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This action is known as unicast flooding. Flood If the destination MAC address is not in the MAC address table, then the frame needs to be flooded and is sent to all ports except for the port through which it arrived.
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In this mode, the firewalls send the frame with the source MAC address, which receives in its LAN interface to its WAN interface, therefore the switches of the LAN and the WAN network, have the MAC addresses of their peers, learning these Through each package that arrives through each of its interfaces, this in layer 2 is known as spanning tree.