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DGS-3200 Series Layer 2 Gigabit Ethernet Managed Switch
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Figure 3 - 4. 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 3 - 5. 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 of 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.
