Amer Networks SS3GR24i - Manual (Page 35)

SS3GR24i/SS3GR48i Layer3 Stackable Gigabit Switch User’s Guide
Figure 4- 1. 802.1Q Packet Forwarding
802.1Q VLAN Tags
The figure below shows the 802.1Q VLAN tag. There 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. The tag is contained in the
following two octets and consists of three bits or user priority, one
bit of Canonical Format Identifier (CFI – used for encapsulating
Token Ring packets so they can be carried across Ethernet
backbones) and twelve bits of VLAN ID (VID). The three bits of
user priority are used by 802.1p. The VID is the VLAN identifier
and is used by the 802.1Q standard. Because the VID is twelve bits
long, 4094 unique VLANs can be identified.
The tag is inserted into the packet header making the entire
packet longer by four octets. All of the information contained in the
packet originally is retained.
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