VLAN forwarding decisions for transmitting frames is determined by whether or not the traffic being classified is or is not in the VLAN‘s forwarding database as follows:
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 the example in Inside the Switch, received the frame, the frame would then be sent to all ports that had VLAN C in their Port VLAN List.
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.
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