How 802.1Q Vlan Works - ZyXEL Communications VES-1616 Support Notes

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TPID has a defined value of 8100 in hex. When a frame has the
TPID:
EtherType equal to 8100, this frame carries the IEEE 802.1Q / 802.1P tag.
Priority: The first three bits of the TCI define user priority, giving eight (2^3)
priority levels. IEEE 802.1P defines the operation for these 3 user priority bits.
CFI: Canonical Format Indicator is a single-bit flag, always set to zero for
Ethernet switches. CFI is used for compatibility reason between Ethernet type
network and Token Ring type network. If a frame received at an Ethernet port
has a CFI set to 1, then that frame should not be forwarded as it is to an
untagged port.
VID: VLAN ID is the identification of the VLAN, which is used by the standard
802.1Q. It is 12 bits long and allows the identification of 4096 (2^12) VLANs. Of
the 4096 possible VIDs, a VID of 0 is used to identify priority frames and value
4095 (FFF) is reserved, so the maximum possible VLAN configurations are
4,094.
Note that user priority and VLAN ID are independent of each other. A
frame with VID (VLAN Identifier) of null (0) is called a priority frame,
meaning that only the priority level is significant and the default VID of
the ingress port is given as the VID of the frame.

How 802.1Q VLAN works

Based on the VID information in the tag, the switch forwards and filters frames on
the ports. Ports with the same VID can communicate with each other. IEEE
802.1Q VLAN function defines three tasks: Ingress Process, Forwarding Process
and Egress Process.
All contents copyright 2008 ZyXEL Communications Corporation.
VES-1616/24FA-5x Series Support Notes
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