What does a Layer 2 switch use to decide where to forward a received frame?
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When a frame is received, the switch looks at the destination hardware address and finds the interface if it is in its MAC address table. If the address is unknown, the frame is broadcast on all interfaces except the one it was received on.
A Layer 2 switch uses the source MAC address of a received frame to decide where to forward it.
When a switch receives a frame, it examines the source MAC address in the frame's header to determine which port the frame was received on. It then updates its MAC address table, associating the source MAC address with the port on which it received the frame.
If the switch has already learned the destination MAC address in a previous frame, it looks up the destination MAC address in its MAC address table and forwards the frame out the appropriate port. If the switch has not learned the destination MAC address yet, it will flood the frame out all ports except the one it was received on.
The destination MAC address is not used to make forwarding decisions because the switch needs to learn it first before it can use it. In contrast, the source MAC address is already present in the frame's header when the switch receives it, allowing for quick forwarding decisions based on the source MAC address.
The other options listed in the answers, such as source IP address, destination IP address, source switch port, and destination port address, are not used by Layer 2 switches for forwarding decisions because they operate at higher layers of the OSI model. These attributes are used by Layer 3 switches and routers to make forwarding decisions.