In Part I I’ve described what Unicast Flooding is and when and why it actually happens. I have also shown you a topology where unicast flooding does not necessary pose an issue.

In this Part II, I will now show you a topology where unicast flooding does actually create a problem; furthermore, I will show you how to fix it.

So here are our two topologies:

ucast-flooding

LEFT-HAND SIDE
RIGHT-HAND SIDE

In order to fix this problem all you need to do is set the ARP timeout timer to be less than the MAC ageing timer. By doing so, the ARP entry will expire first forcing for an ARP request to be sent out which in turn, will refresh the MAC address table.

Though keep in mind the following:

  • an excessively low ARP timeout will also generate frequent ARP requests
  • similarly, by increasing the MAC address ageing timer, we could potentially cause the CAM to fill up too fast

Finding the right balance will be a matter of experience, memory resources and knowledge of the network.

At last,


Thank you,
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