3com 4210 PWR 9-Port 3CR17341-91-ME User Manual
Product codes
3CR17341-91-ME
VLAN Overview
75
In Figure 30 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.
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 31 Format of VLAN tag
As shown in Figure 31, a VLAN tag contains four fields, including the tag protocol
identifier (TPID), priority, canonical format indicator (CFI), and VLAN ID.
identifier (TPID), priority, canonical format indicator (CFI), and VLAN ID.
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TPID is a 16-bit field, indicating that this data frame is VLAN-tagged. By
default, it is 0x8100 in 3Com series Ethernet switches.
default, it is 0x8100 in 3Com series Ethernet switches.
■
Priority is a 3-bit field, referring to 802.1p priority. Refer to “QoS
Configuration” on page 299 for details.
Configuration” on page 299 for details.
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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.
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.
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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.
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.
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.
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 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 received packets and the corresponding
ports receiving the packets for consequent packet forwarding. The process of
recording is called MAC address learning.
packets. So that switches maintain a table called MAC address forwarding table to
record the source MAC addresses of the received packets and the corresponding
ports receiving the packets for consequent packet forwarding. The process of
recording is called MAC address learning.
After VLANs are configured on a switch, the MAC address learning of the switch
has the following two modes.
has the following two modes.
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Shared VLAN learning (SVL): the switch records all the MAC address entries
learnt by ports in all VLANs to a shared MAC address forwarding table. Packets
received on any port of any VLAN are forwarded according to this table.
learnt by ports in all VLANs to a shared MAC address forwarding table. Packets
received on any port of any VLAN are forwarded according to this table.
■
Independent VLAN learning (IVL): the switch maintains an independent MAC
address forwarding table for each VLAN. The source MAC address of a packet
received on a port of a VLAN is recorded to the MAC address forwarding table
of this VLAN only, and packets received on a port of a VLAN are forwarded
according to the VLAN’s own MAC address forwarding table.
address forwarding table for each VLAN. The source MAC address of a packet
received on a port of a VLAN is recorded to the MAC address forwarding table
of this VLAN only, and packets received on a port of a VLAN are forwarded
according to the VLAN’s own MAC address forwarding table.
TPID
DA&SA
Type
Priority CFI
VLAN ID
VLAN Tag