ZyXEL Communications MES-2110, User Manual

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MES-2110 User’s Guide
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H A P T E R
11
VLAN
11.1 Overview
This chapter shows you how to configure 802.1Q tagged and port-based VLANs. A
VLAN (Virtual Local Area Network) is a network that is not limited by the physical
location of a device (such as a switch).
11.2 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 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 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 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 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
2 Bytes
User Priority
3 Bits
CFI
1 Bit
VLAN ID
12 bits