3Com 4500 Configuration Manual page 72

tag is encapsulated after the destination MAC address and source MAC address to show the
information about VLAN.
Figure 1-3 Format of VLAN tag
As shown in Figure 1-3, a VLAN tag contains four fields, including the tag protocol identifier (TPID),
priority, canonical format indicator (CFI), and VLAN ID.
TPID is a 16-bit field, indicating that this data frame is VLAN-tagged. By default, it is 0x8100 in
Ethernet switches.
Priority is a 3-bit field, referring to 802.1p priority. Refer to the "QoS-QoS profile" part of this manual
for details.
CFI is a 1-bit field, indicating whether the MAC address is encapsulated in the standard format. 0
(the value of the CFI filed) indicates the MAC address is encapsulated in the standard format and 1
indicates the MAC address is not encapsulated in the standard format. The value is 0 by default.
VLAN ID is a 12-bit field, indicating the ID of the VLAN to which this packet belongs. It is in the
range of 0 to 4,095. Generally, 0 and 4,095 is not used, so the field is in the range of 1 to 4,094.
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.
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.
1-3