What is frame forwarding?

Frame forwarding is the process by which a switch receives an Ethernet frame and forwards it to the right port based on the destination MAC address.

Briefly: forwarding is the switch's decision about exactly where to send a received frame.

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#How frame forwarding happens (step by step)

#1. Receiving the frame

The switch receives an Ethernet frame on one of its ports.

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#2. Learning

The switch:

• reads the source MAC address
• remembers it in the MAC table, together with the port the frame arrived on

This is needed to know where each device is located.

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#3. Looking up the destination MAC address

The switch checks:

• whether the destination MAC address is in the MAC table

Possible outcomes:

• MAC found → forwarding to a specific port
• MAC not found → flooding (sending to all ports except the incoming one)
• Broadcast / Multicast → sending to the corresponding ports

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#4. Forwarding the frame

The switch:

• sends the frame only to the right port
• or sends it out on several ports (multicast/broadcast)

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#Important details for an interview

• Forwarding happens at the data link layer (L2)
• It is based on MAC addresses, not IP
• The switch does not change the contents of the frame
• The decision is made separately for each frame

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#What is NOT forwarding

• routing (that is L3)
• analyzing TCP/UDP ports
• changing IP addresses

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#Short version for an interview

Frame forwarding is the process by which a switch sends an Ethernet frame out the corresponding port, based on the destination MAC address and the MAC table.