Forwarding Unknown Unicasts and Broadcasts


Bridges forward LAN broadcast frames, and unknown unicast frames, out all ports.LAN broadcasts, by definition, are received by all devices on the same LAN.So the switch simply forwards broadcasts out all ports, except the one on which the broadcast was received. Switches forward unknown unicast frames, which are frames whose destination MAC addresses are not yet in the bridging table, out all ports as well. The switch floods the frame with the hope that the unknown device will be on some other Ethernet segment, it will reply, and the switch will build a correct entry in the address table.

Generally speaking, switches also forward LAN multicast frames out all ports, just like they do for broadcast. However, a few multicast features in switches limit the flooding of multicasts, such as internet group Management protocol (IGMP) snooping.



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