An unknown unicast frame is addressed to one destination host, but the receiving switch has no matching destination MAC address in its MAC address table. The switch floods the frame through every port except the receiving port. section 6.3.2
A switch handles the frame as follows:
- Learns the source MAC address and associates it with the port where the frame arrived. section 6.3.2
- Checks the destination MAC address in its MAC address table. section 6.6
- Floods the frame when the destination entry is missing; devices that are not the destination discard it. section 6.3.2
PC1 sends to PC3
With empty MAC address tables, PC1 sends a frame to PC3. SW1 learns PC1’s address and floods the frame; PC2 drops it, while SW2 learns PC1’s address and floods it again. PC3 accepts the frame. When PC3 replies, both switches learn PC3’s address and forward the reply through the appropriate ports.
Do not confuse a switch’s MAC address table with an ARP table: the former maps MAC addresses to switch ports, while the latter maps IP addresses to MAC addresses. section 10.1.1
