Q Vlan Tags; Port Vlan Id - D-Link DES-3326SR Manual

D-Link DES-3326SR Layer 3 Switch

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.
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.

Port VLAN ID

Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q
compliant network device to another with the VLAN information intact. This allows 802.1Q VLANs to span
network devices (and indeed, the entire network – if all network devices are 802.1Q compliant).
Unfortunately, not all network devices are 802.1Q compliant. These devices are referred to as tag-unaware.
802.1Q devices are referred to as tag-aware.
Prior to the adoption 802.1Q VLANs, port-based and MAC-based VLANs were in common use. These VLANs
relied upon a Port VLAN ID (PVID) to forward packets. A packet received on a given port would be assigned
that port's PVID and then be forwarded to the port that corresponded to the packet's destination address (found
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