XTT 5000 User's Manual
Technology: VLAN Tagging
A virtual LAN (or VLAN) is an independent logical LAN within a physical network. For
example, with in a single enterprise LAN, different divisions may be grouped within
their own VLANs.
VLANs provide a secure means of sub-diving networks, control broadcast domains,
and manage user access. VLANs are defined in the IEEE 802.1p and 802.1q
standards.
VLANs are designated by a VLAN tag that is added to the MAC frame after the MAC
source address.
The 4-byte tag consists of a 2-byte Tag Protocol Identifier (TPID) and 2-byte Tag
Control Information (TCI). The TPID has a value of 0x8100.
VLAN Tag
Note
: The TPID can actually be thought of as an Ethertype designation, identifying
the payload as a VLAN. The original MAC frame’s Ethertype field is moved to the
inside of the VLAN payload, following the TCI.
The TCI contains the 12-bit VLAN identification, 3-bit priority field, and 1-bit
canonical format indicator (CFI). The VLAN ID can have a value between 0 and 4095.
However, values 0, 1, and 4095 are reserved and best avoided. The priority field
allows the network administrator to assign a value from 0 to 7 based on the type of
traffic. The CFI is always set to 0 for Ethernet traffic.
table.
VLAN Membership
Ethernet traffic can be assigned VLAN memberships through several means:
By Port: all traffic through a particular switch port is assigned the same VLAN.
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Fast traffic forwarding
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