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Monday, August 10, 2009
Unexpected Flooding of Switch Ports
On a recent occasion, we had a customer question us about the following confusing situation. An Industrial Ethernet switch had been delivering unicast messages to the proper port for quite awhile when suddenly the messages began flooding to all switch ports.
After a brief investigation, we learned that the customer had been sending a sustained stream of messages to a destination with no acknowledgements from the destination device. The customer had no need of acknowledgement and so had not arranged for a response.
This kind of traffic pattern is seldom encountered and, therefore, rarely causes issues. However, unexpected message flooding can occur under the circumstances that this customer had unwittingly devised. The issue involved the switch address table.
After a MAC address has a prolonged period of inactivity, an Industrial Ethernet switch removes the dormant address from its address table. The switch learns the MAC address of each attached device that transmits to the network. But when a device fails to transmit for a long time, the switch assumes that the device has failed or left the network and so removes the inactive address to improve the efficiency of its address table. This process is called aging and is commonly set to occur after about five minutes.
If you have a situation similar to that described above, you may consider a managed switch. Managed switches generally have the ability to adjust the address table aging time. For example, each model in the several lines of managed switches from Contemporary Controls can have its aging time set to over 12 days.