Configuring Loop Protection on an Interface

To maintain the root port status and status of blocked ports on a network running MSTP, a switch receives BPDUs from an upstream switch. If the switch cannot receive these BPDUs because of link congestion or unidirectional-link failure, the switch re-selects a root port. The original root port becomes a designated port and the original blocked ports change to the Forwarding state. This may cause network loops. To mitigate this issue, configure loop protection.

If the root port or alternate port does not receive BPDUs from the upstream device for a long period, the switch enabled with loop protection sends a notification to the NMS. If the root port is used, the root port enters the Discarding state and becomes the designated port. If the alternate port is used, the alternate port remains blocked and becomes the designated port. In this case, loops will not occur. After the link congestion subsides or unidirectional link failures are rectified, the port receives BPDUs for negotiation and restores its original role and status.

An alternate port is a backup port for a root port. If a switch has an alternate port, configure loop protection on both the root port and the alternate port.

Perform the following steps on the root port and alternate port on a switch in an MST region.