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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
• Egress rules – determines if the packet must be sent tagged or untagged.
Figure 5-8. 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 5-9. IEEE 802.1Q Tag
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