ZyXEL Communications IP PBX X6004 - Manual (Page 139)

Chapter 5 Network Deployment
X6004 User’s Guide
139
5.8 Network Technical Reference
This section contains background material relevant to the Network screens.
IEEE 802.1Q Tagged VLANs
An IEEE 802.1Q tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to
identify the VLAN membership of a frame across bridges - they are not confined to
the device on which they were created. The VLAN ID associates a frame with a
specific VLAN and provides the information that devices need to process the frame
across the network. A tagged frame is four bytes longer than an untagged frame
and contains two bytes of TPID (Tag Protocol Identifier), residing within the type/
length field of the Ethernet frame) and two 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 the
device. 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 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.
IEEE 802.1p Priority Level
IEEE 802.1p defines up to eight separate traffic types by inserting a tag into a
MAC-layer frame that contains bits to define class of service. Frames without an
explicit priority tag are given the default priority of the incoming port.
TPID
2 Bytes User Priority
3 Bits CFI
1 Bit VLAN ID
12 bits