3Com 4200G 12-Port Configuration Manual page 74

Figure 1-3 Format of VLAN tag
A VLAN tag comprises four fields: tag protocol identifier (TPID), priority, canonical format indicator (CFI),
and VLAN ID.
The 16-bit TPID field with a value of 0x8100 indicates that the frame is VLAN tagged. On the Switch
4200G series Ethernet switches, the default TPID is 0x8100.
The 3-bit priority field indicates the 802.1p priority of the frame. Refer to the "QoS" part of this
manual for details.
The 1-bit CFI field specifies whether the MAC addresses are encapsulated in the canonical format
for the receiving device to correctly interpret the MAC addresses. Value 0 indicates that the MAC
addresses are encapsulated in canonical format; value 1 indicates that the MAC addresses are
encapsulated in non-canonical format. The field is set to 0 by default.
The 12-bit VLAN ID field identifies the VLAN the frame belongs to. The VLAN ID range is 0 to 4095.
As 0 and 4095 are reserved by the protocol, a VLAN ID actually ranges from 1 to 4094.
The Ethernet II encapsulation format is used here. Besides the Ethernet II encapsulation format, other
encapsulation formats such as 802.2 LLC and 802.2 SNAP are also supported by Ethernet. The VLAN
tag fields are also added to frames encapsulated in these formats for VLAN identification.
VLAN ID identifies the VLAN to which a packet belongs. When a switch receives a packet carrying no
VLAN tag, the switch encapsulates a VLAN tag with the default VLAN ID of the inbound port for the
packet, and sends the packet to the default VLAN of the inbound port for transmission. For the details
about setting the default VLAN of a port, refer to
MAC address learning mechanism of VLANs
Switches make forwarding decisions based on destination MAC addresses. For this purpose, each
switch maintains a MAC address table, of which each entry records the MAC address of a terminal
connected to the switch and to which port this terminal is connected, assuming that no VLAN is involved.
For the ease of management, a MAC learning mechanism is adopted on switches. With this mechanism,
a switch can populate its MAC address table automatically by learning the source MAC address of
incoming traffic and on which port the traffic is received. When forwarding traffic destined for the learned
MAC address, the switch looks up the table and forwards the traffic according to the entry.
After VLANs are configured, a switch adopts one of the following MAC address learning mechanisms:
Shared VLAN learning (SVL), where the switch records all learned MAC address entries in one
MAC address table, regardless of in which VLAN they are learned. This table is called the shared
MAC address forwarding table. Packets received in any VLAN on a port are forwarded according to
this table.
Configuring the Default VLAN ID for a
1-3
Port.