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VLAN Switch Operation
2-4 VLAN Operation
2.4.1 Receiving Frames from VLAN Ports
When a switch is placed in 802.1Q Operational Mode, every frame received by the switch must
belong, or be assigned, to a VLAN.
Untagged Frames
The switch receives a frame from Port 1 and examines the frame. The switch notices that this
frame does not currently have a VLAN tag. The switch recognizes that Port 1 is a member of
VLAN A and classifies the frame as such. In this fashion, all untagged frames entering a VLAN
switch assume membership in a VLAN.
The switch will now make a forwarding decision on the frame, as described in Section 2.4.2.
Tagged Frames
In this example, the switch receives a tagged frame from Port 4. The switch examines the frame
and notices the frame is tagged for VLAN C. This frame may have already been through a VLAN
aware switch, or originated from a station capable of specifying a VLAN membership. If a switch
receives a frame containing a tag, the switch will classify the frame in regard to its tag rather than
the PVID for its port.
The switch will now make a forwarding decision on the frame, as described in Section 2.4.2.
2.4.2 Forwarding Decisions
The type of frame under consideration and the filter setting of a VLAN switch determines how it
forwards VLAN frames.
2.4.2.1 Broadcasts, Multicasts, and Unknown Unicasts
If a frame with a broadcast, multicast, or other unknown address is received by an 802.1Q VLAN
aware switch, the switch checks the VLAN classification of the frame. The switch then forwards
the frame out all ports that are identified in the Forwarding List for that VLAN. For example, if
Port 3, shown in Figure 2-1, received the frame, the frame would then be sent to all ports that had
VLAN C in their Port VLAN List.
NOTE: A VLAN ID is always assigned to a port. By default, it is the Default VLAN
(VLAN ID = 1).