Multisource Detection is a feature that prevents network disruption due to excessive topology changes caused by a full duplex port transmitting multiple BPDUs with different source MAC addresses, and different BPDU information. When a port is point-to-point, the received priority information comes from the most recently received BPDU. When a port is non-point-to-point, the received information reflects the best priority information out of all the received BPDUs. Typical scenarios for multisource detection are when a switch is connected to a device which has been improperly configured to forward received BPDUs out on other ports or has been configured to not run the Spanning Tree protocol and treats BPDUs as multicast packets by transmitting them out all other forwarding ports. In these situations, the connected port is acting as a shared media device. The way to detect shared media is the duplex setting. Since the port is full duplex it treats the connection as point-to-point.
When Loop Protect is configured for the port, if multisource detection is triggered, the port will go to the listening state and no longer be part of the active topology. Loop protect does not operate on shared media ports.