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Configuring VLANs
About VLANs
The Virtual Local Area Network (VLAN) technology divides a physical LAN into multiple logical LANs.
It has the following benefits:
•
Security
—Hosts in the same VLAN can communicate with one another at Layer 2, but they are
isolated from hosts in other VLANs at Layer 2.
•
Broadcast traffic isolation
—Each VLAN is a broadcast domain that limits the transmission of
broadcast packets.
•
Flexibility
—A VLAN can be logically divided on a workgroup basis. Hosts in the same
workgroup can be assigned to the same VLAN, regardless of their physical locations.
VLAN frame encapsulation
To identify Ethernet frames from different VLANs, IEEE 802.1Q inserts a four-byte VLAN tag
between the destination and source MAC address (DA&SA) field and the Type field.
Figure 1 VLAN tag placement and format
As shown in
, a VLAN tag includes the following fields:
•
TPID
—16-bit tag protocol identifier that indicates whether a frame is VLAN-tagged. By default,
the hexadecimal TPID value 8100 identifies a VLAN-tagged frame. A device vendor can set the
TPID to a different value. For compatibility with a neighbor device, set the TPID value on the
device to be the same as the neighbor device. For more information about setting the TPID
value, see "Configuring QinQ."
•
Priority
—3-bit long, identifies the 802.1p priority of the frame.
•
CFI
—1-bit long canonical format indicator that indicates whether the MAC addresses are
encapsulated in the standard format when packets are transmitted across different media.
Available values include:
0 (default)
—The MAC addresses are encapsulated in the standard format.
1
—The MAC addresses are encapsulated in a non-standard format.
This field is always set to 0 for Ethernet.
•
VLAN ID
—12-bit long, identifies the VLAN to which the frame belongs. The VLAN ID range is 0
to 4095. VLAN IDs 0 and 4095 are reserved, and VLAN IDs 1 to 4094 are user configurable.
The way a network device handles an incoming frame depends on whether the frame has a VLAN
tag and the value of the VLAN tag (if any).
Ethernet supports encapsulation formats Ethernet II, 802.3/802.2 LLC, 802.3/802.2 SNAP, and
802.3 raw. The Ethernet II encapsulation format is used here. For information about the VLAN tag
fields in other frame encapsulation formats, see related protocols and standards.
For a frame that has multiple VLAN tags, the device handles it according to its outermost VLAN tag
and transmits its inner VLAN tags as the payload.
TPID
DA&SA
Type
Priority
CFI
VLAN Tag
FCS
Data
VLAN ID
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