H3C Technologies H3C WX3000 Series Unified Switches - Manual (Page 76)

10-3
Figure 10-2 Encapsulation format of traditional Ethernet frames
Type Data DA&SA
In Figure 10-2 DA refers to the destination MAC address, SA refers to the source MAC address, and
Type refers to the upper layer protocol type of the packet. IEEE 802.1Q protocol defines that a 4-byte
VLAN tag is encapsulated after the destination MAC address and source MAC address to show the
information about VLAN.
Figure 10-3 Format of VLAN tag
TPID DA&SA Type Priority CFI VLAN ID
VLAN Tag
As shown in Figure 10-3 , a VLAN tag contains four fields, including the tag protocol identifier (TPID),
priority, canonical format indicator (CFI), and VLAN ID.
z
TPID is a 16-bit field, indicating that this data frame is VLAN-tagged. By default, it is 0x8100 in the
WX3000 series devices.
z
Priority is a 3-bit field, referring to 802.1p priority. Refer to QoS-QoS profile in H3C WX3000 Series
Unified Switches Switching Engine Configuration Guide of this manual for details.
z
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.
z
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 frame format here takes the Ethernet II encapsulation as an example. Ethernet also supports
802.2/802.3 encapsulation, where VLAN tag is also encapsulated after the DA and SA field. Refer to
Encapsulation Format of Ethernet Data for 802.2/802.3 encapsulation format.
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 the default VLAN ID configuration of a port section in
Port Basic Configuration in H3C WX3000 Series Unified Switches Switching Engine Configuration
Guide .
MAC address learning mechanism of VLANs
Switches forward packets according to the destination MAC addresses of the packets. So that switches
maintain a table called MAC address forwarding table to record the source MAC addresses of the