1-2
2) Improving LAN security. By assigning user groups to different VLANs, you can isolate them at
Layer 2. To enable communication between VLANs, routers or Layer 3 switches are required.
3) Flexible virtual workgroup creation. As users from the same workgroup can be assigned to the
same VLAN regardless of their physical locations, network construction and maintenance is much
easier and more flexible.
VLAN Fundamentals
To enable a network device to identify frames of different VLANs, a VLAN tag field is inserted into the
data link layer encapsulation.
The format of VLAN-tagged frames is defined in IEEE 802.1Q issued by IEEE in 1999.
In the header of a traditional Ethernet data frame, the field after the destination MAC address and the
source MAC address is the Type field indicating the upper layer protocol type, as shown in Figure 1-2
.
Figure 1-2 The format of a traditional Ethernet frame
IEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field, as shown in Figure 1-3
.
Figure 1-3 The position and format of VLAN tag
A VLAN tag comprises four fields: tag protocol identifier (TPID), priority, canonical format indicator (CFI),
and VLAN ID.
z The 16-bit TPID field with a value of 0x8100 indicates that the frame is VLAN-tagged.
z The 3-bit priority field indicates the 802.1p priority of the frame. For information about frame priority,
refer to QoS Configuration.
z The 1-bit CFI field specifies whether the MAC addresses are encapsulated in the standard format
when packets are transmitted across different media. Value 0 indicates that MAC addresses are
encapsulated in the standard format; value 1 indicates that MAC addresses are encapsulated in a
non-standard format. The filed is 0 by default.
z 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.
When receiving a frame, a network device handles the frame depending on whether the frame is VLAN
tagged and the value of the VLAN tag, if any. For more information, refer to section Introduction to
Port-Based VLAN
.
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