Hi, I have read that the basic purpose of spanning tree was to allow redundant physical links between switches, whereby a redundant link was blocked until the main link failed and then the redundant link would move to the forwarding state. The condition of a loop in the network at layer 2 exists because when a device makes an arp request the switch forwards it out of all ports except the entry port, so when you have 2 or more switches connected with redundant links the arp is broadcast back to itself on another port and on it continues looping eternally. However, how does this explain someone patching a phone into a data point causing a blocked port? The VoIP phone has a single port that the PC connects to, the VoIP phone itself connects to the data point, the data point goes to the patch panel, patch panel to the switch. There can only be one physical link to the switch, so why is spanning tree blocking the port? I thought that you can only cause a loop if you either add a layer 2 device (like a switch) in such a way that it has 2 links to a switch, or you accidentally patch a cable so that the switch has redundant links. As the only way out of the switch to the core is via a different network, then the spanning tree problem can only be local. I don't understand why the port is being blocked. What am I missing?