DES-7000/DES-7100 Layer 2 Switch User
’
s Guide
Switch Management
43
The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the
information contained in the packet originally is retained.
The figure below illustrates the elements of the IEEE 802.1Q tag.
Figure 5-8. 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 5-9. Adding 802.1Q Tag to a Packet Header
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
Summary of Contents for DES-7000 Series
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