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VLAN Switch Operation
13.7.1 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.
The switch will now make a forwarding decision on the frame, as described in
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.
NOTE: A VLAN ID is always assigned to a port. By default, it is the Default VLAN
(VLAN ID = 1).
The switch will now make a forwarding decision on the frame, as described in
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
13.7.2 Forwarding Decisions
The type of frame under consideration and the filter setting of a VLAN switch determines how the
switch forwards VLAN frames.
13.7.2.1 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 Egress List for that VLAN. For example,
if Port 3, shown in
Figure
had VLAN C in their Port VLAN List.
13-10
VLAN Operation and Network Applications
13-2, received the frame, the frame would then be sent to all ports that
Section
13.7.2.
Section
13.7.2.
Section
13.7.2.
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