PLANET GSW-4804SF - Manual (Page 58)

User’s Manual of GSW-4804SF
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IEEE 802.1Q VLANs
IEEE 802.1Q (tagged) VLAN are implemented on the Switch. 802.1Q VLAN require tagging, which enables them to span the
entire network (assuming all switches on the network are IEEE 802.1Q-compliant).
VLAN allow a network to be segmented in order to reduce the size of broadcast domains. All packets entering a VLAN will
only be forwarded to the stations (over IEEE 802.1Q enabled switches) that are members of that VLAN, and this includes
broadcast, multicast and unicast packets from unknown sources.
VLAN can also provide a level of security to your network. IEEE 802.1Q VLAN will only deliver packets between stations that
are members of the VLAN. Any port can be configured as either tagging or untagging. The untagging feature of IEEE 802.1Q
VLAN allows 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.
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.
802.1Q VLAN Tags
User Priority CFI VLAN ID (VID)
3 bits 1 bits 12 bits
TPID (Tag Protocol Identifier) TCI (Tag Control Information)
2 bytes 2 bytes
Preamble Destination
Address
Source
Address
VLAN TAG Ethernet
Type
Data FCS
6 bytes 6 bytes 4 bytes 2 bytes 46-1517 bytes 4 bytes