recommends us to use DOT1q instead of ISL.
     • 26 byte ISL header is added in front of Ethernet frame and 4 byte of ISL trailer is added at end.
     • Native VLAN concept is not there in ISL.
• It is recommended to set native VLAN to a VLAN number which is not even being used within the network.
• Native VLAN concept is a bogus concept in today’s world.