ZyXEL Communications XS3800-28, User Manual, Page: 729 / 789

Chapter 88 Networked AV Mode
XS3800-28 User’s Guide
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Figure 581 Shared Server Using VLAN Example
IEEE 802.1Q Tagged VLANs
A tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to identify the VLAN membership of a
frame across bridges
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they are not confined to the switch on which they were created. The VLANs can
be created statically by hand dynamically through GVRP. The VLAN ID associates a frame with a
specific VLAN and provides the information that switches need to process the frame across the network.
A tagged frame is 4 bytes longer than an untagged frame and contains 2 bytes of TPID (Tag Protocol
Identifier, residing within the type or length field of the Ethernet frame) and 2 bytes of TCI (Tag Control
Information, starts after the source address field of the Ethernet frame).
The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. 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. The remaining twelve bits define the VLAN ID, giving a possible maximum number of
4,096 VLANs. 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. Of the 4,096 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.
Forwarding Tagged and Untagged Frames
Each port on the Switch is capable of passing tagged or untagged frames. To forward a frame from an
802.1Q VLAN-aware switch to an 802.1Q VLAN-unaware switch, the Switch first decides where to
forward the frame and then strips off the VLAN tag. To forward a frame from an 802.1Q VLAN-unaware
switch to an 802.1Q VLAN-aware switch, the Switch first decides where to forward the frame, and then
inserts a VLAN tag reflecting the ingress port's default VID. The default PVID is VLAN 1 for all ports, but this
can be changed.
TPID
2 Bytes
User Priority
3 Bits
CFI
1 Bit
VLAN ID
12 bits