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xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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Figure 7- 1. 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 of 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 originally contained
in the packet is retained.
Figure 7- 2. IEEE 802.1Q Tag