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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
76 Switch Management and Operating Concepts
Figure 5-8. 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.