Bill's Blog on Industrial Ethernet

Have you ever installed a switch only to discover that certain communication does not pass through it? One of the most common issues that arises in the installation or reconfiguration of an Industrial Ethernet switching hub (managed or unmanaged) involves the most defining feature of a switch — its address table.

Typically installers attach various CAT5 cables to a switch, power it up and check for initial functionality. Sometimes the initial functionality will be less than perfect for various reasons: a remote device has not been turned on, auto-negotiation was disturbed by some transient condition, initial cable placement needed to be changed, or some other issue caused a glitch.

Any of the issues mentioned above (among many others) could prompt the installer to move a cable from one port to another to see if the second port behaves in the same way as the first one that experienced the problem. Indeed, your natural inclination might be to move a cable from one port to another, then to another, and so on until each port Link LED has been confirmed to glow as expected.

But the port-swapping scenario described above can create headaches because of how the switch address table works. When a source device sends a message through the switch for the first time, the switch learns that the originating device is connected to a specific switch port and the switch records this device/port association in its address table for future use. If you then move that device's cable to another port on the switch, the switch still "thinks" the device is attached to the first port. Consequently, messages bound for the device will be mis-sent to the original port until the switch "realizes" the situation and updates its table — a process that usually takes several minutes. Until the mis-addressing is corrected, you might well think that the switch is defective. Pings to devices that are "lost" due to port swapping will go unanswered until the address table is correct.

If you find yourself confronting this address issue, the situation will self-correct in a few minutes (typically five) — or you can correct it immediately by cycling power to the switch.