Vlan; Chapter 9 Vlan; Overview; What You Need To Know - ZyXEL Communications XGS1250-12 User Manual

9.1 Overview

This chapter shows you how to configure VLAN settings.

9.1.1 What You Need to Know

Read this section to know more about VLAN and how to configure the screens.
9.1.1.1 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 – they are not confined to the switch on which they were created. The VLANs can
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 12 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.
TPID User Priority
2 Bytes 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.
CFI VLAN ID
1 Bit 12 bits
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H A P T E R

VLAN

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