24-Port Managed Gigabit Switch
are four additional octets inserted after the source MAC address.
Their presence is indicated by a value of 0x8100 in the EtherType
field. When a packet's EtherType field is equal to 0x8100, the
packet carries the IEEE 802.1Q/802.1p tag (see IEEE 802.1p
Priority section). The tag is contained in the following two octets
and consists of 3 bits of user priority, 1 bit of Canonical Format
Identifier (CFI - used for encapsulating Token Ring packets so they
can be carried across Ethernet backbones and 802.3 source
routing information), and 12 bits of VLAN ID (VID).
The first 3 bits of the VLAN tag indicate the priority of the traffic
which is included in the packet to allow for some basic QoS to
ensure that critical data can pass through the network quickly with
as little delays as possible. The value of this field can be generated
at the end station and updated on every switch (VLAN aware) on
the way as well. More on the usage of these 3 bits can be found in
IEEE 802.1p standard. The VID is the VLAN identifier and is used
by the 802.1Q standard. Because the VID is 12 bits long, 4094
unique VLANs can be identified. The tag is inserted into the packet
header making the entire packet longer by 4 octets. All of the
information originally contained in the packet is retained.
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