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VLAN Switch Operation
802.1Q VLAN User’s Guide 2-5
The switch will now make a forwarding decision on the frame, as
described in Section 2.4.2, Forwarding Decisions.
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, Forwarding Decisions.
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.
2.4.2.2 Known Unicasts
When a VLAN switch receives a frame with a known MAC address as its
destination address, the action taken by the switch to determine how the
frame is transmitted depends on the VLAN, the VLAN associated FID,
and if the port identified to send the frame is enabled to do so.
When a frame is received it is classified into a VLAN. The destination
address is looked up in the FID associated with the VLAN. If a match is
found, it is forwarded out the port identified in the lookup if, and only if,
that port is allowed to transmit frames for that VLAN. If a match is not
found, then the frame is flooded out all ports that are allowed to transmit
frames belonging to that VLAN.