Product Manual

DES-3526 / DES-3526DC Fast Ethernet Layer 2 Switch
Figure 6- 34. 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.
Port VLAN ID
Figure 6- 35. Adding an IEEE 802.1Q Tag
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 tagunaware.
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
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