We ran into a rather bizarre problem a while back with a VoIP installation. The installation consisted of a cisco 3560 48-port PoE switch feeding 35 Polycom phones. Most of the phones powered up fine but a few wouldn’t boot.
When a working phone was unplugged, another previously-dead phone would come to life, but the one just unplugged would then remain dead when reconnected.
Checking the power requirements showed that the Polycom phones pulled 6 watts. The switch is rated for 370 watts of PoE power. Even with all ports active, 6 watts X 48 phones is only 288 watts, well within the switch capacity. Yet the switch was denying power to more than about 25 phones. What is going on here? (more…)