D-Link DGS-3212SR Layer 2 Gigabit Ethernet Switch
Figure 5-10. 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.
Figure 5-11. Adding an IEEE 802.1Q Tag
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
in the switch’s forwarding table). If the PVID of the port that received the packet is different from the PVID of
the port that is to transmit the packet, the switch will drop the packet.
Within the switch, different PVIDs mean different VLANs. (Remember that two VLANs cannot communicate
without an external router). So, VLAN identification based upon the PVIDs cannot create VLANs that extend
outside a given switch (or switch stack).
Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the switch.
If no VLANs are defined on the switch, all ports are then assigned to a default VLAN with a PVID equal to 1.
Untagged packets are assigned the PVID of the port on which they were received. Forwarding decisions are
based upon this PVID, in so far as VLANs are concerned. Tagged packets are forwarded according to the VID
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