Loop Protect

STP depends on continuous reception of Type 2 BPDUs (RSTP/MSTP) based on the port role. The designated port transmits BPDUs and the non-designated port receives BPDUs. If one of the ports in a physical redundant topology no longer receives BPDUs, then STP assumes that the topology is loop free. This leads the blocking port from the alternate or backup port becomes designated and moves to a forwarding State causing a loop.

Loop Protect protects the network from loops. The Loop Protect feature is achieved by ports receiving BPDUs (RSTP/MSTP only) on point-to-point ISLs before their states are allowed to become forwarding. Further, if a BPDU timeout occurs on a port, its state becomes listening until a new BPDU is received. In this way, both upstream and downstream facing ports are protected. When a root or alternate port loses its path to the root bridge, due to message age expiration, it takes on the role of designated port and will not forward traffic until a BPDU is received. When a port is intended to be the designated port in an ISL, it constantly proposes and will not forward until a BPDU is received. It will revert to listening if it stops getting a response. Loop Protect also overrides the port admin setting. This protects against misconfiguration (such as disabling STP on a port) and protocol failure by the connected bridge.

By default, Loop Protect is disabled on all ports.