DES-3226L Layer 2
Fast Ethernet Switch User’s Guide
Figure 6-12. 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 6-13. 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
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