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VLAN | 1101
VLAN Tagging
Since a port may belong to more than one VLAN, the switch must be able to identify the VLAN two which
a broadcast frame belongs. For this case, IEEE 802.1Q defines a method of marking frames to indicate the
VLAN on which the frame originated.
The marker, called a VLAN tag, is 4 bytes and is inserted after the source MAC in the Ethernet frame
header, as shown in Figure 56-2. The tag is preserved as the frame moves through the network so that
intermediate switches can forward the frame appropriately.
Figure 56-2. 802.1Q VLAN Tag
Ports that belong to more than one VLAN insert VLAN tags into frames and so they are called tagged
ports. Ports that belong to a single VLAN, do not insert VLAN tags into frames and are called untagged
ports. When you add a port to a VLAN, you must specify whether the port should be tagged or untagged.
Figure 56-3. Tagged and Untagged Ports
Ports on either side of the link must have the same tagged/untagged designation, and if tagged, must
belong to the same VLAN. Else, the frame is dropped.
Preamble Destination
Address
Source
Address
Tag
Header
Ether
Type
Data Frame
Check
Sequence
TPID
(0x8100)
Priority
(0-7)
CFI
(0 or 1)
VLAN ID
(0-4095)
Signals that 802.1Q
information will follow
Cannonical
Form Indicator
tagged VLAN 100
tagged VLAN 200
untagged VLAN 100