Flooding Unknown Unicast and Broadcast Frames
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Now again turn your attention to the forwarding process, using the topology in Figure 7-5.
What do you suppose the switch does with Fred’s first frame, the one that occurred when
there were no entries in the MAC address table? As it turns out, when there is no matching
entry in the table, switches forward the frame out all interfaces (except the incoming interface)
using a process called flooding. And the frame whose destination address is unknown
to the switch is called an unknown unicast frame, or simply an unknown unicast.
Switches flood unknown unicast frames. Flooding means that the switch forwards copies of
the frame out all ports, except the port on which the frame was received. The idea is simple:
if you do not know where to send it, send it everywhere, to deliver the frame. And, by the
way, that device will likely then send a reply—and then the switch can learn that device’s
MAC address, and forward future frames out one port as a known unicast frame.
Switches also flood LAN broadcast frames (frames destined to the Ethernet broadcast
address of FFFF.FFFF.FFFF), because this process helps deliver a copy of the frame to all
devices in the LAN.
For example, Figure 7-7 shows the same first frame sent by Fred, when the switch’s MAC
table is empty. At step 1, Fred sends the frame. At step 2, the switch sends a copy of the
frame out all three of the other interfaces.
Figure 7-7 Switch Flooding: Unknown Unicast Arrives, Floods out Other Ports