Understanding How VLANs Operate
Fixed Switch Configuration Guide 9-5
Forwarding Decisions
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:
• Unlearned traffic: When a frame’s destination MAC address is not in the VLAN’s forwarding
database (FDB), it will be forwarded out of every port on the VLAN’s egress list with the
frame format that is specified. Refer to”Broadcasts, Multicasts, and Unlearned Unicasts“below
for an example.
• Learned traffic: When a frame’s destination MAC address is in the VLAN’s forwarding
database, it will be forwarded out of the learned port with the frame format that is specified.
Refer to “Learned Unicasts” below for an example.
Broadcasts, Multicasts, and Unlearned 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 the example in Figure 9-2, received the frame, the frame would then be sent to all
ports that had VLAN 30 in their Port VLAN List.
Learned 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.
Example of a VLAN Switch in Operation
The operation of an 802.1Q VLAN switch is best understood from a point of view of the switch
itself. To illustrate this concept, the examples that follow view the switch operations from inside
the switch.
Figure 9-2 shows the inside of a switch with six ports, numbered 1 through 6. The switch
associates VLAN 10 with FID 10, VLAN 20 with FID 20, VLAN 30 with FID 30, VLAN 40 with FID
40, and VLAN 50 with FID 50. It shows how a forwarding decision is made by comparing a
frame’s destination MAC to the FID to which it is classified.
Figure 9-2 Inside the Switch
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