Planet WSD-800 User Manual page 48

User's Manual of WSD-800 / WGSD-910/WGSD-1022C
headers. The tagging feature allows VLAN to span multiple 802.1Q-compliant switches through a single physical
connection and allows Spanning Tree to be enabled on all ports and work normally.
Any port can be configured as either tagging or untagging. The untagging feature of IEEE 802.1Q VLAN allow
VLAN to work with legacy switches that don't recognize VLAN tags in packet headers. The tagging feature allows
VLAN to span multiple 802.1Q-compliant switches through a single physical connection and allows Spanning Tree
to be enabled on all ports and work normally.
Some relevant terms:
Tagging - The act of putting 802.1Q VLAN information into the header of a packet.
Untagging - The act of stripping 802.1Q VLAN information out of the packet header.
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 Ether Type field. When a packet's Ether Type 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 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 12 bits of VLAN ID (VID). The 3 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 12 bits long, 4094 unique VLAN 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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