Multisource Detection

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 hence 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 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 effectively 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. Multisource detection, which is always enabled, recognizes the multiple source MAC addresses and sets the port‘s operational point-to-point status to false, treating the port as a shared media device. The port is constantly monitored. If the situation is resolved, as determined by receiving a unique address for a sufficient amount of time, the port‘s operational point-to-point status will revert to true.

A syslog message is issued when multiple source addresses are detected.

Note

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.