D-Link DES-3226 User Manual - 802.1Q Vlan Tags

DES-3226 NWay Standalone Fast Ethernet Switch User’s Guide 

Figure 5-6.  IEEE 802.1Q Packet Forwarding 

802.1Q VLAN Tags 

The figure below shows the 802.1Q VLAN tag. There are four additional octets inserted after the source 

MAC address. Their presence is indicated by a value of 0x8100 in the EtherType field. When a packet’s 

EtherType field is equal to 0x8100, the packet carries the IEEE 802.1Q/802.1p tag. The tag is 

contained in the following two octets and consists of 3 bits or user priority, 1 bit of Canonical Format 

Identifier (CFI – used for encapsulating Token Ring packets so they can be carried across Ethernet 

backbones) and 12 bits of VLAN ID (VID). The 3 bits of user priority are used by 802.1p. The VID is the 

VLAN identifier and is used by the 802.1Q standard. Because the VID is 12 bits long, 4094 unique 

VLANs can be identified. 

The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the 

information contained in the packet originally is retained. 

29