Category Archives: Messages

Unicast, Multicast and Broadcast messages ?\.

Unknown unicast

What happens though when the switch receives a frame with a destination MAC address that is not included in the table? In that case the switch will just broadcast/flood the frame with the unknown destination address to all of its ports (apart from the port where the frame came from). This process is called unknown unicast flooding. The same will happen if the switch receives a frame with a broadcast/multicast destination address with the difference that this time the destination of the frames is the well-known broadcast/multicast address. A basic example is described below of how a switch, connected to three servers, builds its MAC address table and forwards or floods frames.

When host A is about to send traffic to host B and it does not know the mac address of host B, it will arp for it, the arp request will be sent in broadcast, so the switch will flood out that frame destined to everyone out of all ports in that vlan except the port on which it received the frame. Unknown unicast instead is being sent when a mac address is aged out from the switch cam table or if the switch has never learned that mac address, so it will flood out that frame destined to a specific host (because the sender knows already the destination mac address) which has the mac address included in the frame out of all ports in that vlan except the one on which it has been received. In the end the result is the same in both cases but for different reasons.

In my view, flooding is a process of sending a received frame to all ports except the port it was received on. Broadcast is simply a name for a particularly addressed frame or a packet, but it does not refer to the process of delivering it.