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User’s Manual of LRP-822CS/LRP-1622CS
115
IEEE 802.1Q Standard
lemented on the Switch. 802.1Q VLAN require tagging, which enables them to span the
n the t).
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.
rity to your network. IEEE 802.1Q VLAN will only deliver packets between stations that
are memb LAN. Any port can be configured as either
tagging
or
untagging
.:
 The untagging feature of IEEE 802.1 itches that don't recognize VLAN tags
in packet headers.
LAN to s switches through a single physical connection and
e to be enabled
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
d is
he VID is 12 bits long, 4094 unique VLAN can be identified.
d
802.1Q Tag
■
IEEE 802.1Q (tagged) VLAN
are imp
entire network (assuming all switches o network are IEEE 802.1Q-complian
VLAN can also provide a level of secu
ers of the V
Q VLAN allows VLAN to work with legacy sw
 The tagging feature allows V pan multiple 802.1Q-compliant
allows Spanning Tre on all ports and work normally.
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 an
used by the 802.1Q standard. Because t
The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information originally containe
in the packet is retained.
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 Source
Address Address
VLAN TAG
Ethernet
Data FCS
Type
6 bytes 6 bytes 4 bytes 2 bytes 46-1500 bytes 4 bytes