ZyXEL Communications OMC-2301, User Manual

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OMC-2301 User’s Guide
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H A P T E R
8
VLAN Profile
This chapter describes how to configure VLAN profiles for the ONUs to manage VLAN
traffic.
8.1 Introduction to 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 device 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 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 Ethernet
switching devices. 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.
8.1.1 Forwarding Tagged and Untagged Frames
VLAN group ID (or VID) is a unique number than identifies a VLAN. A port VID (PVID) is
the VID associated to a physical port. A PVID defines the VLAN group to which a port
belongs.
The ONU is capable of passing tagged or untagged frames. To forward a frame from an
802.1Q VLAN-aware device to an 802.1Q VLAN-unaware device, the ONU first decides
where to forward the frame and it may then strip off the VLAN tag or add a new VLAN tag.
To forward a frame from an 802.1Q VLAN-unaware device to an 802.1Q VLAN-aware
device, the ONU first decides where to forward the frame, and then inserts a VLAN tag
reflecting the ingress port's default VID.
TPID
2 Bytes User Priority
3 Bits CFI
1 Bit VLAN ID
12 bits