• Frame tagging is addition of extra VLAN header of 4 bytes in the frame when a frame travels between switches. This frame tagging is used by the receiving switches to determine the receiving VLANs.
• IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual LANs (VLANs) on an IEEE 802.3 Ethernet network.

Structure of VLAN tagged frame.

VLAN tag is of 4 bytes, consisting of following fields:

Tag protocol Identifier(TPID):

• This is 16 bit field and is used just to distinguish between a non VLAN tagged frame and a VLAN tagged frame.
• It is set to value 0x8100.

Tag Control Information(TCI):

This is also 16 bit fields consisting of following sub fields:

Priority:

• This is 3 bits and indicates the priority of the frame that is it provides a mechanism for implementing QoS at the media access control(MAC) level.
• Its value can range from 0 to 7 where 7 is the highest priority.
• Different types of traffic can be set with different priority such as for Video traffic can be set to 4, Voice traffic set to 5 and Network control set to 7 which is maximum.

Drop Eligible Indicator:

This is 1 bit and indicates which frame need to be dropped when there is heavy congestion in the network.

VLAN Id:

• This is the most important of 12 bits and it identifies the VLAN that the frame belongs to. The VLAN id range from 1 to 4095 as 4096 is reserved.
• The hexadecimal values of 0x000 and 0xFFF are reserved.
• All other values may be used as VLAN identifiers, allowing up to 4,094 VLANs.
• The reserved value 0x000 indicates that the frame does not carry a VLAN ID;

Tagged Frame Format:

Now the total Ethernet frame size will be 1500 + 6 + 6 + 4 + 2 + 4 = 1522

Categories: Networking