LevelOne ProCon GSW-1290, User Manual

Page: 69 / 144

LevelOne GSW-1290 Gigabit Layer2 SNMP Switch
Figure 4- 17. 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.
Figure 4- 18. IEEE 802.1Q Tag
The EtherType and VLAN ID are inserted after the MAC source address, but before the original
EtherType/Length or Logical Link Control. Because the packet is now a bit longer than it was originally,
the Cyclic Redundancy Check (CRC) must be recalculated.
69