What Is Loop Protect?

As described previously in the overview of Loop Protect, this feature prevents or short circuits loop formation in your network. It does this by requiring ports to receive type 2 BPDUs (RSTP/MSTP) on point-to-point inter-switch links (ISLs) before their states are allowed to become forwarding. Further, if a BPDU timeout occurs on a port, its state becomes non-forwarding until a 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 a 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. This protects against misconfiguration and protocol failure by the connected bridge.