...
The unicast-flood process on R5000 devices is the following: if the frame's destination MAC-address is not included in the device unit's MAC switch forwarding table, then this frame is flooded to all interfaces besides the sender interface. The distribution occurs until the device unit receives a frame with this MAC-address as the sender (i.e., the interface to which the frame was destinated to respond). After that, the R5000 will learn: it will add this MAC address to the MAC switch forwarding table and map it with the interface from which it was received. If the R5000 does not learn in 4 seconds, and frames still arrive, then traffic to this direction will be blocked for 4 seconds. Then the process repeats.
...