stp loop-protection

Usage Scenario

On a network running a spanning tree protocol, a switching device maintains the status of the root port and blocked port by continually receiving BPDUs from the upstream switching device. If ports cannot receive BPDUs from the upstream switching device due to link congestion or unidirectional link failures, the switching device will re-select a root port. Then, the previous root port becomes a designated port and the previous blocked port enters the Forwarding state. As a result, loops may occur on the network.

To prevent the preceding problems, deploy loop protection. If the root port or alternate port does not receive BPDUs from the upstream device for a long time, 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 keeps blocked and becomes the designated port. In this case, loops will not occur. After the link is not congested or unidirectional link failures are rectified, the port receives BPDUs for negotiation and restores its original role and status.

• An alternate port is the backup of the root port. When the root port can normally send and receive BPDUs, the alternate port is in the blocked state.

• Between two interconnected switching devices in a spanning tree, the switching device nearer to the root bridge is the upstream device of the other devices.

Precautions

Loop protection and root protection cannot be configured on the same interface simultaneously.

If loop protection needs to be configured on an interface in a process with a non-zero ID, the stp binding process command must have been run to bind this interface to the process.