ZyXEL Communications P-871H Series, User Manual

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P-871H Series User’s Guide
Chapter 6 VLAN 43
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H A P T E R
6
VLAN
The type of screen you see here depends on the VLAN Type you selected in the Prestige
Setup screen. This chapter shows you how to configure 802.1Q tagged and port-based
VLANs.
6.1 Introduction to IEEE 802.1Q Tagged VLAN
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 Prestige 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 the
Prestige needs 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
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 (212) 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.
6.1.1 Forwarding Tagged and Untagged Frames
Each port on the Prestige is capable of passing tagged or untagged frames. To forward a frame
from an 802.1Q VLAN-aware Prestige to an 802.1Q VLAN-unaware Prestige, the Prestige
first decides where to forward the frame and then strips off the VLAN tag. To forward a frame
from an 802.1Q VLAN-unaware Prestige to an 802.1Q VLAN-aware Prestige, the Prestige
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