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- MAC address table overflow (a common problem in large networks);
- Hosts with ARP timers greater than the ARP cache timeout;
- Incorrect STP settings;
- Incorrect switch groups settings (in particular, when the numbers of switch groups for receiving and sending traffic are different).
The unicast flood process on R5000 devices looks like this: if the frame's destination MAC-address is not included in the device switch group's MAC address forwarding table, then this frame is sent out in all interfaces except the sender interface. The distribution occurs until the device receives a frame with this MAC address as the sender (i.e., the interface to which the packet was destined to respond). After that, the R5000 will learn: add this MAC address to the switch group MAC address forwarding table and fix 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.
This process has the following representation in the unit's interfaces and links graphs:
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