Frame flooding sends a frame out every switch port except the port that received it. A switch uses this behavior when a unicast frame’s destination MAC address is missing from its MAC address table, giving the frame a chance to reach its intended host. section 6.3.2
Flooding is different from normal forwarding. A switch forwards a known unicast through the table-selected port, but floods an unknown unicast. Broadcast frames, such as ARP requests sent to ffff.ffff.ffff, are also flooded within the broadcast domain. section 6.6
PC1 sends to PC3
With empty MAC address tables, SW1 learns PC1’s source MAC address and floods the frame. PC2 drops it, while SW2 receives it, learns PC1’s address, and floods it again. PC4 drops the frame, and PC3 accepts it. When PC3 replies, both switches have learned the needed MAC addresses and forward the reply through specific ports instead of flooding. section 6.3.2
Operational points:
- MAC address learning records a source MAC address with the receiving port; dynamic entries age out after five minutes without traffic. section 6.6
- Use show mac address-table to inspect entries; clear mac address-table dynamic commands remove dynamic entries. section 6.6
