ZyXEL Communications XMG1915 Series User Manual page 267

A tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to identify the VLAN membership of a
frame across bridges
be created statically by hand or 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
4096 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 4094.
TPID User Priority
16 Bits 3 Bits
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.
A broadcast frame (or a multicast frame for a multicast group that is known by the system) is duplicated
only on ports that are members of the VID (except the ingress port itself), thus confining the broadcast to
a specific domain.
48.2.0.1 Port VLAN Trunking
Enable VLAN Trunking on a port to allow frames belonging to unknown VLAN groups to pass through
that port. This is useful if you want to set up VLAN groups on end devices without having to configure the
same VLAN groups on intermediary devices.
Refer to the following figure. Suppose you want to create VLAN groups 1 and 2 (V1 and V2) on devices
A and B. Without VLAN Trunking, you must configure VLAN groups 1 and 2 on all intermediary switches C,
D and E; otherwise they will drop frames with unknown VLAN group tags. However, with VLAN Trunking
enabled on ports in each intermediary switch you only need to create VLAN groups in the end devices
(A and B). C, D and E automatically allow frames with VLAN group tags 1 and 2 (VLAN groups that are
unknown to those switches) to pass through their VLAN trunking ports.
Chapter 48 VLAN
they are not confined to the switch on which they were created. The VLANs can
–
CFI VLAN ID
1 Bit 12 Bits
XMG1915 Series User's Guide
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