D-Link DES-3550 User Manual page 64

DES-3550 Layer 2 Fast Ethernet Switch User's Guide
Figure 6- 18. 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 6- 19. 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
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