28.9.3 Loop guard

(1) Overview

When a unidirectional link fault occurs, such as when a one-way line is cut, and BPDU reception is cut off, a loop might have occurred. Loop guard functionality prevents these kinds of loops from occurring.

The following figure shows the problems that occur during unidirectional link faults.

Figure 28-13: Problems when a unidirectional link fails

Loop guard functionality transitions the status of a port for which BPDU reception has been cut off to a non-transferrable status until another BPDU is received. When BPDU reception starts, operation resumes as a normal Spanning Tree target port.

Loop guard functionality cannot be set for ports already set with PortFast functionality for specifying ports connected to terminals, or with root guard functionality.

(2) Notes on loop guard

Loop guards cannot be used for Multiple Spanning Tree.

After loop guard functionality is set, when the following events occur, the loop guard runs to block ports. Loop guards are not cleared until a BPDU is received.

• Starting the switch

• A port goes up (including due to link aggregation)

• The Spanning Tree program restarts

• The type of Spanning Tree Protocol changes (to STP or Rapid STP, PVST+ or Rapid PVST+)

Configure loop guard functionality not only on designated ports, but also on partner switches. When it is configured only on designated ports, even when the above events occur, the designated ports might not receive BPDUs. In cases like this, removing loop guards might take a long time. This is because removing a loop guard requires waiting for BPDU transmission after a BPDU reception timeout is detected on a port on the partner switch.

Also, even if loop guards are set for both ports, removing the loop guard on a designated port might take a long time if no BPDUs are received. Specifically, this happens when a bridge, port priority, or path cost is changes so that the opposing port becomes a designated port, in which case a BPDU timeout is detected on the opposing port, and loop guard operation is performed. If this port is a designated port, BPDUs might not be received, and removing loop guards might take a long time.

When loop guard functionality is set during operation, the loop guard will not run immediately, but instead will run when a BPDU reception timeout occurs.

When a switch that does not forward BPDUs exists between the Switch and a partner switch, and a port is linked up while loop guard functionality is set on both ports, loop guards will continue running on both ports. To perform recovery, BPDU forwarding functionality must be enabled on switches between both ports, and the ports must be linked up again.