DES-3226S Layer 2 Fast Ethernet Switch User’s Guide
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Figure 5 - 4. 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.
Figure 5 - 5. IEEE 802.1Q Tag
The EtherType and VLAN ID are inserted after the MAC source address, but before the original EtherType/Length or Logical
Link Control. Because the packet is now a bit longer than it was originally, the Cyclic Redundancy Check (CRC) must be
recalculated.
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