2.13.3. Frame Flooding

Switches flood frames in three situations:

1. Unknown unicast: Destination MAC not in table
2. Broadcast: Frame addressed to FFFF.FFFF.FFFF
3. Multicast: Unless IGMP snooping is configured

Flooding sends the frame out all ports in the VLAN except the receiving port.

By contrast, a known unicast (destination in the table) goes out only the one port the table lists, and a frame addressed to the switch's own MAC (for example, SSH to its management interface) is processed by the switch's CPU rather than forwarded.